Walter Baade: A Life in Astrophysics 9780691223285, 069104936X, 2001021129

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WA L T E R B A A D E

Walter Baade A LIFE IN ASTROPHYSICS

✶✶✶ Donald E. Osterbrock

✶✶✶ PRINCETON UNIVERSITY PRESS PRINCETON AND OXFORD

Copyright  2001 by Princeton University Press Published by Princeton University Press, 41 William Street, Princeton, New Jersey 08540 In the United Kingdom: Princeton University Press, 3 Market Place, Woodstock, Oxfordshire OX20 1SY All Rights Reserved British Library Cataloging-in-Publication Data is available Library of Congress Cataloging-in-Publication Data Osterbrock, Donald E. Walter Baade : A life in astrophysics / Donald E. Osterbrock. p. cm. Includes bibliographical references and index. ISBN 0-691-04936-X 1. Baade, Walter, 1893–1960. 2. Astrophysicists— United States—Biography. I. Baade, Walter, 1893–1960. II. Title. QB460.72.B22 O77 2001 523.01′092—dc21 [B] 2001021129 This book has been composed in Palatino Printed on acid-free paper. ∞ www.pup.princeton.edu Printed in the United States of America 1 3 5 7 9 10 8 6 4 2

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CONTENTS

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PREFACE 1. The Preparation: Go¨ ttingen and Hamburg, 1893–1927

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2. The Path toward the Two Populations: Hamburg, 1927–1931

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3. Before the War: Mount Wilson, 1931–1938

49

4. War and a Great Discovery: Mount Wilson, 1939–1947

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5. Young Stars and Old: Palomar and Princeton, 1948–1953

112

6. Radio Astronomy and the Size of the Universe: Palomar and Pasadena, 1948–1958

147

7. Telling the Good News: America and Europe, 1953–1959

177

8. The Finale and After: Australia and Go¨ ttingen, 1959–1960

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ABBREVIATIONS

229

NOTES

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BIBLIOGRAPHY

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INDEX

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WALTER BAADE was the great observational astronomer of the middle years of the twentieth century. Edwin Hubble was much better known to the general public, probably deservedly so, for his discovery of the expansion of the universe and his confirmation of the fact that our Galaxy is but one of myriads of roughly similar star systems, spread through space as far as we can “see” with our largest telescopes. Baade, much more voluble with other astronomers but much more publicity-shy with reporters, writers, and broadcasters, “discovered” the two stellar populations which turned out to be young stars and old, and thus opened up the fields of stellar evolution, star formation, and the evolution of galaxies which have contributed so much of our present knowledge of the universe. These fields provide the tools that astronomers use today, to push that knowledge out in space and back in time. Baade’s research achievements have recently been recognized most appropriately by the Carnegie Institution of Washington, which operated Mount Wilson and Palomar Observatories in Southern California, where both Hubble and Baade made their great discoveries. The CIW president announced that its new 6.5-meter (260-inch) stateof-the-art reflecting telescope, nearing completion in the Chilean Andes as I began this preface, would be named the Walter Baade Telescope. This new research instrument in the mountains has now joined the Edwin Hubble Space Telescope in space in helping to acquire the quantitative, factual knowledge that will still further extend our understanding of the universe and its evolution. Certainly Baade deserves a biography of his own. As a research astronomer turned historian of astronomy I have long wished to provide it. This book is the result. My aim in writing it is to present the known facts of Baade’s life and scientific career in interesting and readable form, and to let the reader draw his or her own conclusions about him and the astronomers and astrophysicists with whom he interacted. I have tried to guess at Baade’s thinking from my knowledge of him and of the astronomy of his time, but I have

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been wary of vast generalizations. Of course Baade’s biography includes the astronomical setting in which he worked. It was very different from today, with many fewer astronomers and only a handful of important research institutions. There were no telescopes in space, no ultraviolet or X-ray astronomy, very little infrared, and until the end of Baade’s career no radio astronomy. The ground-based telescopes were smaller, but Baade’s Mount Wilson and Palomar Observatories had all the biggest ones. He and his fellow staff members were an elite group; only they had access to the 100-inch and later the 200-inch reflectors on a regular, continuing basis. There were no national observatories except the specialized Naval Observatory, and no federal funding until just at the end of Baade’s career. All these aspects of the astronomy of his era I have tried to weave in throughout the book. As a graduate student at Yerkes Observatory of the University of Chicago half a century ago, I learned from several teachers, especially Thornton Page, William W. Morgan, Otto Struve, Bengt Stro¨mgren, Gerard P. Kuiper, and William P. Bidelman, about Baade’s recent research accomplishments, and especially of the importance of the populations concept to all branches of stellar and galactic research. I first heard Baade lecture in 1950, at the dedication of the new Heber D. Curtis Schmidt telescope of the University of Michigan, and was entranced by his personality, his way of speaking, and his many new (to me) insights on galaxies and the stars they contain. As a postdoc at Princeton in 1952–53 I worked closely with Martin Schwarzschild; both he and Lyman Spitzer, Jr., the observatory director whom I also came to know very well, were unstinting in their praise of Baade. Following that year at Princeton, in 1953 I participated in the month-long “summer school” (or workshop, as we would call it today) on astrophysics at Ann Arbor, in which Baade was one of the main lecturers. It was a wonderful experience for me, and for all the other postdocs, graduate students, and young faculty members who were there. All of us were inspired by Baade, and many of the participants became outstanding research astronomers and astrophysicists who treasured the insights they had gained from him.

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That summer of 1953 I was on my way west to California, where as an instructor and then an assistant professor at the California Institute of Technology and a staff member of Mount Wilson and Palomar Observatories I saw Baade frequently for five years. He taught me to take direct photographic plates with the 100-inch and 200-inch telescopes and discussed my research with me frequently, even after I became a nebular spectroscopist. Baade’s knowledge, experience, and ideas covered a wide range of astrophysics, and he was always affable, lively, and stimulating. I had a number of long conversations with him, mostly about astronomy, spiced with stories he told me about himself and other astronomers. My last personal contact with Baade was at the American Astronomical Society meeting in Madison, Wisconsin, June 29–July 3, 1958. I had just resigned my position at Caltech to take up a new one at the University of Wisconsin, and Baade was there to give the Henry Norris Russell Lecture of the society as he retired from the CIW research staff. He, Albert E. Whitford (who was leaving the University of Wisconsin to become director of Lick Observatory in California), and I were all in the lounge of Baade’s motel near the campus at midnight on June 30, exchanging toasts with “good Wisconsin beer” (Baade’s phrase) as our jobs changed. I was stunned at the news of Baade’s death in 1960; I had imagined that he would live many more years, and that we would meet again somewhere. After his posthumous book, Evolution of Stars and Galaxies, edited by Cecilia Payne-Gaposchkin, came out in 1963, I used it for twenty-odd years as a supplementary text in graduate courses on galactic structure and stellar populations at Wisconsin and at the University of California, Santa Cruz. Thus I knew Baade well. This book is not based on what he told me, however, although I believe that I understand him better from my contacts with him. Baade was famous for, in the words of Whitford, “never letting a few facts get in the way of a good story.” The facts of his life and career are in his published scientific papers, his book, his correspondence, and his working papers, including notes and reading manuscripts for lectures and talks. Baade saved little of his correspondence and copies of only a few of his own most

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important letters, but by the time of his retirement he had accumulated more than thirty boxes of working notes on stars, novae, supernovae, Cepheid variables, radio sources, peculiar nebulae, galactic and globular clusters, galaxies, clusters of galaxies, selected areas, and measurements of all of these. He also saved the manuscript or notes for practically every colloquium, journal club report, invited paper, lecture, or series of lectures he gave. This material is preserved at the Huntington Library in San Marino, California, as part of its magnificent Mount Wilson Observatory Collection. I am most grateful to Ronald Brashear and Dan Lewis, successively curators in charge of this collection, for the help they gave me in using it. I am also grateful to the Huntington Library for awarding me a Mayer Fellowship, which made it possible for me to spend a full month there in 1993, examining these materials. I have returned many times since to work further with them. The most fruitful sources of Baade letters are other archives, at research institutions and universities where his correspondents’ papers have been preserved. I am especially grateful for help from Dorothy Schaumberg of our own Mary Lea Shane Archives of the Lick Observatory, Judith A. Bausch (Yerkes Observatory Archives), Clark Elliott (Harvard University Archives), Bernard Schermetzler (University of Wisconsin Archives), and several other archivists who are named below. Baade was born and educated in Germany; he lived there until he was thirty-eight years old and always remained a German at heart and by citizenship. His student days at Go¨ ttingen University ended just after World War I, and he carried out some of his most significant research in America during World War II. Thus conditions in Germany, Hitlerism, Nazis, and two great wars in which he was on the losing side, though he lived and worked in the United States for twenty-seven years, are all parts of his story. I am exceedingly grateful to Jochen Schramm, both for his excellent book Sterne u¨ber Hamburg: Die Geschichte der Astronomie in Hamburg, which tells, with many excellent illustrations, the complete story of the observatory where Baade worked from 1919 to 1931, and for sending me copies of all of the correspondence with him that is in the files there. These include a set of personal letters between Baade

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and Richard Schorr, the observatory director who first hired him, and who tried to lure him back from America to be his successor. Dr. Schramm tracked down these letters himself and generously shared them with me. I am also grateful to Theodor Schmidt-Kaler, who has written and published two most interesting articles on Baade and his teachers in Germany, and who has kindly sent me much important information about Baade’s family, which he learned through his own research in Germany. Hilmar Duerbeck and Dieter Reimers were also especially helpful with photographs and information from Germany. Thanks to my family heritage (all German), my high-school teachers, and the University of Chicago language examinations, I can read German fairly well (especially if some of the long words are astronomical), so most of the translations are my own. However, for expert help with the more difficult (to me) passages I am most grateful to Peter Bodenheimer, Andreas Burkert, Alfred Gautschy, Wolfgang Hillebrandt, Wilhelm Kley, and Harold Yorke. My friends Allan Sandage and Halton Arp, Baade’s two Ph.D. thesis students at Caltech, were most helpful with their thoughts, memories, and other informative material. Albert E. Whitford, in many personal discussions, and the late Martin Schwarzschild and Lyman Spitzer, Jr., in their letters, provided extremely important information from their personal knowledge of Baade. Owen Gingerich, whom I first met as a fellow participant in that 1953 summer school, and who has selflessly encouraged my historical endeavors in later years, very kindly made available to me copies of letters which the late Cecilia Payne-Gaposchkin had given him from her scientific correspondence with Baade. Spencer Weart informed me of the microfilm at the the Niels Bohr Library of the American Institute of Physics containing some of the correspondence between Baade and Jan H. Oort, and made it possible for me to use it. Adriaan Blaauw and J. K. Katgert-Merkelijn arranged for me to obtain copies of all the other surviving letters between Baade and Oort from the University of Leiden Library. W. Butler Burton was most helpful in sending me several photographs of Baade from the Leiden Observatory. To all of them I am most grateful.

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Others who have helped with notes, letters, photographs, and other material include Horace W. Babcock, Roger Bell, Ray Bowers, Arthur D. Code, George V. Coyne, S. J., David DeVorkin, Richard D. Dreiser, Ronald D. Ekers, Michael Feast, Roy H. Garstang, Karl Grandin, Janice Goldblum, Dorrit Hoffleit, Karl Hufbauer, I. Khan, Tom Kinman, Helen Knudsen, Douglas N. C. Lin, Sabino Maffeo, S.J., Tina McDowell, Jeremy R. Mould, Joseph F. Mulligan, Leos Ondra, Roswitha Rahmy, Kenneth W. Rose, Walter L. Sanders, Maarten Schmidt, Irwin I. Shapiro, Maxine F. Singer, Don C. Skemer, Tom Steman, William G. Tifft, George Wallerstein, John Whiteoak, Lo Woltjer, and the late Hendrik C. van de Hulst. Finally, let me say that I have used the English-language names of cities and regions in Germany, such as Munich and Westphalia. I have generally used the names of people that they used themselves in adult life, such as Walter Baade (though he was probably christened Walther, the accepted German spelling at the time of his birth) and Cecilia Payne-Gaposchkin. Rudolph Minkowski spelled his first name Rudolf before he emigrated to the United States, but I have used Rudolph throughout the book. Johanna Baade was the name of Walter’s wife, and her nickname, which she evidently used with her friends in Germany, was Hanni, more or less the equivalent of Jo in English. But Baade, at least in his later life in America, called her Muschi, equivalent perhaps to Kitty or Cozy, and most of his American friends who knew them well also called her by that name. Hence I have used that name throughout almost all of the book, although in fact she signed her letters in English to important contemporaries such as Oort and Ira S. Bowen as Hanni Baade, especially after her husband’s death. This book is based on a series of three long articles, published in four installments in the Journal for the History of Astronomy in the years 1995 through 1998. I have rewritten them carefully, including substantial amounts of additional material.1 I am most grateful to Michael Hoskin, editor of JHA, for his excellent editorial assistance with the original articles and for his kind permission to draw on them freely for this book. All the photographs are reproduced with the permission of the institutions, observatories, and individuals credited, for which I am most grateful.

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The Preparation ¨ TTINGEN AND HAMBURG, GO 1893–1927

Introduction Walter Baade was one of the great astronomers of the twentieth century. He opened up the fields of study of stellar and galactic evolution that have made up so much of astronomy in our time, but which were sterile and unproductive before his discovery of the two stellar populations, young stars and old. Baade was lucky in being the right man in the right place at the right time, but he was also able to seize the situation and make the most of it in a way that none of his contemporaries could. Baade was a unique person, a great scientist who was also a warm, friendly human being; a German who was widely admired, loved, and respected in America, which had twice fought bloody wars with his country; a very good teacher who claimed he did not like to teach; a research scientist who was not a professor but who left a generation of astronomers he had advised and inspired behind him. Widely considered “only” an observational astronomer, he had in fact had an excellent training in astrophysics and collaborated in research with astrophysicists all his life. His aim was to understand the universe, and he took us far along the path toward it. Baade’s great discovery of the two stellar populations did not come to him out of the blue at Mount Wilson Observatory in the mountains of Southern California in 1944. His whole life was a preparation for it; the discovery was the culmination of his career, begun twenty-five years earlier, as an observational astronomer who sought physical understanding. A little, undated notebook he started as a young scientific assistant at the Hamburg Observatory,

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probably in 1921, bears the title “Stellar Evolution.” What it actually contains are charts and reductions of variable star measurements in the globular cluster M 53 and references to papers on the spectra of “nebulae,” including not only gaseous nebulae but also galaxies like M 31. But this is in fact the path he followed to open up the whole field of stellar evolution. When Baade gave his first, big, post–World War II invited paper on the two stellar populations, at the American Astronomical Society meeting held at Perkins Observatory in Ohio in 1947, he began by reviewing the steps “which led to the recognition of two distinct types of stellar population. What I want to show is that this conception emerged gradually during the last 25 years.”1 That quarter of a century began in Germany, where Baade was born, educated, and trained in research, and where he began his own work on variable stars, globular clusters, local-group galaxies, and distant clusters of galaxies. Hence Baade’s preparation, mostly in Germany but including one year spent in the United States on a Rockefeller Fellowship, up to the time he left his native country to take a permanent position on the Mount Wilson Observatory staff, was a very important part of his scientific career.

Early Life and Education Walter Baade was born in Schro¨ ttinghausen, a small town in Westphalia, in northwestern Germany in 1893, early in the reign of the young Emperor Wilhelm II. Otto von Bismarck had resigned the prime ministry just three years before Baade’s birth. The little baby was christened, in the German Lutheran custom of those days, with a long string of names, Wilhelm Heinrich Walter Baade, but as an adult he was always known simply as Walter Baade. His father, Konrad, was a schoolteacher and later principal, whose first two wives had died childless. He and his third wife, Charlotte (ne´ e Wulfhorst) had four children. Walter was the oldest, then his brother Martin, and then two sisters, Katherine (“Ka¨ the”) and Elisabeth (“Betti”). The future astronomer received an excellent classical education in Schro¨ ttinghausen and at the Gymnasium in Herford, a larger city where his family moved when he was ten. Besides

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French and English, he studied Latin, Greek, and Hebrew; in later life he liked to say that his parents raised him to become a theologian, but in fact he received very good training in mathematics and science as well. Baade began to show an interest in astronomy at about the age of fourteen. In 1912, when he was nineteen, Baade entered the nearby University of Mu¨ nster for one year, and then transferred to Go¨ ttingen in 1913. It was the Harvard of Germany, with a tradition in astronomy going back to Carl Friedrich Gauss, and in his first year there Baade attended the noted mathematician David Hilbert’s lectures on “mechanics,” actually more like a course on differential equations.2 Baade had been born with a congenital hip defect, which made him walk with a pronounced limp and prevented him from running. It saved him from field service in the German army in World War I, and perhaps saved his life.3 At Go¨ ttingen Baade, an excellent student, took courses in astronomy, mathematics, physics, and geophysics. His teachers included Johannes Hartmann and Leopold Ambronn in astronomy, Hilbert and Felix Klein in mathematics, and Emil Wiechert in geophysics, all of them famous in their fields. Baade worked for Klein as his assistant, and during the war also spent two years in the army auxiliary service at Ludwig Prandtl’s Institute for Experimental Aerodynamics in Go¨ ttingen, while continuing as a part-time astronomy student. Although Hartmann had been a pioneer in photographic astronomical spectroscopy at Potsdam, he was in his sixties when Baade began his graduate work, and Ambronn had devoted his entire career to visual positional measurements. Go¨ ttingen Observatory’s equipment was obsolete, and Baade learned to observe visually with transit instruments and a heliometer, but had no opportunity to use even a moderately large telescope or to take direct photographs or spectrograms of stars. In 1916 he began his astrophysical thesis on the spectrum and orbit of β Lyrae, a bright eclipsing and spectroscopic binary, under Hartmann. Baade measured, reduced, and analyzed the spectroscopic plates of it which his professor had taken at Potsdam years earlier. There were twenty-six spectrograms in all, covering the years from 1900 to 1909, but concentrated especially in the autumn

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of 1908. Baade learned all the necessary measuring techniques, using a high-powered microscope mounted with a hand-operated, precision screw-driven moving stage as a “measuring machine” to identify and determine the stellar and “comparison” lines’ positions on each plate. The next step was to make the long numerical computations necessary to derive the observed stellar lines’ wavelengths, and from their minute differences from their known “rest” (laboratory) wavelengths to compute the radial velocity of the star or stars. Then he could move on to interpreting their meaning. Baade’s progress was slow, because of his duties for Klein and Prandtl, and Hartmann by now was devoting most of his time to teaching, lecturing, and writing on the history of astronomy. In 1919 he became dean of the faculty and head of its mathematical and scientific division, especially demanding administrative posts in that chaotic postwar year, and had little time to guide his student’s work. Baade’s main results were to confirm in large part the results Ralph H. Curtiss had found on β Lyrae from his own similar spectroscopic observations, obtained at Allegheny Observatory in America. He had taken most of his spectrograms in 1907, but in contrast to Hartmann had worked them up and published his paper quickly. According to both Curtiss’s and Baade’s papers, β Lyrae consists of two stars, a brighter B5 one with a strong-emission line spectrum associated with it, and only much weaker absorption lines, and a fainter B8 star, with strong, easily measurable absorption lines. Baade, like Curtiss, derived the period and orbit of the B8 star about the center of mass of the system. This again involved long, complicated numerical calculations, using well-known but sophisticated formulae and fitting procedures. The two astronomers’ results for these orbital parameters were very similar. But Baade, examining the lines’ appearances carefully and how they varied through the period, was able to prove that the weak B5 metallic absorption lines had the same radial velocities as the hydrogen and helium emission line, but varied differently in strength from them. This meant that the source of the emission lines moved with the B5 star and was bound to it, but was not centered on it, because these

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lines were partially eclipsed by the B8 star at a different phase from the B5 continuum and absorption lines. Baade thus proved that one of Curtiss’s two possible interpretations (that the B8 star was in orbit about a postulated much more massive gas cloud which surrounded the B5 star) was untenable. The alternative, that the B8 star was in orbit about the B5, led to reasonable masses for the two stars. But Baade’s interpretation left the question of just where the source of the emission lines is located with respect to the B5 star unanswered. He could not interpret this puzzle then; β Lyrae remained an object of intense study for more than fifty more years as the solution was worked out by numerous astronomers using bigger and better telescopes, spectrographs, computers, and theories. Baade finished his thesis and received his Ph.D. in the summer of 1919, nine months after the Armistice which marked the defeat of Germany and the end of World War I. He clearly had not gained much inspiration from Hartmann and Ambronn, but had learned many techniques from them, and above all had proved to himself that he liked research and could work hard at it and get results, even from old data. His thesis was not published in full, and remained unknown to most later researchers. It fulfilled the formal requirements for the degree, but the main new ground it broke was in asking questions that he could not answer with only the spectrograms Hartmann had taken more than a decade earlier.4 Baade’s degree really meant that his professors knew that they had taught him all they could, that the war was over, and that he was ready for a research career. Evidently he had been doing a lot of reading on his own, including copies of whatever journals reached Go¨ ttingen from America, for he knew about some of the research that was going on with the big telescopes there. This independence, selfreliance, and absolute dedication to research were to prove to be three of Baade’s strongest characteristics throughout his life. The following April, less than a year after Baade’s degree was granted, Harlow Shapley and Heber D. Curtis held their “Great Debate,” actually a pair of lectures, in Washington before the assembled members of the National Academy of Sciences. Curtis upheld his view that the spiral “nebulae” were really “island universes”

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or star systems, while Shapley maintained that they were indeed nebulae. He was wrong, and Curtis was right. Understanding the structure and makeup of island universes, which we call galaxies today, was to be Baade’s life work.5

Hamburg Observatory In 1919 Mount Wilson was the most famous observatory in the world. Its 60-inch reflector had begun work in 1908. An outstanding staff, directed by George Ellery Hale and headed by Walter S. Adams, had begun making brilliant discoveries on the astrophysical nature of stars. In 1919 the 100-inch went into operation, the largest telescope in the world (which the 60-inch had been until 1917). Baade, who was especially interested in astrophysics and in stellar spectroscopy, wanted to go to Mount Wilson immediately, to do research with the 100-inch as an unpaid volunteer “assistant,” more or less the equivalent of a modern “postdoc.” This is just what Henri Chre´ tien had done in 1909–10, with the 60-inch, under the sponsorship of Nice Observatory. However, Baade’s professors advised him that such a program would be quite impossible for a German citizen less than a year after the Armistice. The twenty-sixyear-old new Ph.D. would have to get a job in his own, defeated country. He had heard through his friend Heinrich Rauschelbach, a fellow 1919 Go¨ ttingen Ph.D., of an opening for an assistant at Hamburg Observatory, and wrote to its director, Richard Schorr, to apply for the job. Baade’s teachers, Ambronn and Hartmann, recommended him strongly to Schorr, emphasizing his thorough training in classical astronomy and his strong interest in astrophysics. Baade was “clever and industrious,” “very apt and thorough,” and “a very pleasant and amiable person” (a recurring description of him all his life). At the end of his paean of praise for Baade, Hartmann advised Schorr to “grab him.” The Hamburg director did so, and Baade started work on October 1, 1919.6 Edwin Hubble, who had completed his Ph.D. thesis on “faint nebulae” (mostly galaxies) at the University of Chicago in 1917, then served as an infantry officer in

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the American Expeditionary Forces in France, had begun working at Mount Wilson just a month earlier.7 The Hamburg Observatory was actually located in Bergedorf, a village outside the smoke and the worst of the light pollution of the big port city. With his job, Baade was assigned a two-room apartment at the observatory where he was required to live; Rauschelbach had not been hired because he was married and all the apartments in the “observatory dwelling” were for single men only. Thus did Baade qualify for his first observatory position! Luckily for Rauschelbach, who had been much more interested in measuring positions of the moon and planets than Baade, and in making time determinations with a meridian circle, there was also an opening for an astronomer in the Time Service of the German Naval Observatory at Hamburg, so he got a job too.8 Schorr, then forty-eight years old, had been the director of Hamburg Observatory since 1902. He was not famous for any research he had done, but he had earned his Ph.D. at Munich under a great teacher of the previous generation, Hugo von Seeliger. Schorr, a short figure, always erect and well dressed, concentrated on keeping on good terms with the rich Hamburg merchants and the important local and national officials who controlled his institute’s budget. Its main instrument was its 1-meter (40-inch) reflector, the largest telescope in Germany. Baade, young and eager, intelligent and quick to learn, soon made himself a master observer. Schorr, who had supposedly been using the telescope himself, was busy and greatly occupied by his administrative tasks. In 1920 he turned all the observing with the reflector over to Baade.9 The young Ph.D. was patient and skillful in guiding the long photographic exposures, yielding the accurate positions of comets and asteroids which Schorr, a classical astronomer, considered the most important work in his observatory. But Baade, reading the Astrophysical Journal and the Mount Wilson Observatory Contributions, was much more interested in variable stars, globular clusters, and other astrophysical research he could do by direct photography. One minor project Schorr assigned to Baade was to take a few exposures of the variable star SS Cancri, which Kasimir Graff, a senior Hamburg astronomer, was observing visually with the “great”

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60-cm (24-inch) refractor of the observatory. His long series of measured magnitudes determined its visual light curve, and Baade’s photographic magnitudes then gave its photographic light curve.* In this little program, Baade cut his teeth on the variable-star measurements that were to be the basis of so much of his research at Hamburg, Mount Wilson, and Palomar.10 In 1919 Schorr had taken sixty-seven direct photographic plates with the 1-meter reflector; the next year Baade, devoting full time to observing, obtained more than five times as many. Nearly all of Schorr’s were exposures on comets and asteroids, to measure their positions accurately, for determining or correcting their orbits. The next year Baade, working under Schorr’s supervision, obtained more plates of comets and asteroids than his director had the previous year, but in addition began his research on the Orion nebula, star clusters, novae, and variable stars. Among the “novae” or “new” stars, Baade took five plates of the one which Max Wolf, the dean of German photographic observers, had discovered at Heidelberg, in the little spiral galaxy NGC 2608. Following the practice of that time, Wolf sent postcards announcing his discovery to other European observatories, including Hamburg, and Baade pounced on the nova. With his 1-meter telescope he could observe it to considerably fainter magnitudes than Wolf could with his smaller instrument at Heidelberg. Both astronomers recognized that the “new” star, nearly as bright as the whole galaxy in which it was located, was a most unusual nova. Baade was able to obtain one last plate of it in 1922, before it faded into invisibility. It was in fact the first supernova he observed, a class of objects much more luminous than “ordinary” novae, and Baade was to become the leading researcher on them in America once he got to the really large telescopes of Mount Wilson.11 In 1921, given a little additional freedom in his choice of objects, Baade worked even harder and took 456 plates, a hundred more * A star’s visual magnitude, mv, is basically a measure of its brightness in yellow light, because the human eye is most sensitive to that color, while the photographic magnitude, mpg, depends on its brightness in the violet and blue spectral region. The color index, CI = mpg − mv, is a quantitative measure of its color, negative for blue stars, near zero for white stars, and positive for yellow and red ones.

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than in his first year. Schorr had allowed him to begin working on his own much more astrophysical projects. One was a study of the globular cluster M 53; Baade obtained 73 plates of it to search for variable stars. He also took a few plates each of the globular clusters M 92 and NGC 4147, and nearly 50 more of the Orion nebula. In addition Baade took 35 plates of the globular cluster M 3 for Johannes Larink, then a student at Hamburg but soon to become a junior staff member, to use in a massive study of the variable stars in it. Schorr certainly assigned Baade to this project, but the young enthusiast had probably initiated it himself by calling his director’s attention to the pleas of the former Harvard College Observatory director, Edward C. Pickering, for more data on this variable-rich cluster. Larink worked up all the plates, measuring the light variations of 137 variable stars in M 3, many of which he had found himself on these plates. He derived light curves and periods for 110 of these variables, which became the standard of comparison for other globular clusters.12 That same year Baade also made a first, tentative step into galaxy research. He took six plates of the large spiral “nebula” M 33, in reality a giant star system similar to the Milky Way. “Blinking” or comparing these plates with one another, he found three variable stars in the “nebula.” This was a real discovery by Baade, made in 1921, a year before John C. Duncan published his independent discovery of three variables in this same spiral. Except for novae, Baade’s and Duncan’s variables were the first objects definitely identified as stars like those in our Galaxy in any other galaxy except the Magellanic Clouds. Baade never published his discovery, probably because he wanted to take more plates of these variables before doing so, and after Duncan’s paper appeared it was too late. The next year Wolf revealed that he had photographs going back twenty years showing another variable in M 33. All three of Baade’s variables were probably like two of Duncan’s and Wolf’s one, highly luminous irregular variables which Hubble found in M 33 with the 100-inch telescope and published in 1925, confirming beyond anyone’s doubt that spiral “nebulae” were remote star systems.13 In 1921 Baade wrote to Harlow Shapley, now the new director of Harvard College Observatory, and to the older Solon I. Bailey, its

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former acting director, both variable-star experts, to describe his own discoveries in the globular clusters M 53 and M 92 and in the Orion nebula. These research projects were closely related to the concept of the two stellar populations, then still a quarter of a century in the future. Bailey replied, congratulating Baade on his results and encouraging him to go on with this work, while Henrietta S. Leavitt, the discoverer of the period–absolute magnitude relation for Cepheid variables which was to form the basis for so much of Baade’s later research, replied along similar lines for Shapley.14 In the summer of 1922 the Harvard director went to Europe for the International Astronomical Union meeting in Rome, and met Baade at Bergedorf. Shapley encouraged the young German to continue his observational program on the variable stars in globular clusters. Baade’s results, which Shapley recognized as highly accurate, were useful to him in mapping the distribution of the clusters in the Galaxy and measuring the distance to its center. At Baade’s request, Shapley emphasized the importance of this work to Schorr, urging him to let his impatient young observer spend more time on it. Baade began a concerted program to find variable stars in the Milky Way and to study their distribution, another problem related to his future two-population concept.15 Even in these first real research projects of his own, Baade went well beyond Shapley’s scientific ideas. In the globular cluster M 53, Baade found many additional “cluster-type variables” (RR Lyrae variables in today’s terminology), more than doubling the number discovered at Harvard. Shapley had shown that all the variables of this type in a given cluster, pulsating stars with periods shorter than a day and with a range in brightness of the order of 0.5 to 1.0 magnitude, have the same median absolute magnitude (or intrinsic luminosity). Furthermore, he had assembled convincing evidence that this median absolute magnitude is the same for all globular clusters. Thus the cluster-type variables formed the primary basis of Shapley’s distance determinations of globular clusters. He used the fundamental relationship between apparent magnitude, absolute magnitude, and the distance, expressing the fact that more distant stars are fainter than nearby stars of the same type.

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Nearly every globular cluster had at least a few cluster-type variables (Baade, like Shapley, adopted their presence as the definition of a globular cluster in doubtful cases) and only a few fairly bright examples, such as RR Lyrae itself, were known to exist outside globular clusters. However, Baade, searching his first plates of M 53, found two faint variable stars over half a degree from its center, far outside the recognized limits of the cluster. Recognizing their importance, he switched to larger plates, to cover a larger field and find other possible variable stars. With the Hamburg F/3 reflector, the coma (an inherent aberration or blurring of the stellar image of a reflecting telescope) was very bad near the edges of this 2.5° < 3.4° field, but Baade, a careful, diligent observer, was able to focus, center, and guide the telescope so that at least the images were the same on every exposure. Eventually he discovered five more variable stars in the larger area which he would have missed if he had continued to use the standard-size plates. Baade had made himself as expert as it was possible to be in the photographic photometry of his day, and set up a magnitude sequence of local standard stars in the sky near the cluster by comparison with one of the published Harvard regions. He could thus determine accurately the magnitudes of the seven variables on each exposure, construct their light curves, and determine their periods. Five of the seven turned out to be cluster-type variables (one of the other two was an unknown type and the other was red and had a period longer than twenty days, and was thus presumably a longperiod variable). From their apparent magnitudes and Shapley’s standard value for the median absolute photographic magnitude of a cluster-type variable, Mpg = −0.23, Baade could then calculate the distances of the individual stars, ranging from 16,300 light years (the units he used) for the nearest to 63,800 light years for the farthest. The cluster itself was at 72,400 light years, measured in the same way. From the unavoidable measurement errors, and the small range in absolute magnitudes of cluster-type variables (as then known), Baade wrote that it was possible that the furthest of the five might be at approximately the same distance as the cluster, but the other four

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clearly were far outside its limits not only as seen in projection on the plane of the sky, but also in distance from the sun. Furthermore, their distances were very large, and since M 53 and the field around it are at high galactic latitude, b = +79° (the reason Baade was studying it), these isolated cluster-type variables were very far from the galactic plane, contradicting the received ideas of the time. Baade, no doubt with a touch of irony, wrote that from the investigations of Jacobus C. Kapteyn and others “we know that in the regions of the galactic poles the star density of our Milky Way system falls practically to zero at distances of 10,000 to 12,000 light years.” But the variable stars he had discovered “show that, far outside the proper Milky Way, in the system extended by the globular clusters, individual stars occur whose distances are comparable with those of the globular clusters.” This was Baade’s first published statement on a subject to which he was to come back time after time for the rest of his life. It culminated in his announcement of the two stellar populations twenty-two years later, and he was still following it up in the last year of his life, observing RR Lyrae variables (as “cluster-type variables” were by then known) in “Baade’s window” near the galactic center with the Mount Stromlo 1.9-meter telescope. In his 1922 paper Baade merely wrote that he was investigating another, even more distant cluster, NGC 5053.16 Along with the galactic structure work, Baade kept up his assigned observations of minor planets, and discovered a new comet, for which he received a medal from the Astronomical Society of the Pacific. By now Schorr was pleased with his skilled, hard-working assistant and gave him raises regularly, though the runaway German inflation made the value of his salary uncertain.17 Baade had also earned more freedom in choosing his own programs, and he photographed many more globular clusters and “nebulae,” both planetary nebulae and galaxies, to see for himself what they were like and what he could do with them. In 1923 he began observing M 31, the Andromeda galaxy, which was to be the subject of so much of his later work at Mount Wilson and Palomar, because it is relatively close to our Galaxy and more similar to it than M 33 is. That same year the Swedish astronomer K. Gustav Malmquist, just Baade’s age, came to Hamburg from Lund University to work with

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him as a volunteer assistant. He learned Baade’s photometric methods, taking plates to measure the color indices of large numbers of stars photographically. This involved using “isochromatic” plates, the earliest type sensitive to yellow light, with a yellow filter to match the color sensitivity of the eye.18 The Hamburg director, like many other heads of observatories, was a great believer in accumulating observational data for future use, even for programs in which he was not directly involved himself. Thus he participated in, or sent out, many solar eclipse expeditions. On January 24, 1925, an eclipse was to occur whose path of totality ran mostly over the North Atlantic Ocean. Schorr arranged to observe it from a freighter of the Hamburg-American Line, which would steer its course to be in the track at the right moment. He took Baade with him to help operate the cameras with which they would record the corona and prominences. They sailed from Hamburg and were at sea for more than a week before the eclipse. The day was stormy and the ship was rolling, but the sky began clearing and they managed to take a series of photographic plates at totality; twenty minutes later the sky was completely overcast again. After another two weeks they reached Philadelphia, where Schorr and Baade debarked. They spent ten days visiting observatories in the East, Sproul Observatory at Swarthmore College, Allegheny Observatory in Pittsburgh, the Naval Observatory in Washington, and Harvard. It was Baade’s first sight of the United States. He and Schorr had to sail back to Hamburg from New York in midFebruary, but he wanted to return to America as soon as he could.19 Schorr and Baade had seen Shapley in Cambridge. They both thought that the time was ripe for Baade to come to America, to broaden his astronomical experience. Schorr asked Shapley, already one of the most powerful men in American astronomy, for his help. The Harvard director told him that he thought that by now a German could get a fellowship to come to the States and work, although he was not certain. He would recommend Baade to the International Education Board, the arm of Rockefeller philanthropy which provided the main source of American financial support for European scientists abroad and in the United States. If Schorr could make part of the support available from Hamburg, the Rockefeller

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Walter Baade (left) and Richard Schorr (right) with the ship’s captain in the North Atlantic Ocean during the 1925 solar eclipse expedition. (Courtesy of Hamburg Observatory.)

Foundation would probably pay the rest, Shapley wrote. He himself recommended Baade strongly to the board, as “one of the very best of the young German astronomers,” and went on to boast that American observatories were the acknowledged world leaders in astrophysics, so that rather than Americans going abroad to study in that subject as they had a generation before, now the Europeans wanted to come to the United States. There was no question, Shapley wrote, of Baade’s “ability and energy.”20 Baade filled out an application blank (his first communication written in English) in which he stated he could speak German and

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English, and could read those languages as well as French, Latin, Greek, and Hebrew. He proposed to do “astrophysical observations and investigations” during a one-year visit to several American observatories. Schorr (who had undoubtedly insisted that Baade return that soon) recommended him strongly as “a very gifted young astronomer, well trained in both the mathematical and physical sides,” with excellent practical astronomical experience and observing skills. He stated that he would continue Baade’s salary while he was in America on leave, if he got the fellowship, but that because of “present conditions” (the Weimar Republic inflation) he could do no more.21 Shapley had advised Schorr well; Baade got the fellowship and an important factor was the 295 marks (approximately $75) per month in salary which the Germans would provide. It showed the right “attitude . . . in not wishing to load off the whole . . . cost on the I.E.B.” and would serve as a good example “for the stimulation of others” in the eyes of the foundation executives. The board would also give him $75 per month plus his travel expenses for the round trip from Europe and within the United States.22 Baade was overjoyed; he professed to consider the fellowship grant a “miracle” and thanked Shapley profusely. His plan was to spend some time at each of the four major observatories in the United States, and to visit the Dominion Astrophysical Observatory in Victoria, British Columbia, as well. Baade was working hard, finding distant variable stars in the Milky Way and globular clusters, and puzzling over the observational criteria that distinguished the latter from “open” or galactic clusters, another forerunner of his population concept still two decades in the future. Shapley was already spending much of his time in administration, fund raising, and advising others; his comments on Baade’s work were generally diffuse and vague. The hard-working young observer’s, in contrast, were sharp, concise, clear, and insightful.23 Baade had originally intended to go to America to begin his fellowship year in the fall of 1925, but he postponed his departure from Hamburg until the end of the following winter, to write up and publish some of his observational results.24 He had joined the German Astronomical Society in April 1925 when he applied for

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the fellowship, perhaps to convince Schorr that he would really return to Gemany.25 One of these papers was on a subject dear to Schorr’s heart, a long list of the positions of comets and asteroids Baade had measured from photographs he had taken with the 1meter reflector. Among them were several objects which he had discovered himself, including Comet Baade (1922 II); 944 Hidalgo, an unusual asteroid with a mean distance near Jupiter’s but a large eccentricity and hence an aphelion closer to Saturn’s orbit; 1924 TD, another unusual asteroid (later named Ganymede); and 966 Muschi, a more typical one which he had somehow managed to get named for his future wife, in spite of the then rigid rule that minor planets must be given names of goddesses and other female figures from mythology (except Trojan asteroids, which had to be named for male heroes of Homer’s Trojan War).26 She was Johanna (often shortened to Hanni) Bohlmann, a “technical assistant” (meaning a computer who worked with a hand calculating machine). Throughout her life she was known to her friends as “Muschi,” a nickname something like “Kitty,” implying that she was clever, cozy, and comfortable. Hanni had begun work at the Hamburg Observatory in February 1920, four months after Baade.27 The other three papers which he finished before leaving for America were on the subject he greatly preferred, cluster-type variables in and near globular clusters. One purely observational paper listed the seventeen additional variables he had found in M 53, the globular cluster for which he had earlier published his more exciting discovery of the five cluster-type variables in the field outside it. His new variables approximately doubled the number known within this cluster, originally published by Shapley.28 The other two papers dealt with NGC 5466, another distant globular cluster at high galactic latitude, like M 53. NGC 5466, because of its relatively small number of giant stars and its relatively weak central concentration could be considered in some ways a transition object, intermediate between a globular and an “open” (galactic) cluster. Baade, however, searched it thoroughly and found fourteen variable stars in it, twelve of which he was able to establish as cluster-type variables. Their median apparent magnitudes, with

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Shapley’s value for the mean absolute magnitude of these variables, Mpg = −0.23, gave a distance to the cluster 19,000 parsecs (the distance unit Baade was now beginning to use, equal to 3.2 light years). The presence of these variables established for him that NGC 5466 is a globular cluster, though a relatively sparse, low-luminosity one. Nevertheless Shapley’s secondary distance method, based on the angular diameter of the cluster, worked satisfactorily, Baade showed.29 In his other paper Baade announced the discovery of five clustertype variables in the extended field around this same cluster, which he had begun studying after finding the variables outside M 53. He stated that as a check, he had then taken similar plates centered on the north galactic pole, but he found no cluster-type variables there. Hence he was surprised, he claimed, to find the five variables in the NGC 5466 field, somewhat brighter than the variables within the cluster and therefore certainly outside it in distance from the earth as well as in the plane of the sky. Here then was a second example of a field at high galactic latitude, with an unexpectedly large number of field cluster-type variables, far beyond the bounds of the conventional Milky Way, in the region of the globular clusters. Further observations were necessary, he stated, to see whether there was a connection between the globular clusters and the field cluster–type variables.30 Baade finally completed all four of these papers within a monthlong period near the beginning of 1926. He finished the observational paper at the end of January, before sailing from Hamburg early in February. He must have made the final revisions on the other three while he was crossing the Atlantic, for he landed in New York on February 14, and the dates on the published papers show that he completed them (or they were received at the observatory in Bergedorf) by March 1. From New York Baade headed north to Cambridge and Harvard College Observatory, where he was to begin his fellowship year.31 He was the first German astronomer to come to the United States on a Rockefeller Foundation fellowship, although a few German physicists, including Otto Laporte, had preceded him, beginning in 1924, and Friedrich Hund, later another

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well-known physicist, came in the same year as Baade. The next German astronomer or astrophysicist to receive one of the fellowships after Baade was Albrecht Unso¨ ld, for 1928–29.32

Wanderjahr in America Harvard College Observatory was but the first stop in Baade’s fellowship year, which he had planned to spend in working visits to the major American observatories. At each he talked with everyone, saw what they were doing, and participated in the work, including observing. He did not stay at any place long enough to carry through a research project on his own, nor did he even collaborate with any single American astronomer sufficiently fully to coauthor a paper. But Baade did learn a tremendous amount about observational techniques, some of which he adopted or improved, others of which he recognized as not as good as his own. He also learned what the Americans considered the most important research problems of the day, on which they were working themselves. Even more important, Baade learned to speak and write American English very expressively, and he became acquainted with most of the leading observational astronomers in the United States. His own obvious skills and interest in research, together with his attractive, outgoing personality, made them all admire, respect, and remember him. Baade stayed at Harvard for two and a half months, until the end of April. He saw Shapley, then just forty years old, almost daily. The young Harvard director dominated his large staff and carried out a huge volume of research, based on the effort of a corps of hard-working assistants, many of them women, who alternately loved him for his warm personality and hated him for exploiting them. Most of the observational data came from relatively small cameras, operated by assistants at Cambridge, at a nearby darkersky site, and at the Harvard southern station at Arequipa, Peru.33 Although Shapley had done excellent globular-cluster research, and was beginning his work on galaxies, which at the least was quite important in defining the problems, Baade was clearly not impressed by his mass-production approach.34 Baade must have

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Walter Baade at Yerkes Observatory in 1926. (Courtesy of Yerkes Observatory.)

met Cecilia Payne (later Payne-Gaposchkin), the brilliant young astrophysicist, deeply interested in stellar spectroscopy, whom Shapley had brought to Harvard on a fellowship after she had finished college at Cambridge, England, in 1923. After she completed her Ph.D. thesis at Radcliffe, which became the influential book Stellar Atmospheres, Shapley put her to work on (photographic) photometry, the technique Baade was particularly interested in exploring. No record of their impressions of each other survives, but years later she was to worship Baade’s research on galaxies, and to edit his posthumous book, Evolution of Stars and Galaxies.35 From Harvard Baade moved on to Yerkes Observatory of the University of Chicago for the months of May and June. Its director,

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Edwin B. Frost, had been a graduate student in Germany for two years, and knew Schorr well. The Hamburg director had carefully prepared the way for his young prote´ ge´ , Baade, whom Frost welcomed to the observatory. Scientifically, it was in its long, slow decline under Frost, a singularly uncreative astronomer locked to the traditions of the past.36 However, Baade met Frank E. Ross there, an outstanding expert in astronomical photography and photometry, and the young Otto Struve, who had come to Yerkes as a graduate student in 1921 straight from the remnant of the defeated White Russian army in Turkey, had earned his Ph.D. in 1923, and had immediately become the most productive member of the Yerkes staff. Ross was an irreverent, completely unpretentious scientist, with a keen wit and a sharp eye for the foibles of humanity, and he and Baade got along famously all the rest of their lives.37 From Yerkes Baade continued to the Dominion Astrophysical Observatory, in Victoria, British Columbia, for a few days. Its 72inch reflector had been the largest telescope in the world from 1917, when it went into operation, until 1919, when the Mount Wilson astronomers began making regular observations with the newly completed 100-inch. The DAO director, John S. Plaskett, welcomed Baade and showed him what was still the second-largest reflector in existence. It was devoted largely to measuring radial velocities. Baade was no doubt interested, but anxious to move on to the California observatories.38 From Victoria he traveled to San Francisco, probably by ship, and then on to Lick Observatory of the University of California which Schorr had visited himself in 1923, along with Mount Wilson, Yerkes, and Harvard, after a jaunt to a total solar eclipse in Mexico. Baade arrived at Mount Hamilton on July 5, and spent the entire month there. Schorr had introduced him in advance, as at all the other American observatories, as a very well prepared scientist, eager to learn, who was also “ein sehr netter Mensch” (a very nice person). Robert G. Aitken, the associate director, was pleased to welcome him to the little mountain-top community. Lick had earlier been the most important observatory in the West, but its antiquated 36-inch refractor and its Crossley reflector (also a 36-inch) had been overshadowed since 1908 by the bigger telescopes on

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Mount Wilson. Aitken was a classical visual double-star observer, and Baade apparently worked most closely with William H. Wright, who was doing nebular research with the Crossley reflector, and to a lesser extent with Joseph H. Moore, who supervised the radial-velocity program, and Robert J. Trumpler, the young Swiss who had been Ambronn’s student at Go¨ ttingen before Baade. In his report after his return to Germany in 1927, Baade put Lick in the second rank of American observatories, with Yerkes, below Mount Wilson, and in his later reminiscences it figured largely as the location of stories about hard-working astronomers who still managed to do respectable work with old telescopes.39 Finally, at the end of July, Baade arrived at Mount Wilson, his primary goal ever since he had gotten his Ph.D. Walter S. Adams, its director, was generally less enthusiastic about visiting astronomers from abroad than Shapley or Frost, but he too welcomed Baade to take part in the ongoing research at Mount Wilson. Adams already knew of Baade from his published work on photographic photometry with the Hamburg reflector.40 Photometry was a Mount Wilson specialty, the elderly Frederick H. Seares the expert in it. His chief program was determining an accurate magnitude sequence to as faint a limit as possible at the north celestial pole, and transferring it to the Kapteyn selected areas, small, well-defined regions all around the sky. Seares, like Adams, Frost, and many other top American research astronomers of their generation, had done graduate work in astronomy in Germany. Baade worked closely with him, concentrating especially on standard sequences in the selected areas he would use himself, and secondary sequences in the globular clusters and Milky Way fields he had observed or would observe at Hamburg. After his return to Germany Baade corresponded frequently with Seares about his latest photometric results, and used them in his own papers.41 Baade had begun to observe two fields in the Milky Way before leaving Hamburg, and in Pasadena was particularly struck by one of the variables he had found in the Cygnus field. Its period was 15.8 days, and its light curve an “exact copy” of W Virginis, a Cepheid variable, but its apparent magnitude, together with its absolute magnitude from the standard period-luminosity relation,

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corresponded to a distance of 163,000 light years, so large it was “next to impossible.” He obtained additional exposures at Mount Wilson, and confirmed the period and apparent magnitude. He believed that probably an “obscuring cloud” (interstellar extinction) was the problem, but noted that this variable star was interesting in itself, because the Danish astronomer Einar Hertzsprung had recently found that the absolute magnitude of W Virginis did not seem to agree with “the” period-luminosity relation. This discrepancy lay at the root of the difference between the period-luminosity relations of populations I and II, and the consequent doubling of the scale of the universe, which Baade was to come to understand a quarter of a century later. During that same six months, Milton L. Humason obtained a 75hour exposure of the spectrum of the nucleus of M 31, the Andromeda “nebula,” which appeared to show that it is composed of dwarf G stars like the sun. He had earlier found the same result for the slightly brighter nucleus of M 32, its companion. And Hubble confirmed, with the 100-inch, that the recently discovered WolfLundmark-Melotte “nebula” was indeed an irregular galaxy of the Magellanic Cloud type.42 It was a glorious time to be at Mount Wilson, especially for a German astronomer who had only read about the Mount Wilson reflectors before, but had wanted to work with them on galaxies and globular clusters since 1919. Baade must also have had many discussions with Hubble, then carrying out his epoch-making work on spiral and irregular “nebulae” as galaxies, based on identifying Cepheid variables in them. He was famous even then, and not a good mixer, but he was always ready to talk about his own research, and Baade was a good listener. Certainly when he returned to Germany in 1927, Baade carried with him not only the 60-inch plates of his fields which he had taken himself, but also some plates Hubble had obtained with the 100-inch, and lent to him.43 During his six months in Pasadena, Baade also wrote and published two important semitheoretical papers. One was his announcement of the method (now called the Baade method) to determine the radius and absolute magnitude (as functions of time) of a pulsating variable such as a Cepheid variable, by integrating the

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observed radial velocity curve. Shapley had demonstrated, only a few years previously, that Cepheids are not eclipsing binaries, as had earlier been supposed, and Baade’s paper showed how the presumed “orbital parameters,” derived from the entire velocity curve, could be used to give the most accurate values of the necessary integrals. The key idea of Baade’s method is that the time integral of the velocity between two phases of equal temperature gives the difference in radius between them, while the magnitude difference between them gives the ratio of radii.44 The method was later improved by A. J. Wesselink, who removed Baade’s assumption that the star radiates as a black body, and it is in this later form that the Baade-Wesselink method is used today.45 Baade and Wolfgang Pauli, then a young docent and brilliant theorist of relativity and quantum mechanics at Hamburg University, wrote the other paper. They had met when Pauli first came to Hamburg in 1921, and they remained close until the great physicist died in Zurich in 1958. Baade supplied the idea and the stimulus for their paper; Pauli provided the know-how in theoretical physics necessary to carry out the calculations. They showed that using the best known data on the f-values (essentially, spectral line and band strengths) of atoms and ions observed in comets, including Na, CO+, and N2+, the observed curved shapes of comet tails could be quantitatively understood to result from light pressure from the sun’s observed spectrum. The idea of light pressure as the repulsive force operative on comet tails dated back many years, but it was generally believed to operate on dust; their paper showed that it could also be understood to act directly on atoms and molecules.46 Both these papers demonstrated Baade’s easy familiarity with the latest, most quantitative observational data of astronomy, and his strong desire to understand and explain them in physical terms. Before he left Pasadena, Baade met Hale, the retired Mount Wilson director, then living in seclusion but still involved in astronomy. Pauli, greatly interested in Albert Einstein’s attempts to develop a “fundamental” theory which would link gravitation and electrodynamics, had charged Baade to ask Hale if he had succeeded in measuring the general magnetic field of the sun, his postretirement project. Almost any theory which connected gravitation and elec-

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tromagnetism would predict a relation between the sun’s known rotational velocity and its magnetic field, which could be derived (approximately) on simple dimensional grounds. Pauli wanted to know the value of the solar magnetic field to check if any such fundamental theory could be true. Unfortunately Hale’s team of colleagues and assistants had not been able to measure the small field we now know exists, so Baade could not take back an answer to Pauli. but he was briefly involved in the resulting correspondence between Einstein and Hale, two titans of science.47 Baade published the two papers he had written at Mount Wilson Observatory in Germany, to which he was soon to return. He left Pasadena in mid-January 1927, stopped at Harvard for ten days to work on the plates which had been taken for him there of two fields he was investigating in the Milky Way, and sailed from New York on the Thuringia in early February.48 In his year in America, Baade had obtained a great deal of scientific data, which formed the basis of several of his later papers. He had met many American astronomers, and they had all formed a good impression of him. He had learned to speak and write fluently in English, which he now began using in his letters to the United States. Financial problems were even more severe in Germany than they had been the previous year, and the frequently cloudy skies were no better than ever. Baade was disappointed by Europe when he first got back, and confessed to feeling “very miserable” (somewhat depressed), an unusual state for the normally ebullient astronomer. Very probably he was already hoping for a job at Mount Wilson, which in fact was to come just four years later. But in March 1927 he buckled back down to work at Hamburg Observatory.49

✶

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The Path toward the Two Populations HAMBURG, 1927–1931

Back at Hamburg-Bergedorf Walter Baade arrived at the Hamburg Observatory in Bergedorf from his year in America on February 15, 1927, and by March 1 he was back to a full observing program with the 1-meter reflector. Now, however, his horizons were widened by his stimulating visit to Mount Wilson. One of the first objects he photographed, as soon as it came around in the sky, was NGC 6822, the “nebula” which Edwin Hubble had just proved was an “extragalactic nebula,” or as we would say today, a galaxy. It was the first “nebula” for which Hubble had published all his detailed observational data on the Cepheid variables, luminous blue stars, and gaseous nebulae within it, proving it was actually a “remote stellar system” within what came to be called the Local Group. At Mount Wilson Baade had certainly discussed this paper with Hubble, and his later, even more important publications on the spiral “nebulae” M 33 (published in 1926) and M 31 (still work in progress while Baade was in America, and published in 1928). The young German astronomer wanted to jump into this work himself as deeply as he could with his smaller telescope in a poorer climate.1 Richard Schorr, his director, realized that Baade’s growing reputation would soon bring him attractive job offers from rival observatories. He recommended Baade strongly for promotion to Observator, the post which traditionally led to the directorship. Baade got the appointment in September 1927, and the job became officially his after he had taken all his “papers” to the central police station, required because the Hamburg Observatory operated under the

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Astronomers and senior staff personnel at Hamburg-Bergedorf Observatory around 1928, probably soon after Walter Baade was named Observator. Richard Schorr is standing in front, just to left of center, Baade is just to right of center, and Arnold Schwassmann, the senior astronomer, stands on a step between them. (Courtesy of Hamburg Observatory.)

city government. The following summer when Kasimir Graff, the senior Observator, left Hamburg to become the director of Vienna Observatory, Baade thus became Schorr’s heir apparent.2 A year later Johannes Hellerich, formerly an assistant at Kiel, was appointed to succeed Graff as the junior Observator, but Schorr always considered him well below Baade in potential for the future.3 That same year Baade published a paper, based mostly on his earlier Hamburg material (now strengthened by magnitude se-

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quences he had obtained at Mount Wilson) on the number of variable stars in his two Milky Way fields, in Sagitta and Cygnus. In both fields, he reported, there were about nine variables per square degree down to magnitude 14.2 or 14.4, and in each case by far the most numerous type was eclipsing binaries, followed by longperiod variables, Cepheids, and irregular variables. These fields, near the galactic plane, are representative of what Baade was to call population I fifteen years later, and the eclipsing variables he was to use as a marker of its presence.4 He also published his paper on NGC 5053, which he had begun years earlier at Hamburg, and had continued at Mount Wilson with help from Edwin Hubble in the form of a 100-inch plate of this star cluster. It is a relatively loose cluster, with not many stars visible in it, and thus apparently an “open” or galactic cluster. But its high latitude means it is far from the galactic plane, a property of globular clusters. That was the reason Baade had investigated it. He found nine variable stars in it, all “cluster-type” variables, clearly indicating that it was a sparse globular cluster, 19 kpc (kiloparsecs) from the galactic plane (according to the accepted distance scale), not an open cluster. He verified this by showing that the nonvariable stars had the same luminosity function (numbers per unit magnitude interval) as in M 3, a typical rich globular cluster, but that there were only about onefourth as many stars in NGC 5053 as in M 3. Such extreme cases were always interesting to Baade, and at the end of his paper he pointed out that NGC 6366 is an even lower-luminosity globular cluster than NGC 5053.5 Harlow Shapley, to whom Baade had been writing in English since his return from the United States, called Baade’s paper “one of the most interesting contributions to the problems [of understanding star clusters] of the last year or so.”6 At about this time Baade was pointing out to Shapley the growing evidence which Robert J. Trumpler was finding at Lick Observatory that interstellar extinction by dust particles exists, modifies the color of stars, and makes them appear systematically more distant than they actually are. Shapley did not want to accept these observational results, because he knew they would vitiate his distance determinations to clusters, and hence the scale of the Galaxy which

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he had determined. The younger Baade was eager for new knowledge; the older Shapley was fighting to preserve old triumphs.7 In the summer of 1927 Baade had a relaxing interlude from observing with the reflector, a second total solar eclipse expedition with Schorr. This one was in Lapland, just a little north of the Arctic Circle. The Hamburg party, consisting of Schorr, Baade, Bernhard Schmidt (the optician), and a mechanician, made the trip from Germany to Narvik, Norway, by coastal steamer, and then continued to the site Schorr had chosen at Jokkemukk, Sweden, by train and finally by car. Four other Hamburg astronomers and the young Dutch astronomer Willem Luyten, then at Harvard, came by the land route through Sweden and joined them there. The eclipse day, June 29, was clear and they got all their plates; they then returned by the road, sending their equipment back on the slower ship.8 Schmidt, an Estonian by birth, had lost his right hand and arm “experimenting” with gunpowder as a boy, but in spite of this handicap had become an outstanding maker of telescope mirrors. Fourteen years older than Baade, Schmidt had been trained in vocational schools in Sweden and Germany, and had been making professional-quality mirrors since 1901. He had first come to Hamburg Observatory in 1918, as a freelance optician who contracted to build a horizontal reflecting telescope for Schorr. The postwar inflation had driven him back to Estonia, but in 1926 he returned to Bergedorf, while Baade was in America. Schmidt carried out several more or less independent photographic projects at Hamburg Observatory, and produced fine mirrors to the director’s order.9 Two years later Schorr stayed home himself, but sent Baade and Schmidt to the Philippine Islands to observe the solar eclipse of May 9, 1929. They sailed from Hamburg in early February, via the Suez Canal, and reached Manila a month and a half later. From there it was on by a smaller ship to the island of Cebu, where they located their telescopes near the University of Manila group’s site. May 9 was clear, but as the eclipse progressed, clouds began to form, and during the last 20 seconds of totality covered the sun. Nevertheless, the photographs they obtained were usable. Because of the heat, they took the plates to Cebu City, where a commercial laboratory allowed them to use their cooled darkroom to develop

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Bernhard Schmidt (left) and Walter Baade at the Philippine Islands eclipse in 1929. (Courtesy of Hamburg Observatory.)

them. Then, after packing their equipment for shipment, they left Cebu on May 28, and Manila on June 4, and finally arrived back in Germany after another voyage of a month and a half.10 The lively young Baade and the morose Schmidt hit it off well, and on the long voyages out and back, and the boring two months in the little village of Sogod, Cebu, they had time to discuss many subjects under the sun—and stars. During their five months together Baade and Schmidt went over, time after time, the need for a fast, wide-field, coma-free reflecting telescope, to photograph large areas of the sky, in order to search efficiently for variable stars, nebulae, galaxies, or even planets. According to Baade’s enthusiastic reminiscence two decades later, Schmidt had told him even before they reached Manila that he already had the solution, “the perfect mirror.” It seems unlikely that Schmidt really had the full idea then, but he may have been close to it.11 A few years earlier he had been experimenting with unconventional two-mirror systems, but cer-

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tainly by a year after their return, he had hit on the brilliant idea of using a primary spherical mirror (which is coma-free) with a thin glass corrector-plate at its center of curvature to remove spherical aberration. Thus the Schmidt camera (or telescope) was born, and in 1931 Schmidt himself was able to publish photographs taken with the “original Schmidt,” a 44-cm aperture, f/1.75 instrument.12 The 1929 trip to the Philippines was Baade’s third and last eclipse expedition. Schorr had assigned him to go on all of them because he knew Baade had the observing skills and cool judgment necessary to get data during the tense few minutes of totality, with no chance to postpone the observations or to readjust the equipment and try again later. No scientific results were ever drawn from any of these data; Schorr believed it was Hamburg Observatory’s duty to obtain them so they would be available for posterity, but he made no effective efforts to get anyone to reduce and analyze the spectrograms and direct photographs of the chromosphere and corona which he, Baade, and Schmidt brought back from their trips to the North Atlantic, Lapland, and Cebu. Various Hamburg business firms, especially steamship lines, subsidized the expeditions, which, in terms of their results, were more combined publicity ventures and junkets than anything else. Baade had no choice but to go on these trips, and he doubtless enjoyed them at the time and kept Schorr, Schmidt, and the other members of the parties in high spirits. But he regarded taking data for unspecified research purposes as a complete waste of time all of his life, in contrast to many of his contemporaries among both German and American astronomers, many of whom continued routine observing programs well past the point of diminishing returns.13 Baade did in fact receive an important job offer in Germany soon after his return from his fellowship year in the United States. Just as he arrived back at the observatory in Bergedorf, his friend Wolfgang Pauli sent him a note reporting that he had learned that Jena University wanted to appoint Baade as director of its observatory, to succeed Otto Knopf, who was retiring. Always sarcastic, Pauli wrote that he did not know how large a telescope Jena had (in fact it was an 8-inch refractor), but he was sure that its weather could not be worse than Bergedorf’s.14 The official “call” came in January

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1928, and Baade went to Jena to look over the situation and negotiate. His demands were high; he would come, he said, if Jena would get a 1-meter reflector (as large as he had at Hamburg) at a good dark site outside the city, with a Cassegrain spectrograph, a staff of four assistants and shop personnel, new measuring machines, and a greatly expanded astronomical library. He provided a detailed conceptual plan of his dream observatory. Baade wanted to do frontier research, not become the figurehead director of a tiny, old-fashioned observatory.15 Jena was the home of the Zeiss Optical Company, and Baade and the Thu¨ ringen educational authorities (where it was located) hoped that its Carl Zeiss Foundation would finance the new observatory. It was not to be, however; the amount required was far too high, particularly in those uncertain days of Weimar Republic inflation. When Baade learned he would not get the new observatory in Jena, he tried to use the offer to persuade Schorr and the Hamburg authorities to move their 1-meter telescope to a better site, but that too was out of the question.16 In the end Baade declined the Jena directorship. Knopf, who had hoped that Baade would come as his successor, was sorry to learn of his decision, but believed that the young man had proved his idealism.17 However, Baade “respectfully” reported to the Hamburg authorities that he had given up a 2,000-Mark raise to remain at Bergedorf, and hoped that they would “consider this circumstance” in the near future. Schorr was overjoyed that Baade was staying, and recommended him strongly for a raise, describing him as “one of the most proficient young astronomers, both in the theoretical field as well as in the theoretical physics and mathematics that are so important for the development of modern astronomy, who also is outstandingly gifted in the practical observing art, and has a series of important results to show.” Baade would not have been happy if he had seen that in this letter the first three of these important results were his discoveries of the two “especially interesting” asteroids Hidalgo (far outside the normal asteroid belt, beyond Jupiter’s orbit) and Ganymede (whose orbit is highly eccentric and whose perihelion distance is just outside the earth’s orbit), and of Comet Baade. These were the traditional subjects dear to Schorr’s heart, but after them the director also cited Baade’s latest “nebular”

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work.18 Very probably Baade received the raise, although there is no accessible record of it. More important in the long run, the Jena offer had included an associate professorship as well as the directorship of the observatory. Thus the Jena faculty had demonstrated their opinion that Baade, who had a strictly research position at Hamburg and did no teaching there, was competent to lecture at the university level. Hence Baade could apply for his “Habilitation,” the right to teach as a professor in a German university, and it would be granted to him practically automatically. He forwarded the necessary documents to the dean of the Mathematical-Physical faculty at Hamburg University, and was given his Habilitation immediately, in December 1928.19 The next month Baade delivered his inaugural lecture, “The Extragalactic Nebulae as Stellar Systems.” In it he described how astronomers had not understood the nature of the spiral and elliptical “nebulae” until Heber D. Curtis, using Lick Observatory’s Crossley reflector, and George Willis Ritchey, with the Mount Wilson 60-inch, had found novae, very bright “new” stars, easily recognized as they flared up to maximum light in the nearest large spiral, M 31, the Andromeda “nebula.” Then Hubble, pushing down to fainter stars with the new 100-inch, had discovered Cepheid variables, stars less luminous than novae at maximum light but more numerous, and also recognizable by their periodic light variations. Baade also told of the unusual “new” star which had flared up in 1885 in M 31, much brighter than the other novae in it. It reached nearly nakedeye visibility, with absolute magnitude M = − 15, contrasted with the average M = − 5 of “ordinary” novae. Baade called this one especially bright nova a “Hauptnova” in German (chief nova, his early word for a supernova, a very new concept at that time). In 1920, Baade said, Max Wolf, searching photographs of the skies he had taken at Heidelberg Observatory decades earlier, had found another of these rare Hauptnovae in a more distant spiral “nebula,” NGC 2608, proving it too contained stars. These topics were to be the subjects of much of Baade’s research for the rest of his life. His observational investigations of this nearest of the large “extragalactic nebulae” was to lead directly to his discovery of the two stellar

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populations fifteen years later, and his research at Mount Wilson and later Palomar was to lead the way toward understanding supernovae, and using them to measure distances far out in the universe.20 The day after the lecture Johanna Bohlmann resigned her position as a technical assistant at Hamburg Observatory, and she and Baade were married a few days later, just before he embarked for the Philippines. He was then thirty-six years old; the vivacious, attractive “Hanni” was just six months younger. A year earlier Schorr, returning unexpectedly to the observatory late on what was undoubtedly a very cloudy January night, had seen Hanni arrive in her own car, alone, just after he got out of his taxi. She gave him a cheeky “Good evening,” and walked straight into the observatory building where Baade’s apartment and the telescope were located. Schorr said nothing then, but called her in to his office the next morning, and asked her where she had been going the night before. “To Dr. Baade’s” was her unabashed response. The old-fashioned director chastised her severely, and declared that if he ever learned that she had entered Baade’s apartment again, day or night, or the telescope dome at night, she would lose her job.21 Evidently Schorr never caught her again; perhaps they were more careful the next time. When they were married, Baade had to move out of his bachelor’s quarters in the observatory, and for the rest of their lives they were to live in rented apartments or houses, always ready to move on.22 Walter and Hanni Baade never had children, but they remained close until the day he died, thirty-one years later, and she revered his memory for another twenty-eight years after that. Baade’s 1929 inaugural lecture was closely related to two semitechnical talks which he gave. The first, “The Distances and Dimensions of the Extragalactic Nebulae,” was delivered before the Scientific Society of Hamburg University in December 1928, the second, “Recent Investigations on Extragalactic Stellar Systems” in 1930, at a meeting of a German scientific society roughly equivalent to the American Association for the Advancement of Science. The manuscripts of these two excellent lectures by Baade show that they were on the whole universe, aimed at scientists with no professional

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knowledge of astronomy. In the 1930 talk Baade mentioned the eighteenth-century speculations of the pioneer cosmogonist Thomas Wright of Durham, the philosopher Immanuel Kant, and the astronomer Johann H. Lambert that there were other star systems than our Milky Way, then continued through William Herschel’s nineteenth-century surveys of the sky to Hubble and Cepheid variables in the 1920s. Baade estimated that there were roughly two million “extragalactic star systems” within reach of the largest telescope of that time, the Mount Wilson 100-inch reflector. He told of clusters of galaxies, and of the velocity-distance proportionality which Hubble had discovered only a few years before. How to understand the result was a question for the future. Probably this understanding would come through the questions raised recently by the theorists Arthur S. Eddington and Willem de Sitter. Perhaps the answer would be found in the expanding solution which the Belgian mathematical physicist Georges Lemaitre (a priest) had recently discovered to Einstein’s general relativistic equations of the universe. Whatever the solution turned out to be, which was not at all certain, Baade said, the overriding meaning was that in the observed redshifts* of extragalactic systems “we possess for the first time observational results in which the structure of the universe [has made] itself felt.”23 This investigation of the evolving structure of the universe through observational studies of the redshifts of the galaxies (and quasars) became one of the most active areas of research from the 1960s to the 1990s. Even before his inaugural lecture, Baade had begun publishing new, important results on “extragalactic nebulae.” In 1928 he had * Astronomers measure the velocity of a star, nebula, or galaxy toward or away from us (its “radial” velocity) from its spectrum. A positive radial velocity (away from the observer) “stretches out” the light, lengthening all its wavelengths, and the greater the velocity the longer each wavelength becomes. This is exactly the principle by which police radar sets measure automobiles’ velocities, except that a star (for example) emits light on its own rather than reflecting a radar pulse transmitted from the police car. Long wavelength light is red (short is blue), so the lengthening of light waves is called a “redshift,” although the amount of it is so small there is no perceptible color change. But astronomers can measure the radial velocity in the spectrum, and that is how we know the universe is expanding, Hubble’s great discovery of the 1920s. It was often called the “law of the redshifts.”

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first photographed IC 1613 with the Hamburg reflector, and recognized that it was not at all a nebula, as Wolf had reported, or a group of small nebulae, as Curtis, at Lick Observatory, had thought. With his superior photographs Baade could see in fact that it was a “star cloud” (or dwarf irregular galaxy) of the same type as the Magellanic Clouds and Hubble’s NGC 6822. The brightest stars in it were very faint, with magnitudes from seventeenth to eighteenth. The small nebulae Curtis had seen were giant emission nebulae within it. Since the brightest stars were more or less the same magnitude as in NGC 6822, and the small nebulae roughly the same size, IC 1613 was at approximately the same distance as it. Thus it was one of the nearest galaxies known, a result “not without interest,” as Baade put it.24 That same year he also discovered, with the Hamburg reflector, a “noteworthy new cluster of nebulae in Ursa Major.” He had noticed what appeared to be a “rare concentration” of faint “nebulous stars” off the axis of one of his plates taken for another purpose. Following it up with long exposures centered on the concentration, he could see that it was a cluster of small, faint (extragalactic) “nebulae” containing about 160 objects to his plate limit. He showed it was a real concentration, and that its luminosity function was similar to that of the well-known Virgo cluster, which had been studied by Shapley and Adelaide Ames. From the relative magnitudes of a local maximum in the two luminosity functions, he calculated that his new cluster was fifteen times more distant than the Virgo cluster, or using Shapley and Adelaide Ames’s distance for the latter, 15 < 106 light years away. Before publishing his paper on this cluster of galaxies, Baade gave an oral version of it at the meeting of the German Astronomical Society in Heidelberg in July 1928. It was the only meeting of the society he attended before leaving Germany three years later. His new cluster of galaxies, which a few years later was dubbed UMa 1, was to play an important role in Hubble and Milton L. Humason’s observational extension of the redshift-distance relation at great distances. Baade had attended the Heidelberg meeting along with 173 other members and guests of the society. Most of them were Germans, but many other nationalities were represented as well. Eddington,

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Walter Baade (lower left), seated in the observing chair at the guide telescope of the 1-meter reflector of the Hamburg-Bergedorf Observatory around 1930. (Courtesy of Hamburg Observatory.)

the English theorist, often came to the meetings in Germany; at this one he presented a paper on the main sequence in the HertzsprungRussell diagram, trying to locate it as accurately as possible from all the available observational data on the absolute magnitudes of stars, much of it of relatively low precision. Elis Stro¨ mgren, director of Copenhagen Observatory, with very close ties to German astronomy, was also present, as was his twenty-year-old son Bengt, already a graduate student of astronomy with several published papers to his credit. Nearly all of the thirty oral papers presented at Heidelberg were on stars, positional astronomy, or instrumentation;

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besides Baade’s there were only two other papers on galaxies, and only one on nebulae.25 In that same highly productive year, 1928, Baade also for the first time obtained direct plates with the Hamburg 1-meter telescope of two objects he was to work on very effectively for the rest of his life. One was NGC 147, which he was to resolve into stars with the Mount Wilson 100-inch in 1943, showing that it was a dwarf elliptical companion of the giant Andromeda spiral galaxy and one of the defining examples of a pure population II system. The other was NGC 1952, the Crab nebula, which he and others were to prove was the remnant of a supernova of nearly a thousand years ago, observed by Chinese astrologers, and one of the brightest radio sources in the sky.26 In 1930 Baade published an important paper on the very distant, high-latitude globular cluster NGC 4147, based on plates he had taken all the way back to 1920. Shapley, who had discussed this object previously, had not found any “cluster-type” variables in NGC 4147, and had assigned distances of 51 kpc and 24 kpc to it in successive papers, illustrating graphically the uncertainties of secondary distance-determination methods. Baade did find three cluster-type variables in it (they are very faint in this distant cluster), and from his magnitude transfers from Selected Area 82, newly measured by Seares, determined the distance more accurately as 20 kpc, very nearly perpendicular to the galactic plane.27 Besides the RR Lyrae variables within NGC 4147, Baade found five more outside the cluster but in the extended field around it which his plates covered (about 8 square degrees). Of these only one or at most two were at roughly the same distance as NGC 4147; from their apparent magnitudes one of the others was certainly much closer than it, and two others more distant. This confirmed his previous result, found in the fields around the other two highlatitude globular clusters M 53 and NGC 5466. The variables at roughly the same distance as each cluster could conceivably have escaped from it, but the others could not have done so. Baade emphasized this important result, that isolated “cluster-type variables” occur in the same large volume as the globular clusters, extending out to distances of 25 kpc perpendicular to the galactic

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plane. This connection is “expressed,” he wrote, by the high space velocities which the members of both groups have. These isolated RR Lyrae variables are distributed everywhere in the sky, he wrote, for he had found roughly comparable numbers of them even in his Milky Way fields in Cygnus and Sagitta.28 As he put it more graphically in a letter he wrote to Shapley while he was working up these results: Perhaps you will remember that some years ago I found rather high numbers of faint cluster variables in high galactic latitudes. Just now I have finished the search for faint cluster variables in two other fields of high latitude. Result: again 5 resp[ectively] 6 faint variables of this type per 8 [square degrees]. Since in my fields in Cygnus and Sagitta I found practically the same numbers it seems reasonable to assume that I observed only the general distribution of cluster variables and that the concentration is spurious. The cluster variables in the Cygnus cloud then would be interlopers of the large nonrotating system as they are in our own neighborhood. The fact that no cluster is known which belongs physically to our nearer system whereas they are frequent in that sparse but very e[x]tended system of globular clusters (nonrotating system of Lindblad & Oort) makes me always wonder.29

Baade was already very close to his idea of the two stellar populations here, and he kept “wondering” about it (and working on it) until he could announce the full concept in 1944, the flat, rotating population I system, identified by the blue stars of the Milky Way, exemplified here by the Cygnus and Sagitta fields, and the spherical, nonrotating (or slowly rotating) population II system, exemplified by the globular clusters and RR Lyrae variables. In his correspondence with Shapley, Baade always discussed his quantitative results in detail, giving estimates of the probable errors of the magnitudes and derived distances and outlining their physical meaning as he understood it at the time. Shapley, in contrast, tended to gloss over details, but was fond of striking phrases, such as “delightfully faint” RR Lyrae variables, and the “nice thrilling puzzle” of the meaning of the isolated RR Lyrae variables. Baade could admit his own mistakes (which often led, when recognized, to further understanding) such as the distance he had derived for

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the Cygnus cloud, which now appeared “meaningless” to him, while Shapley tended to blame his failure to find the RR Lyrae variables in NGC 4147 or to recognize the importance of “the fairly uniform distribution of cluster type variables” on one of his former assistants, who had been “not very expert.” But Shapley was very glad to have Baade’s “interesting” and “important” results, which he said he planned to mention in a colloquium discussion very soon (before Baade had published them), and wrote that he was sure that Baade’s value of the distance to NGC 4147 was “essentially correct.”30 Baade by this time was a very well known, highly respected research worker, a superb observer who knew how to get every bit of data that was possible out of the detectors of his day, photographic plates. He began experimenting with orthochromatic (green-sensitive) and panchromatic (red-sensitive) plates combined with various color filters for several years and had learned just what they could and could not do. In 1928 Baade began using the primitive panchromatic plates then available in attempts to penetrate through the thick dust extinction (“absorption”) in the Orion nebula and in several “dark nebulae,” recognized by the apparent absence of stars in and behind them. The next year he started experimenting with orthochromatic plates and a filter to obtain images of gaseous nebulae, especially planetaries, in the light of their two strong green emission lines, now known to be omitted by doubly ionized oxygen, [O III]. Baade also tried the red-sensitive plates on NGC 147, the small, faint, enigmatic nebula or galaxy near M 31, which showed neither resolved stars on ordinary “blue” direct exposures nor any emission lines in its spectrum. Undoubtedly he wanted to see if anything would show in the red. None of these experiments was successful then, for he never reported results from any of them, but he continued trying as photographic laboratories began producing better panchromatic films and plates. These early unsuccessful trials provided Baade with the experience and knowledge for his later successes with faster photographic emulsions at the Mount Wilson and Palomar Observatories in the 1940s and 1950s. He was also an expert on wide-field photography, and on the recently developed Ross lens, the best wide-field system then

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in existence (just before Schmidt had discovered the principle of his system, and announced it). Frank E. Ross had invented his own four-element lens system, and Baade had discussed it with him at Yerkes Observatory, tried Ross’s own camera there, and persuaded Schorr to have a 4-inch model made for Hamburg Observatory.31 When the minor planet Eros, so small that it is almost always too faint to observe, swung very close past the earth in 1930, it was quickly recovered and several visual observers detected rapid variations in its apparent magnitude. These, if true, would be evidence that the asteroid was not spherical and was spinning on an axis, presenting different areas to the sun and earth as it did so, thus modulating the light it reflected. Baade, the expert in photographic photometry, devoted three nights to measuring these variations accurately using his well-honed techniques, including setting up sequences of standard stars by comparison with the north polar sequence. He confirmed the variations, which had an amplitude of 1.0 magnitude and a period of 2 hours, 38 minutes, or double that value if the light curve had a more complicated form, with alternate maxima different. He also measured Eros’s mean photographic and photoelectric magnitudes accurately, confirming that its color index was approximately the same as the sun’s, and hence that its reflectivity (or albedo) was more or less the same as for other previously known asteroids, establishing that the new asteroid with the very peculiar orbit was “like” them in some sense.32 Earlier that same year, Clyde Tombaugh had discovered Pluto at Lowell Observatory in Flagstaff, Arizona, and in the fall Baade determined several highly accurate positions for it, as other observers had done before him. The next spring he measured the distant planet’s magnitude accurately.33 These were not highly creative projects, but they both required a fairly large telescope and a skilled observer to carry them out. They were exactly the kind of astronomy which Schorr understood, approved highly, and praised in his recommendations for Baade. Most of Baade’s research at Hamburg was along much more astrophysical lines. He had met Rudolph Minkowski, a physicist who specialized in optics and spectroscopy, at Hamburg University in the early 1920s. Minkowski had been interested in astronomy from

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childhood (his father was a well-known professor of medicine, and his uncle, Hermann Minkowski, was the world-famous professor of mathematics, first at Zurich, then in Go¨ ttingen). Through their friendship Minkowski began collaborating with Baade on an astrophysical research program, to measure the line profiles of some of the strongest emission lines in the Orion nebula at very high spectral resolution. They had probably begun discussing the physical idea of the program before 1927, when the origin of these lines (except for the well-known hydrogen and helium lines) was unknown. Measuring the widths of the lines would provide a chance to determine the atomic weights of the elements which emitted them, if the main internal motions in the nebula were thermal rather than mass motions. However, in 1927 the Caltech laboratory spectroscopist Ira S. Bowen (many years later to become Baade’s director at Mount Wilson and Palomar Observatories) broke the puzzle and identified the other strongest lines as “forbidden” lines of ions of oxygen, nitrogen, and neon, never produced in the laboratory. After that Baade and Minkowski’s motivation for the program changed to learning something about the mass motions in the nebulae from the line profiles, which was possible now that the atomic weights of the gases which emitted them were known. The instrument they used, with the Hamburg reflector, was a Fabry-Perot interferometer, an especially advantageous device for obtaining very great spectral resolution on a single spectral line at a time, together with reasonable sensitivity, particularly for an extended light source such as a laboratory gas tube or an area in the Orion nebula. Besides taking part in their discussions of this important astrophysical problem, Baade’s other main contributions were to provide the link with Schorr and hence permission to use the reflector, and to help with the observations in the initial stages. Minkowski provided the ties to the Fabry-Perot interferometer and to the other two Hamburg laboratory spectroscopists who collaborated on the project, Fritz Goss and Peter P. Koch. Using the delicate interferometer under observing conditions in the dome required a long learning process and many modifications to the instrument. The paper was not completed and published until two years after Baade had left Hamburg. Nevertheless, he was listed as the first

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author (the four were named in alphabetical order), but it is obvious that he had little to do with the instrumental ideas which went into the paper, or with the data reduction, analysis, and discussion. For many years this was the only paper which contained results on nebular line profiles at very high resolution, and it was widely quoted and referenced.34 By the end of his years at Hamburg, Baade was recognized as one of the most important observational astrophysicists in Germany. In 1924–25, when Herman Zanstra, the young Dutch theoretician, spent a year at the Hamburg Physical Institute, Baade not only suggested that he work on planetary nebulae but also in fact supplied the crucial idea for the “Zanstra method” which has proved so fruitful for the study of these objects. Zanstra returned to the Hamburg Observatory in 1930 to work with Baade in applying the method at the telescope. The next year the equally young Albrecht Unso¨ ld came to work with Baade on measuring the profile of the strong Hα absorption line in A stars.35 Both Zanstra and Unso¨ ld were to become world experts in their respective fields. In the summer of 1931, as Adolf Hitler and his Nazi party were gaining wide support in Germany, Baade completed a paper on a second, even fainter and therefore more distant cluster of “nebulae” which he had discovered in Ursa Major. He had recognized it on one of his plates, which he followed up with several more long exposures. Even on the best of them the individual galaxies were barely brighter than the increasingly light-polluted night sky at Bergedorf.36 Five years later Humason, at Mount Wilson, succeeded in measuring the radial velocity of this cluster as 4.2 < 104 km sec −1, the largest redshift then known.37 That same summer Baade also sent to press a paper listing brief descriptions of many “extragalactic nebulae” he had observed, fifty-four in all, in the style used earlier by Curtis and Hubble to describe the nebulae and galaxies they had photographed. Baade’s paper summarized the results of many long nights of observing, in which he had honed his skills with the reflector and learned the detailed forms of many galaxies, the similarities among them, and the differences between them. Among them was NGC 147, which he could only describe as “Very faint, structureless oval 7′ < 2′, P-

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A 155°, spiral?” He did not understand this object, but when he did, thirteen years later, on the basis of excellent plates he had taken of it with the 100-inch, it was the key to his recognition of the two stellar populations. Among the nebulae he reported on in his Hamburg paper, four were actual gaseous nebulae, one of them IC 443, which Baade described “as a smaller example of the well known Cirrus nebula in Cygnus (NGC 6995) to which it has a striking similarity in form.”38 Later, at Mount Wilson and Palomar Observatories, Baade was to recognize that both these nebulae are the optically visible manifestations of expanding shock waves about the remnants of supernovae tens of thousands of years old.

Mount Wilson to Stay Baade finished his last three papers at Hamburg, including one on the variable star RS CVn, just before leaving permanently for the United States.39 Adams, the director at Mount Wilson Observatory, had offered him a regular staff position, and Baade had jumped to accept it. He was finally getting the opportunity he had wanted so badly twelve years before, to go to America and observe with the big telescopes, and this time he was going to be paid to do it (instead of coming as a volunteer, as he had hoped back in 1919) and to stay. Mount Wilson Observatory was a research institution, not connected with any university but operated by the Carnegie Institution of Washington, endowed by funds from the enormously successful Scottish-American entrepreneur, Andrew Carnegie. Adams, an extremely productive stellar spectroscopist, had been the director since 1923, and he was responsible for recommending appointments of new staff members to John C. Merriam, the CIW president, a paleogeologist. Adams had begun his campaign to hire Baade in the fall of 1930, no doubt after many quiet conversations with his senior colleagues at Pasadena. Frederick H. Seares, then fifty-eight years old, was tired of observing. He had begun when the 60-inch went into operation at Mount Wilson in 1909, and he had had enough of the long, tiring nights at the telescope. He planned to continue in charge of the observatory’s photometric pro-

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gram, but with a younger man to help him, who could take the plates at the telescope and work with him on reducing them to measure the magnitudes and colors of the stars. Baade was probably the greatest expert in the world in this subject outside the Mount Wilson staff, and a most respectful, friendly, attractive figure as well, whom Seares had come to know and like during the months the young German had spent in California. In frequent correspondence Baade had asked intelligent questions, reported his results, and kept the older man informed of his progress. Whom better could Seares recommend to Adams for the job? Adams could truthfully describe Baade to Merriam, in a preliminary memo, as “one of the most promising younger astronomers of the world.” His “observational experience” was “wide,” his “knowledge of theoretical astrophysics . . . excellent.” Baade had “used reflecting telescopes for many types of work and has shown marked skill and resoucefulness,” while “[m]uch of his best work has been in photometric investigations, a field in which additional assistance is needed greatly at Mount Wilson.” Best of all, Baade had already spent some time at the observatory “and his ability and personal characteristics made a strong appeal to the members of our staff,” not least to Adams himself. Superficially friendly to visiting foreign astronomers, the director was at heart hostile to most of them, as he revealed in personal letters to his closest friends, including Charles G. Abbot, a solar radiation expert who shared Adams’s New England background. Adams had been outspokenly anti-German during World War I, and his attitude had not changed, although he had become less vehement. Yet Baade’s engaging personality had won him over. Adams wrote Merriam that he was “not certain that it would be possible to secure Dr. Baade at Mount Wilson” but he thought there was a chance, because he knew Baade was discouraged about his opportunities for research at Hamburg “where a very elderly director of conservative tendencies gives him little encouragement in developing his methods and ideas.” Hence, Adams suggested, with the future of Mount Wilson in mind, that “if the plans and resources” of the CIW “justifi[ed] the expenditure,” he would like to offer Baade a position at a salary of $3,000

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a year.40 Adams’s analysis of Baade’s qualifications and potentialities was an excellent one and in fact he was eager to come. Adams requested Ross, at Yerkes Observatory, to ask Baade if he would accept the job if it were offered to him. Ross had been Baade’s mentor during his two months at the Wisconsin observatory, and now that the 200-inch project was under way, he was spending increasing amounts of time in Pasadena, as an optical consultant and designer. Baade replied directly to Adams that he was “deeply obliged” for the “splendid opportunity,” and “[o]f course . . . would be very glad to accept such a position.” He enclosed a short curriculum vitae, and stated that his salary at Hamburg was $3,000 a year, but that the conditions there were “so abnormal” (the inflation and continuing crisis of the Weimar government) that he did not know how to compare it with a U.S. salary and would prefer for Adams to “make a proposition.”41 The Mount Wilson director, now sure his man would come, then made a firm recommendation to Merriam that Baade be hired at a salary of $3,300 per year, emphasizing his importance for Seares’s photometric program. Merriam approved, Adams cabled the offer to Baade in Hamburg, and he accepted.42 Adams, an experienced director, knew exactly how to handle the head of his institution in Washington and the research astronomer in Germany he wanted to hire for his staff. He had prepared the way perfectly with both of them, and the well-oiled negotiations went through flawlessly. Yet it is probable that Adams did not reveal all he was thinking. Mount Wilson Observatory’s research program in those years was devoted almost entirely to amassing data on the sun and the stars, in order to understand their physical nature, just as at all other advanced observatories of the time. Hubble had begun his pioneering studies on the galaxies and the universe, and had already made great strides in opening up new horizons. His work had quickly caught the attention of astronomers everywhere, and of the public. Already in 1931 he was linked in the press with Einstein, the greatest scientific personality in the public imagination of the time. Adams and Hubble were almost compete antitheses, with their New England and Southern backgrounds

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respectively, one dour, retiring, organizationally oriented, operating behind the scenes, dedicated to long, hard work, the other personable, outgoing, individually oriented, flamboyant, a loose cannon who worked hard but played hard as well. Within a few years Hubble would be making end runs around Adams and Merriam to get what support he needed directly from the Carnegie trustees themselves. Sixteen years later, after he had retired, Adams, in a bitter letter to Ira S. Bowen, his successor, revealed his deep distaste for Hubble, whom he claimed had “done little work of the first order for twenty years” (that is, after 1927) and “at sixty is still eager for notoriety and has his press agent continuously at work.”43 Almost certainly Adams realized well before 1930 that it would be prudent to have another astronomer on his staff who could be counted on to do important research on galaxies and globular clusters, but who would be a respectful team player, more amenable to direction from above than the prima donna from Missouri. Baade fitted the bill perfectly, or at least he did in 1931, and that was no doubt the “future of Mount Wilson Observatory” which Adams had in mind when he recommended him for the job. Schorr realized that there was no chance to hold Baade at Hamburg against this offer from the land of clear skies and big telescopes. The previous year he had recommended Baade strongly for the title of professor at the university, which would bring him an additional salary of 1,000 Marks per year.44 Baade had not received the appointment, probably because of the economic situation. Now Schorr transmitted Baade’s resignation, to be effective at the end of September, to join the Mount Wilson staff. It was a great honor that a German astronomer, and particularly one from Hamburg Observatory, had been appointed to the permanent staff of the largest telescope in the world, Schorr wrote. Clearly Baade should go not merely as Observator of the Hamburg Observatory, but also as a professor in the Hamburg University. Furthermore, since the appointment would end on September 30, it would not cost much to make the appointment now, he added. The canny Hamburg senators could understand that reasoning easily enough, and Baade finally was promoted, just two months before his job ended.45

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Because of the German inflation, Baade had no savings, and he received no traveling expenses from the Carnegie Institution. It was impossible for him to borrow the money for his and his wife’s passages to America, and he had to appeal to Adams for an advance on his salary. Although the director professed to be surprised by the thought that Baade “had had no opportunity to lay aside any savings,” he passed on this request to Merriam, emphasizing that the young German’s case was “special” and therefore “[c]ertainly . . . need not be taken as a precedent for the future.” Although Merriam did not authorize a direct loan from Carnegie funds, Adams was able to tell Baade he could advance him $900 “from some private funds which we have available at the Observatory” and that he could buy his steamship tickets.46 With the Carnegie influence behind him, and his professorship in hand, Baade had no trouble getting an immigration visa to America. All his letters to Adams were polite and respectful, but not fawning; he dealt with the issues directly and pleasantly.47 Evidently he made the same impression in person on the American consular officials in Hamburg. On August 31, 1931, Baade and Muschi sailed from Bremen on the Hamburg-America steamship Los Angeles, bound for San Pedro, on the month-long voyage via the Panama Canal.48 When he reached America, Baade was superbly trained for the research that lay before him. He had had a long schooling at the outstanding German university of the time, where his teachers had been among the leaders of German science. On completion of his Ph.D., he had gotten a job in which he almost instantly became the main observer with the largest telescope in Germany, a reflector like the really big telescopes at Mount Wilson. Although he chafed under Schorr’s conservative direction, Baade had plenty of time for his own research programs, on the globular clusters which were at that time the key to mapping our Galaxy, and which were to become, in his hands, the key to recognizing the two stellar populations. He had the skill, drive, and ability to carry out these programs, and the pleasant but persistent personality which enabled him to meet and become confidants of important scientists such as

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Shapley and Ross in his own field of research, as well as Adams and Edwin B. Frost among the directors. Baade’s year in America as the first German International Education Board fellow in astronomy had introduced him to many other, younger astronomers as well, and had broadened his horizons to include research on galaxies and clusters of galaxies. On his return to Germany he had been offered a superficially more important directorship, but had turned it down to stay in active telescopic research. He had already begun to make important new contributions on the local group and on distant clusters of galaxies with the Hamburg reflector. He had already discovered, in incomplete form, many of the ideas of the two stellar populations. Baade was better prepared to use the big telescopes at Mount Wilson than Hubble had been when he arrived there in 1919, than Shapley in 1914, than Seares in 1909, even than Adams himself in 1904. Probably Baade was better prepared than anyone else had ever been, when he arrived at Mount Wilson Observatory in October 1931.

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Before the War MOUNT WILSON, 1931–1938

Introduction When Baade and his wife Muschi arrived in Pasadena in September 1931 he was thirty-eight years old. Mount Wilson Observatory, created by George Ellery Hale in 1904, was one of the most important departments of the Carnegie Institution of Washington, a privately funded foundation, devoted entirely to pure research. Walter S. Adams, who had been Hale’s student, assistant, “first astronomer,” assistant director, and frequently acting in his place, had succeeded him as director in 1923. Now nearly fifty-five, Adams was an outstanding stellar spectroscopist and a skillful, quiet, diplomatic but iron-willed scientific leader. Edwin Hubble, three years older than Baade and a much more colorful figure than Adams or the younger German, was already famous among astronomers for his observational research on “nebulae.” By his pioneering observational studies he had proved without a doubt that some of them were really galaxies, remote star systems comparable to the Milky Way, and that the universe was expanding. He opened up the whole field of cosmology and broadened astronomy to include the study of the entire universe, rather than being confined to the study of planets, stars, nebulae, and interstellar matter, as it had been before his time. To the general public Hubble was most famous for discovering “the expansion of the universe,” even if he did not use those words himself. Some of the other leading Mount Wilson Observatory staff members were Frederick H. Seares, the assistant director, a few years older than Adams; Alfred H. Joy and Paul W. Merrill, both stellar spectroscopists; and Seth B. Nicholson, an expert on solar and planetary research.

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Mount Wilson Observatory was unquestionably the most important observational astronomy research center in the world. Its 100-inch reflector was the largest telescope in existence; it and its 60-inch were both superb instruments at an excellent site, well equipped with the photographic spectrographs and direct plateholders which were the prime data-collecting systems of the time. Results flowed from them, published as Mount Wilson Observatory Contributions at the same time they appeared in the Astrophysical Journal. Briefer papers could be published even more quickly in the Proceedings of the National Academy of Sciences. Lick Observatory, seventy miles south of San Francisco, had a good site but two older, smaller telescopes, the 36-inch refractor and the antiquated Crossley 36-inch reflector. They restricted its research to long-term radial velocity programs, or astrophysical problems the Mount Wilson astronomers did not want to do themselves. Yerkes Observatory, with its 40-inch refractor at a much poorer Midwestern site, was in the doldrums, awaiting the retirement of its longtime director Edwin B. Frost, locked to the ideas of the past. He had not tried to add Baade to his staff, although two of his most productive colleagues, George Van Biesbroeck and Frank E. Ross, had urged him to do so years before Adams made his move.1 Harvard College Observatory, directed by Harlow Shapley, who had left the Mount Wilson staff in 1920, had an assortment of small telescopes in Cambridge and in South Africa, which could not compete with the big Mount Wilson instruments but were highly effective in discovering new objects. Shapley was at the apex of a horde of assistants and computers and epitomized the mass-production, assembly-line type of research which contrasted strongly with the Mount Wilson skilled-craftsman approach. The Harvard director had been the great pioneer of globular-cluster work with the big Mount Wilson reflectors, and was a leader in galaxy research with smaller telescopes, so Baade had corresponded with him frequently from Germany, as he continued to do in America. But Shapley, heavily involved in national scientific and educational advisory and advocacy groups, was already finding it difficult to spend large amounts of time in doing research himself, although he was highly effective in directing others.

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Many other astronomers made significant but smaller contributions to research, based almost entirely on observations they made themselves at their various university observatories, on or near their campuses, in the time they could spare from their teaching duties. Only at Lowell Observatory, another private but badly underfunded research institution, at the Naval Observatory in Washington, specializing entirely in positional work, and at the Dominion Astrophysical Observatory in Canada did astronomers do full-time research as at Mount Wilson, Lick, Yerkes, and Harvard.

Photographic Photometry Baade’s primary job when he began work at Mount Wilson was direct photography and photometry of “nebulae” (a term then still used to mean both real nebulae and galaxies, which the astronomers knew were actually star systems) and star clusters.2 He had made himself an expert in these fields with the Hamburg Observatory 40-inch reflector, and Seares had wanted to bring him to Mount Wilson as his own successor in photographic photometry. Seares and his collaborators had defined a zero point and measured accurately their “photographic” (essentially blue light) magnitudes of the stars in the “North Polar sequence,” a small area of the sky near it, then transferred this sequence to the stars in the small Kapteyn “selected areas” evenly spaced around the sky. The difficulties in doing this accurately (so that a difference of ∆m between any two measured stars, wherever they were in the sky, corresponded exactly to a ratio of brightness 100.4∆m) were enormous, but Baade, from his careful comparisons of selected areas with one another, knew that these Mount Wilson magnitudes were reasonably accurate down to mpg = 17.2 or so.3 However, he needed to go much fainter than this to measure the magnitudes of faint stars in distant globular clusters and of faint, distant galaxies, the ultimate aim of his program. For somewhat fainter stars the magnitudes in the selected areas were not accurate, and Seares and his collaborators, taking short exposures with the 60-inch reflector, had not been able to measure magnitudes of the faintest stars within reach of the 100inch at all.

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Baade’s strategy was to choose a very few, well-placed selected areas, and extend the magnitude sequence in each to as faint a level as he could. He tried many methods, just as Seares had; in each the idea was to compare two exposures of the same area with a “known” difference in brightness of the stars, artificially imposed. Baade’s methods for doing this included reducing the aperture of the telescope, inserting a neutral filter in front of the photographic plate, and comparing longer and shorter exposures. But the nonlinear response of the photographic effect, the background sky fog (which limited the exposure time and hence the faintness to which stars images could be recorded), and changing conditions, not only of sky transparency, but the more subtle effects of “seeing” and changing thermal distortion of the large primary mirrors, all worked to make each method less accurate in practice than it seemed in principle. Baade never simply took data and hoped for the best; he took many plates of each of his selected areas, compared them carefully, analyzed every failure, improved his methods, and came back to the telescope prepared to do better the next month, the next season, or the next year. Thus in the end he knew quantitatively the probable errors of his results, rather than hoping and extrapolating the magnitudes scale “enthusiastically” as he thought his colleague Hubble was doing.4 Baade’s own work was much less spectacular, slow because he was so careful, but as a result correct in the end. He followed exactly the motto of the great mathematician and onetime Go¨ ttingen Observatory director Carl Friedrich Gauss, “pauca sed matura” (few [papers] but ripe [ones]).5 By 1937 Baade had found that the best method was to use a “halffilter,” so that half the field was reduced in magnitude and exposed simultaneously with the other, unreduced part, doing away with some of the seeing and mirror-distortion effects (which he tried endlessly to reduce by various methods of ventilating the dome). The following year he was able to report that he at last had his “final” magnitude scale complete down to mpg = 21 in selected area (SA) 68, and close to ready in SA 57 and SA 61. He was also starting to get results on photovisual magnitudes (measured in yellow light) down to mpv = 20 in the same selected areas.6 Shapley and

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Walter Baade, 1935. (Courtesy of the Huntington Library, San Marino, California.)

other astronomers were eager to use his faint-magnitude standards, but Baade would only release them when he was certain they were accurate.7 The more Baade worked on setting up magnitude scales, the more convinced he became that photographic photometry was not the right way to do it. The nonlinear photographic effect imposed tremendous difficulties. Joel Stebbins, the American pioneer of photoelectric photometry, was already a research associate of the Carnegie Institution, enabling him to come out to Pasadena on a regular basis in the summer from his faculty position at the University of Wisconsin and use his photometer on the big Mount Wilson telescopes. In those early days photoelectric cells and their associated electrometers were relatively insensitive, but Baade, like Hubble, carefully monitored each improvement and urged Stebbins to push

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toward measurement of very faint stars, which could be used as magnitude standards.8 He and his assistant Albert E. Whitford, originally trained as a graduate student in experimental physics at Wisconsin in the latest vacuum-tube circuits and amplifiers, did do as much of this work as they could. In 1945 Baade, who always regarded his own photographically determined standards as merely a stopgap until photoelectric systems replaced them, wanted to add Whitford to the Mount Wilson staff, but that was not to be. Baade had to continue using his photographic standards for several more years after that.9 Seares retired in 1940 at age sixty-seven, but continued as a research associate, without administrative responsibilities. Adams did not appoint anyone else to be in charge of the photometric work at Mount Wilson, but Baade became the person on whom Adams called for expert advice in that field.10

Early Stellar Population Research Baade did not set up faint-magnitude sequences as an end in themselves, but because he needed them for his own research. He was working on globular clusters, concentrating especially on measuring the distances to the faint, distant ones, some of them in outlying regions of our Galaxy, others near its center. In each he would try to find RR Lyrae variables, then measure their mean magnitudes accurately and thus determine their distances. He always used a magnitude scale from one of the selected areas he had measured himself, which he transferred photographically to a sequence of local standard stars around the outside of the cluster, and then compared the variable stars with them.11 In some globular clusters, such as NGC 5694, which had been too faint for Shapley to study, Baade could find no RR Lyrae variables, in spite of his very careful search; that meant there were none in it. Then instead he measured the magnitudes of the twenty-five brightest stars, and after carefully classifying the type of this cluster, derived its distance as 40 kpc (with the mean absolute magnitude of RR Lyrae variables he was then using). NGC 5694 is in a direction close to the longitude of the galactic center, but about 30° above it; Baade carefully studied the

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field and from the number of faint galaxies, which was “normal,” concluded that it was unaffected by interstellar extinction. Thus the distance he had determined was the true distance, and the globular cluster lay far beyond the nucleus of our Galaxy.12 An even more distant globular cluster was NGC 2419, which lay in the anticenter direction and did have RR Lyrae variables. Baade put it at 56 kpc from the sun, 64 kpc from the center of our Galaxy, further than the distances to the two Magellanic Clouds, and comparable with the distances to M 31 and M 33 according to the extragalactic distance scale of the time. Thus whether it was an “independent system” in intergalactic space or a “far-outlying member” of our system was largely a matter of definition. Shapley referred to it as an “intergalactic tramp,” or (in a letter to Baade) as “Baade’s tramp.”13 Likewise, on his plates of the globular cluster NGC 5634, but outside it and far behind it, Baade found an isolated RR Lyrae variable 40 kpc from the sun and 34 from the galactic center.14 His discussion of these clusters and this “field variable,” like his earlier work at Hamburg on the high-latitude clusters NGC 4147, M 53, and NGC 5466 and the RR Lyrae stars outside and beyond them, make it clear that he had already recognized the vast, more or less spherically distributed “population II” whose discovery he was to announce in 1944. The RR Lyrae variables, which could be found by their characteristic light variations, and whose absolute magnitudes were “known,” were the best markers of it. In 1934 Baade published the results of his long program of finding and studying the variable stars in a field in Cygnus, near the central plane of the Milky Way. He had begun the work in Hamburg, and hoped some day to publish all the detailed observational data there, but never did so.15 Baade found that the most frequent types of variables in it were eclipsing variables and long-period variables, and from them estimated the distance of what he called the “Cygnus cloud,” but far behind it he detected four very faint Cepheid variables at much larger distances. These were all typical luminous members of what he was to name “population I” in that same paper a decade later. Furthermore, in his 1934 paper he noted that he had found five isolated RR Lyrae variables in the Cygnus field, roughly the same number per unit area as in high-latitude

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fields. Hence, he concluded, these RR Lyrae stars were actually representatives of the “general [later he would call it ‘spherical’] distribution of [such] variables,” a key concept which he was to generalize to population II in the 1944 publication.16 Baade was well on the way to several of its main ideas just a few years after joining the Mount Wilson staff.

Clusters of Galaxies Some of the first research Baade did at Mount Wilson was finding previously unknown faint, distant clusters of galaxies. At Hamburg he had discovered a few with its 40-inch reflector; now he could reach further out into space with the 100-inch. Baade had not made a systematic search, photographing blank fields, but instead inspected each plate he had taken for any purpose, looking carefully at everything that was on it. Since he was an expert in guiding and focusing the telescope critically, and correcting the focus slightly for temperature changes during the long exposures, his plates were particularly good and revealed faint, small galaxies near the limit of resolution. Hubble and Milton L. Humason, the former mule driver, janitor, and night assistant who had become an excellent spectroscopist of faint “nebulae,” realized that clusters of galaxies were the best objects for pushing out the redshift-distance relationship as far as possible. The brightest galaxy in a cluster might be intrinsically considerably more luminous than the “average,” and thus spectroscopically observable, while the more numerous, fainter ones could be measured for magnitude, providing a good distance determination. In his first year at Mount Wilson Baade found the Andromeda cluster of galaxies, a fairly close one; Leo II, a rich cluster containing three hundred detectable galaxies above the plate limit, mpg = 19; and Coma II, the faintest then known, well beyond the limit at which individual field galaxies could be measured. The next year he discovered another very faint one, in Cetus, and two years later still another, in Cygnus, not far from the Milky Way and showing signs of interstellar extinction. Baade measured the colors of the galaxies in several of the unobscured clusters and in others which

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Hubble had discovered, and found that they all had only a relatively small range of color index, indicating that in at least this respect they were physically similar and hence possibly similar in absolute magnitude also. Hubble had assumed this; Baade, always skeptical, was trying to check the assumption. None of these discoveries or tests was important enough to Baade to publish them as papers or notes; they appear only in the annual reports of the observatory for his early years at Mount Wilson. He was working his passage by helping observe for Hubble’s and Humason’s program. But he never joined wholeheartedly in it as a collaborator, and gradually dropped it altogether as he became more and more involved in his primary stellar-population research. Always friendly, diplomatic, and helpful, he had nevertheless resolved, probably long before he arrived at Mount Wilson in 1931, to be his own man there as he had been at Hamburg Observatory.

Supernova Research with Fritz Zwicky The study of supernovae became a hot research topic in the 1930s, and Baade was one of its pioneers. The concept that there are two types of novae, the “ordinary” ones and at least one example of a much more luminous type, S Andromedae, which reached visual apparent magnitude 7.5 near the center of M 31 in 1885, had been put forward by Heber D. Curtis in 1917. Knut Lundmark, just three years later, further discussed this idea and called the two types “giant” and “dwarf” novae. In 1929 Baade referred to S And as a “Hauptnova” (“chief nova”) in his inaugural lecture at Hamburg University.17 Then in 1933 Curtis, in an influential review in the Handbuch der Astrophysik, amassed the observational evidence that there are two classes of novae, those with absolute magnitude at maximum about −5, and a much rarer class which reached absolute magnitude about −15. He listed six more examples in addition to S And 1885, one of them in our Galaxy, Tycho Brahe’s “nova” of 1572, and five in other spiral galaxies, four discovered by Hubble and one by Williamina Fleming at Harvard.18 Because they are so very luminous they can be observed to great distances; hence they offered the possibility of being used as distance indicators in the

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universe. Baade, a voracious reader of astronomical research papers, particularly those on galaxies, was undoubtedly aware of all this long before the publication of Curtis’s Handbuch article. Soon after arriving in Pasadena in 1931, Baade met Fritz Zwicky, a Swiss theoretical physicist on the California Institute of Technology faculty. Hale had built Caltech up from tiny Throop College of Technology, and brought Robert A. Millikan from the University of Chicago to head it in 1921. The Mount Wilson Observatory offices on Santa Barbara Street were less than two miles from the Caltech campus, and there were many links between the two institutions. Zwicky, five years younger than Baade, had first come to Caltech as a Rockefeller postdoctoral fellow in 1925–1927, and the two may have met during the second year while the German astronomer was in Pasadena as well. Zwicky, who stayed at Caltech, first as an instructor, then as an assistant professor from 1929 until 1942, dabbled in research in almost every field of physics from ferromagnetism and crystal structure to electrolytes and a “gravitational drag” theory of the redshifts of the galaxies. However, he was a flamboyant, highly successful teacher of theoretical physics, with an interest in astrophysics. Baade and Zwicky became friends, and frequently discussed the two types of novae; they used and thus popularized the name “supernova” for the more luminous class, a word which Lundmark had just invented.19 Together Baade and Zwicky wrote a series of three papers, published in 1934, summarizing the fruits of their discussions and establishing supernova research as an important field. The first paper, clearly drafted largely by Baade, outlined the observational distinctions between ordinary novae, which occur at a rate they estimated as about 10 to 30 per year in our Galaxy and M 31, with mean absolute magnitude at maximum about −5.8, and range of 3 or 4 magnitudes about it, and supernovae, with absolute magnitude about −13 at maximum, which flare up only one per several hundred years in a typical spiral galaxy. By integrating the light curves of supernovae over time, allowing for the unobserved spectral regions, Baade and Zwicky estimated the total energy released in a supernova outburst as possibly of the order of a stellar rest mass, comparable with the mass of the star itself.20

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Their second paper, no doubt expressing more of Zwicky’s ideas, put forward the concept that supernovae were the sources of cosmic rays, then a very hot subject in experimental physics, with Millikan one of its leading practitioners. Baade and Zwicky noted that in a supernova explosion a large mass of gas, probably comparable to a stellar mass, was expelled with high velocity, perhaps nearly the velocity of light, and conjectured that this mass contained the incipient cosmic particles. Although their suggested mechanism differed in detail from the one Millikan favored, it agreed with his general idea that cosmic-ray particles were accelerated in “God’s laboratories in the stars.” Baade and Zwicky further noted that the final, stable configuration of matter would be a neutron star, and that supernovae could represent transitions of stars to this state.21 A third joint, short paper, even more clearly originating in Zwicky’s thinking and published in the Physical Review, mostly called attention to the earlier two, and pointed out that if their ideas were correct, some of the primary cosmic rays incident on the earth would be protons and heavier ions. (At that time the composition of the cosmic rays was still unknown, and the existence of a galactic magnetic field was not suspected.)22 Clearly more observational data on supernovae were needed, and to find just a few each year would require a fast, wide-angle photographic survey, covering hundreds if not thousands of galaxies on a single exposure. Comparing or “blinking” exposures taken a few weeks to a month apart would reveal any bright supernova which had flared up in the field. Hubble had begun a half-hearted supernova search in the Virgo cluster of galaxies in 1928, using the 10-inch photographic Cooke telescope on Mount Wilson. Baade had continued this search after his arrival, but the telescope was not large enough and its inherent aberrations made it unsuitable for the task.23 Baade, who had brought the news of Bernhard Schmidt’s newly invented wide-field camera to Mount Wilson Observatory in 1931, undoubtedly realized that a Schmidt telescope would be ideal for such a supernova search.24 Furthermore, Zwicky would be able to provide the way to get it. Baade cultivated him, and in either 1933 or 1934 Zwicky began searching the Virgo cluster on a more regular basis, with a 3.5-inch,

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commercially available Wollensak camera, stopped down to f/4.5, mounted alongside a visual telescope (used to guide the exposures and for its mounting) on the roof of Robinson Laboratory, the astronomy building on the Caltech campus. It was even less satisfactory than the Cooke 10-inch, but it whetted Zwicky’s appetite and gave him his first serious observing experience. He was in a very good position because Caltech, not Mount Wilson Observatory, controlled Palomar Mountain and the 200-inch telescope which was to be erected there, after its mirror had been completed on its campus. When Hale had secured the funds to build the giant new telescope from the Rockefeller Foundation in 1928, its executives refused to give them directly to Mount Wilson Observatory, part of the rival Carnegie Institution, Corporation, and Foundation empire. Instead the Rockefeller grant for the 200-inch went to Caltech, which already had large chemistry and biology operations financed from the same source. Hale moved John A. Anderson from the Mount Wilson staff to Caltech as executive secretary in charge of the project, and the Mount Wilson astronomers, especially Adams and Hubble, were heavily involved as advisers, but it was Caltech’s project, site, and money, and it would be Caltech’s telescope. The 200-inch design group, mechanical shop, and optical shop were all on its campus, but it had no astronomers or astrophysicists on its faculty. Clearly that was a niche that Zwicky was ready to fill. He had first visited Palomar in 1930, soon after it had been selected as the eventual location for the “Big Eye.” Now, coached by Baade, he helped persuade the Caltech authorities to provide the money to build an 18-inch Schmidt camera and erect it there. At Palomar, an excellent dark-sky site, he could carry out a truly effective supernova search. Zwicky later claimed that he personally convinced Hale to allot $25,000 for this telescope, but it seems likely that with Anderson, Hubble, and Baade, the leading authority in America on Schmidt cameras, all for it, the old “honorary director” could hardly have resisted.25 The project was approved by the Observatory Council in 1935, and in June of that year Baade wrote Richard Schorr, still director of Hamburg Observatory, introducing Zwicky, “a good friend of mine,” for whom the 18-inch Schmidt

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was already under construction. Schorr welcomed him on a visit to the observatory later that summer, and Schmidt exhibited his camera to him, and some of the spectacular photographs he had taken with it.26 The 18-inch Schmidt corrector plate was difficult to make, but by September 1936 the telescope was in operation on Palomar, and Zwicky, assisted by Josef Johnson, began taking films with it. They had a serious search program under way by November, and within a year Zwicky found his first two supernovae with the new Schmidt.27 But by the time Baade and Zwicky published their joint paper on the light curves of these two objects in 1938, tension between them had grown. They never wrote another paper together. Zwicky, an isolated physicist in a new field, not unnaturally feared that the Mount Wilson astronomer would get all the recognition and that his own contributions would not receive the credit he felt they deserved. He was eager for the publicity that would bring him the salary raises and promotion he had long anticipated.28 Zwicky was a very prickly, combative person, quick to take offense, who eventually got on bad terms with almost every scientist whom he could consider, in any sense, a rival. In 1936 Cecilia Payne-Gaposchkin, greatly interested in the astrophysics of all types of variable stars, studied Baade and Zwicky’s papers carefully and published one of her own, mildly criticizing some of the details of their estimate of the total energy released in a supernova outburst, and hence their final numerical result. Zwicky, affronted by the prepublication copy of the paper she sent them, dashed off a hot letter representing himself as speaking for the two of them, in which he criticized the preprint violently and called her a fool. (“We wonder whom you really wish to make fools of—if not yourself.”) Baade, who admired her work in general, and never insulted anyone, in fact had not joined the argument, and soon began dissociating himself from Zwicky.29 The latter published a short paper on his own, refuting to his own satisfaction Payne-Gaposchkin’s criticisms more temperately.30 In 1938 he published another paper describing the results of his supernova search that year, giving few of the observational details and minimizing the contributions Baade had made.31 Their division of effort from the beginning had been that Zwicky

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would find the supernovae with the 18-inch Schmidt, and Baade would follow them with the 60- and 100-inch, obtaining their light curves by transferring the magnitudes he had set up in one of his well-measured selected areas to local standards near the new object. The first supernova he observed this way was in NGC 4273 in the Virgo cluster, discovered by Hubble and night assistant Glenn Moore with the 10-inch Cooke telescope in January 1936, the next two were in Baade’s joint paper with Zwicky, and by 1938 he was able to publish preliminary light curves of eighteen supernovae, including these three, one more found in the interim by the Caltech astrophysicist, and several of the objects previously recorded, for which Baade had redetermined improved magnitude sequences. One of them was the supernova in NGC 2608 which he and Max Wolf had observed in 1920. Using distance moduli provided by Hubble for the Virgo cluster and for a few nonmember galaxies, presumably from their brightest stars, Baade ended up with ten supernovae with reasonably well determined absolute magnitudes at maximum. The mean value was −14.3 ± 0.4, with a dispersion of 1.3; it could be used as a first approximation to determine the distance to the galaxy in which a supernova occurred. Baade was rapidly collecting the observational data which, with low-dispersion spectra, would lead to the classification of supernovae and their use as distance indicators. As he emphasized in his paper, their absolute magnitudes were so much more luminous than the fifty novae Hubble had observed in M 31, all with absolute magnitudes near −5, that there could be no doubt that supernovae were completely different from “ordinary” novae.32 Baade also noted that by far the great majority of supernovae were found in late-type spirals, 72% in Sc and SBc, 16% in Sb and SBb, and only 12% in all the other types together. This was the kind of observational correlation for which he was always looking; in 1944 he would realize that this meant that these supernovae belonged to his then-named population I, associated with O and B stars, and interstellar matter. Finally, in this paper, Baade stated that there was observational evidence of two supernovae in our Galaxy, not only Tycho’s object of 1572 but the Chinese “nova” of 1054, whose remnant was the Crab nebula, expanding at a rate of 0.18″/

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yr, with a radial velocity of 1300 km/sec, as described only the previous year by Nicholas U. Mayall. A young member of the Lick Observatory staff, Mayall had done his undergraduate work in astronomy at Berkeley, graduating in 1928, and had continued for one year as a graduate student there. Then, tiring of classes and needing to earn some money, he had taken a job as an assistant at the Mount Wilson Observatory offices in Pasadena. His main work was measuring and reducing observational data for various staff astronomers, but he managed to get to the mountain with them too, and learned to observe with all the telescopes. He was inspired by Hubble and became especially close to Humason. Mayall returned to the University of California in 1931, did a thesis with the 36-inch Crossley reflecting telescope, earned his Ph.D. in 1934, and joined the Lick staff himself, as an assistant first, but soon in a regular research post. There he designed and soon had built a very fast, low-dispersion spectrograph, well matched to the Crossley reflector, and forming with it an extremely efficient combination for observing globular clusters, galaxies, and nebulae.33 Mayall’s interests fitted exactly with Baade’s, and before long the two were in frequent correspondence. The older German was quick to furnish whatever information the young observer needed on specific clusters and nebulae, and Mayall reciprocated with plates from the Lick files of galaxies with supernovae in them, many of them taken by Curtis years earlier. Baade always added helpful, supportive advice, never seeking credit for the results Mayall obtained while following it. He was the first of many hard-working, young American astronomers whom Baade was to befriend, advise, and inspire throughout their careers.34 In 1937 Baade had encouraged Mayall to use his fast spectrograph to make a “thorough radial velocity” study of M 1, the Crab nebula, a highly peculiar nebula known to be near the position in the sky where Chinese astrologers had seen and recorded a “new star” in A.D.1054.35 Curtis had included it in his list of planetary nebulae, but had described it as peculiar and unlike any other one. Because of its faintness, Vesto M. Slipher at Lowell Observatory had been able to obtain only barely usable spectrograms of it, but May-

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all could take good exposures with his fast instrument on the Crossley, with the slit along the major axis of the nebula or in any other position. These spectra clearly showed that the nebula was expanding with a velocity 1300 km/sec; John C. Duncan’s recent proper motion study of it, with the 100-inch, ruled out the alternate possible interpretation of contraction, and gave 0.18″/yr as the rate. Simply comparing these two values, Mayall immediately obtained the distance to the Crab nebula, approximately 1500 pc. Always conservative, he interpreted it as a nova which had had an outburst in A.D.1054, because the expansion velocity was comparable to those observed in nova shells, and Duncan’s angular rate of expansion gave approximately the observed size of the nebula in 1937, if it had started from a point that long ago.36 However, the fact that the Chinese had seen and recorded the star suggested it had been bright; Baade suspected that it had been a supernova, and by 1938 he thought there was “little doubt” of this conclusion. The Crab nebula appeared nothing like the little expanding nebular shells that were the remnants of ordinary novae, which all seemed to fade to invisibility within a century or less. Baade had begun obtaining direct plates of the Crab nebula at Hamburg, and started again at Mount Wilson, but did not want to publish anything himself that would interfere with Mayall’s work on it. Through letters and in discussions when the Lick and Mount Wilson “nebular” astronomers got together, Baade continued to encourage Mayall’s observational study of the Crab nebula, and in 1939 the latter did publish a popular article on it, in which he called it a probable supernova at last.37 Three years later, after Mayall had left Lick Observatory and its telescopes, on leave to do wartime research “for the duration,” Baade did publish a definitive paper on the Crab nebula as a supernova remnant. Then a few years after the war, the onetime radar “boffins” who energized the new field of radio astronomy discovered that the Crab nebula was one of the brightest radio sources in the sky. This startling result drew Baade into their science, in which he was to become a leader and elder statesman. His close collaborator in those roles was his friend and fellow Mount Wilson staff member Rudolph Minkowski.

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Rudolph Minkowski, 1942. (Courtesy of The Observatories of the Carnegie Institution of Washington.)

Rudolph Minkowski Minkowski had been Baade’s friend and collaborator in Hamburg, an experimental physicist who was a specialist in spectroscopy and optics, drawn into observational astrophysics initially to study the emission-line profiles in the Orion nebula. He was an associate professor of physics at Hamburg University, but he was a Jew according to Hitler’s racial laws, although his family had been assimilated and he was a baptized Lutheran. Minkowski’s father was a medical professor, whose research on diabetes and the pancreas had made him the “grandfather of insulin,” while his uncle Hermann had been a world-famous mathematician at Zurich and Go¨ ttingen. Rudolph, two years younger than Baade, had served as an officer in the German army during World War I, but that made no

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difference after Adolf Hitler came to power as chancellor of the German Reich in 1933. Minkowski knew his days there were numbered.38 He wrote to his friend in Pasadena, and Baade went straight to Adams. Although he had been a staff member at Mount Wilson Observatory only two years, Baade already had learned exactly how to approach his director most effectively. Would Adams be willing to invite Minkowski to work at the observatory for a year, with the prospect of applying the same interferometric methods he had worked out and perfected in his study of the Orion nebula, now using the big Mount Wilson telescopes to apply them to the planetary nebulae? Baade did not breathe a word about Hitler, Jews, or racial laws, but he did report that Otto Stern, the Hamburg physicist famous for the Stern-Gerlach experiment (and soon to leave Germany himself for the United States), had mentioned Minkowski’s case to Max Mason, the president of the Rockefeller Foundation, who “thought it would be possible” to provide funds for his salary for a year.39 Adams understood exactly what Baade’s letter meant, and he was prepared not only to welcome Minkowski if he could come to America with a salary, but to help him get it too.40 The Mount Wilson director preferred to explain all his actions on the basis of scientific research alone, but Minkowski’s famous family name meant a lot, and the fact that Baade wanted to bring him over was all-important. Adams already regarded him as the hope of the future. In addition, the Rockefeller Foundation was funding Palomar, and Mason’s interest in Minkowski further whetted Adams’s willingness to have him at Mount Wilson. It turned out that Mason had spoken impulsively; since Minkowski still had his job in Hamburg the Rockefeller Foundation could do nothing for him then. However, at the end of March 1934 Minkowski was discharged from his faculty post under Hitler’s edict “For the good of the Government Service.” Many other German scientists were trying to get to America too, and the Rockefeller Foundation had only limited funds. It was further hampered by a widespread feeling that Jews or other refugees from Germany should not take jobs which young American scientists were seeking in that Depression year. Adams argued very effectively against this reason for not helping Minkow-

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ski, writing to Edward R. Murrow, the young assistant secretary of the Emergency Committee in Aid of Displaced German Scholars who later became a famous radio war correspondent, that it seemed “unfortunate[ly]” close to the Nazi principle that Jews were in direct competition with young Nordic scientists for jobs in Germany.41 This powerful statement, plus the support of Stern and of Rudolph Ladenburg, another well-known German physicist who had already made his way to Princeton, tipped the balance and in October 1934 the Emergency Committee granted $2,000 to support Minkowski for one year at Mount Wilson Observatory. In November the Rockefeller Foundation itself added a further $1,000, making Minkowski’s total salary for the year $3,000. Adams was sure this would be enough for him (although he had not been paid since March, and was planning to bring his family and belongings with him, and stay in the New World at least until Hitler and the Nazis no longer ruled in Germany).42 Minkowski arrived in New York in May 1935, but now a new complication arose. John C. Merriam, the aging president of the Carnegie Institution, had not been included in the long, complicated negotiations, and now when he was, he refused to accept a check from the rival Rockefeller Foundation to pay Minkowski. It was contrary to policy to do so, Merriam said, and he was a great believer in following policy. Adams was on his way to an international scientific meeting in Europe, and Seares, the acting director in his absence, had to spend hours writing long, persuasive letters and telegrams to Merriam. Finally the Carnegie president agreed to “act as agent,” take the Rockefeller money each month, and pay it to Minkowski.43 While all this was going on, the German scientist had continued on to Pasadena with his family, settled in with Baade’s help, and started work on June 1, 1935, as a “visiting investigator.” He began observing at Mount Wilson very quickly, and in 1936 published his first paper in America, on the spectrum of Comet Peltier, jointly with Baade.44 Before Minkowski’s arrival, Baade had taken spectra of the faint stars in the Cygnus cloud, for instance, but now he turned that side of the work over to his friend, and concentrated almost exclusively on direct photography. Their first big joint program was on the stars and dust in the Orion nebula. Reasonably fast infrared-sensitive

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photographic plates were just starting to be available in the late 1930s, and Baade realized that they were much better for penetrating the dust clouds of interstellar space than the standard blue-sensitive plates long used by astronomers. He knew that the Orion nebula contained dust, and to investigate what stars were near its center, took a long exposure of it in the infrared region λλ7200– 9000. There he found a previously unknown small cluster, containing some eighty stars within a small region centered on the brightest O star in the Trapezium, θ1 Orionis. Baade and Minkowski analyzed this region in detail, measuring the extinction of light by the dust by spectrophotometry, comparing the continuous spectra of the B stars in it which were heavily reddened with others which were not. Using available computed optical properties of various hypothetical types of interstellar particles, they estimated sizes and tentatively confirmed their composition as small, impure “iron” (and other metal) particles with a large admixture of hydrogen. Though their detailed conclusions were superseded a decade later, these papers did much to define the properties of the associations of hot stars and interstellar clouds, which Baade was later to define as population I.45 Minkowski, with his excellent training and experience in spectroscopy, designed a fast nebular spectrograph, built around a Schmidt camera, soon after his arrival in Pasadena. It was constructed in the Mount Wilson instrument shop, and he was using it on the big telescopes within a year. Adams recognized his value as a research scientist and instrumentalist and wanted to keep him on the staff, but there were no vacancies, and no new positions expected in those grim Depression days. Hence the Mount Wilson director recommended him strongly for a second year’s support by the Emergency Committee. This time it allotted Minkowski $2,000, but pressed Adams for a commitment for a permanent appointment for him. The director explained why he could not make it, but emphasized that Minkowski’ s research was so valuable that if the observatory had the money, it would take him on. The Rockefeller Foundation’s firm policy was not to renew any refugee scientist’s grant beyond the first year, so Minkowski’s salary in his second year in America was $1,000 less than his first year’s.46

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A year later that grant ran out, and there was no hope of further support from the Emergency Committee. Adams by now considered Minkowski “a remarkably able physicist with all the modern developments of quantum and wave mechanics at his fingers’ ends, . . . [and] one of the few physicists I have ever known who has also become an admirable astrophysicist.” The German refugee was giving a series of lectures on theoretical spectroscopy to a dozen-odd members of the Mount Wilson staff (including Adams himself), for which they “chipped in” to pay him and thus supplement his income. Adams wanted to keep him on the staff with a temporary appointment, funded by the salary of another staff member who was going on leave for a year, but Merriam would not agree. He had no doubt already earmarked the savings for another project. Thus for his third year in the New World, the only solution was for Minkowski to take a lowly assistant’s job, paying $1,500 a year, another decrease in salary. Adams let him continue actually working mostly on his own research, in spite of the change in title.47 However, Minkowski’s deliverance was at hand, though from an unfortunate source. In February 1938 Francis G. Pease, an elderly staff member who had come to Pasadena with Hale and Adams in 1904, died, creating the first vacancy since Minkowski’s arrival. Adams had meant what he said, and recommended him strongly to Merriam for the position. Minkowski was “the most competent theoretical physicist we have, . . . an excellent . . . observer, [with] marked ability in the design of instruments . . . [and] a very pleasing personality.” Merriam was “favorably disposed,” but wanted to know more about the German refugee’s “immigration status.” Adams was pleased to report that since Minkowski had entered the United States on an immigration visa, he could stay without any danger of being deported, and that he had taken out his first papers and intended to become a citizen as soon as he could. This satisfied the Carnegie Institution president, and Minkowski became a regular scientific staff member on July 1, 1938, at a salary of $3,000 a year.48 Minkowski had earned these accolades (if not more money) especially by his observational work on the spectra of supernovae. Beginning in 1937 with the supernovae in IC 4182 and NGC 1003, the

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first two Zwicky had discovered with the 18-inch Schmidt, Minkowski obtained spectra of every supernova he could, from as soon after discovery as possible until they again became too faint to record. His spectra complemented the light curves Baade was measuring. Previous Mount Wilson observers, especially Humason, had obtained some spectra of a few supernovae earlier, but with Zwicky and Johnson finding them, Baade measuring their magnitudes, and Minkowski recording their spectra, within a few years they obtained a much better idea of the observational aspects of these very high luminosity stellar outbursts. In these first two supernovae he observed, Minkowski confirmed that their spectra consisted of broad emission features, whose redshifts increased with time, reaching a maximum, then decreasing somewhat. None of the emission features could be identified; they did not seem to be lines of either hydrogen or helium, so common in novae, nebulae, and hot stars. Whether the redshifts were due to radial velocities of gas in the shells (as in novae) or were gravitational in origin he could not tell by any test or analysis, but once he had arranged the spectra of these two supernovae in time sequence, he could see that at similar time intervals after maximum light, their spectra were quite similar. With this knowledge he could then interpret the fragmentary spectral information on earlier supernovae, and could see that six of the seven were similar in their development to these two. Only S And, the 1885 supernova in M 31, seemed from the available description of its spectrum to be physically different.49 In 1940 Minkowski observed a supernova in NGC 4725 with a completely different type of spectrum from those he had previously observed. Soon after maximum and continuing for about a week, it had a purely continuous spectrum, very blue, indicating an extremely hot star. As it decreased in light, the broad emission features began to appear faintly, gradually strengthening. Their spectra were thus much more reminiscent of the spectra of ordinary novae. By June 1941, when he gave a paper summarizing his work on supernova spectra at a meeting of the Astronomical Society of the Pacific in Pasadena, Minkowski had enough data to outline a spectral classification scheme for them. There were two basic types,

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type I, like the first two he had studied, and type II, like NGC 4725. There were now nine supernovae whose spectra put them in type I, five in type II. Within each type the spectra were similar in their evolution with time. He had isolated two physical classes of supernovae. Only S And did not seem to belong to either, and it was unclear whether that was the consequence of a real difference, or if the observational data from 1885, so early in the days of scientific photography, were faulty. Minkowski’s classification scheme formed the framework for Baade’s ongoing photometric study of supernovae.50 In spite of his pathbreaking results on supernovae, and his spectrographic design skills, so important at Mount Wilson not only for his own work but for Humason’s measurements of the redshifts of distant galaxies, Minkowski was still receiving $3,000 a year in 1941. Only in 1942, after he came to Adams and told him that his family was having difficulty living on his salary, did he get a raise— to $3,300. The way of a refugee scientist with no competing offers was hard in prewar America.51

Hamburg Directorship In contrast to Minkowski, Baade had come to America to accept a job, not to find a refuge. Schorr, his director at Hamburg Observatory, had wanted him to stay there; he had visualized Baade as his eventual successor in the post as early as 1927, when he had arranged for him to be appointed Observator. Then Schorr had been sixty years old; in 1934, one year from retirement age, he began his campaign to have his former star staff member named as the next director. He wrote Baade, and asked if he would return; the younger man, after weeks of indecision “(in the first sleepless nights of my life!)” replied that he would come back only if he had the great honor to be “called” as Schorr’s successor. The Mount Wilson astronomer was planning to travel to Europe to take part in the International Astronomical Union’s General Assembly in Paris the following year, and he would certainly visit Hamburg and discuss the question with him. The next spring, when Baade received an official letter from Hamburg, asking if he were willing to have

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his name included on the list of candidates for the post, he replied along the same lines, that only a call for the directorship there would cause him to bid Mount Wilson farewell, and that he therefore allowed his name to go forward. But, he now told Schorr, he would not be able to attend the Paris meeting, because under his contract with the Carnegie Institution, to have his trip abroad paid for he would have to declare that he did not intend to resign his post.52 Later that spring Schorr let Baade know, confidentially, that his retirement had been postponed until the fall of 1936, and that the Hamburg faculty had been asked to send nominations for his successor to the Ministry of Science and Education in Berlin. They had named Baade as the single, first, and only candidate. Schorr had then stated that Baade was “nationally disposed” (a loyal German), not a member of any political party, and of “Aryan origin,” along with his wife. He sent Baade the necessary forms on which to list his and his wife’s ancestry and attest to this statement, and emphasized that they should be sent back directly to him, along with their birth certificates, marriage certificate, and their parents’ marriage certificates (all of which would state their religion). That was part of the process for being named to any academic post in Nazi Germany. Schorr confided that Berlin might refuse to accept a single name, and that the Hamburg faculty had so far been unable to agree on alternate candidates for this eventuality. The next day Schorr sent Baade a shorter, more formal letter, stating the faculty requested his advice, as a former member, on the other potential candidates for the directorship. Of the four whose names had been suggested, Baade said he could not judge his friend Johannes Hellerich, the only one who was at Hamburg, praised Otto Heckmann strongly, as by far the best of the younger German astronomers, and was much less enthusiastic about the other two. In another letter, he privately told Schorr that he had arranged for the Carnegie Institution to bring Minkowski to Pasadena, and that he could not leave himself for at least a year and a half, and that after two years would be better. As the reality of leaving the land of clear skies and big telescopes came nearer, he was beginning to draw back. But he enclosed the documents for which Schorr had asked, and wrote that

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their relatives in Germany would deliver the rest to him there. He was willing to play the Nazis’ game, as every astronomer still in Germany was.53 In his correspondence with Schorr, except for the one private letter, Baade barely mentioned Minkowski and gave no hint that he himself had been the impetus in making it possible for him to come to Mount Wilson, preserving the fiction that it would be a one-year visit. In contrast, Baade severely criticized Hans Rosenberg, an elderly pioneer of photoelectric photometry and a refugee from Hitlerism, who had spent some two years at Yerkes Observatory, also supported by the Rockefeller Foundation and the Emergency Committee. He had not produced any scientific results, and failed to land a permanent job in America, finally ending up as director of the Istanbul Observatory in Turkey in 1938. Baade never mentioned that Rosenberg (who had served as an officer at the front in the German army in World War I) was Jewish (as he and Schorr both well knew), but was critical of his sparse research results.54 In this same letter, Baade sent Schorr the good news that he would be able to visit Germany over Christmas and January 1936. Adams was taking him to the annual Carnegie Institution board meeting in Washington in December to describe his latest results to its president and board members. As a reward he would get a three-month leave, and since he would pay for his own passage on the ship, there would be no bar to his resigning later, if he so decided. Schorr notified the Ministry of Science and Education that Baade, the single candidate named by the faculty for the Hamburg directorship, would be in Germany and available for an interview, but the Nazi bureaucrats ignored it and sent a letter to Baade in Pasadena (which he received only after his return) requesting him to send all his papers proving that he and his wife were Aryans. They made no inquiry into his scientific accomplishments. Baade went to Germany, visited his mother and brother in Minden, gave a lecture at the university on “Spiral Nebulae as Star Systems,” and returned to California in March without ever being interviewed.55 Schorr continued pushing for Baade’s appointment, and in September 1936 reported that the dean of the Hamburg faculty thought that “under present circumstances” it would be “good” if Baade

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Walter Baade (left) and Johannes Hellerich in Hamburg, 1936. (Courtesy of Hamburg Observatory.)

joined the Nazi party immediately. Baade’s reply was masterly. He wrote that he could not take such an important step on opportunistic grounds; if he did, it would have no moral value. But if he came back to Hamburg as director, he would show, he believed, that he could work together with the regime. Even before Baade replied, Schorr secretly advised him not to take the “suggestion” too seriously, as only two professors in the entire Hamburg faculty of mathematical sciences were then party members.56 Baade, although he did not go directly to Adams, naturally let his friends on the Mount Wilson staff know of the offer which Schorr, at least, was sure he would soon be getting. They wanted him to stay at Mount Wilson, and warned the director of the danger of his leaving. Adams badly wanted to keep Baade, and talked seriously with him. By then Baade’s salary had risen to $3,600, but he expected it would become the equivalent of $7,500 at Hamburg. Adams recommended him for a $500 raise for the next year, writing to Merriam, the Carnegie Institution president, that Baade “would be quite capable of doing the work which Hubble has done equally well, if not better.” The Mount Wilson director also asked for funds

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to allow Baade to travel to Germany more frequently, which he said was his main desire. Merriam authorized the raise, but added a warning that although “the striking prosperity in Germany, the unity of the people, and the great military strength which is now being developed are appearances which deceive many into thinking that the future of Germany is secure . . . ,” he “ha[d] the feeling that Germany may be in a more dangerous situation today than at any time in the past half century. I would not wish to see any friend of mine deliberately choose to locate himself for life in Germany at this time.” Adams passed the warning on to Baade, who realized “that the situation is a very uncertain one, and may change radically in a very short time,” but said that many considerations would enter into his final decision. Adams had strong hopes that Baade would stay at Mount Wilson, but realized he might not.57 More practically, the Mount Wilson director had Humason, Baade’s closest friend on the staff and at the same time Adams’s trusted confidante, take his colleague for a “ride in the country” in his car, and tell him all the wonders that might be. Adams was committed to remaining as director until the completion of the 200inch telescope, then forecast for 1941, when he would reach retirement age. Everyone, including Humason and Baade, expected that Hubble would then become director of Palomar. Seares, the Mount Wilson assistant director, was slated to retire before that, and if Baade would take out his first papers to become a U.S. citizen, Humason said, Adams would have him appointed to that post, and he would be in line for the directorship himself three years later. The two lectures Baade had given in Washington had convinced Merriam and the Carnegie board of directors that Adams was right, and that the German researcher (if he became an American citizen) was just the man for the job. Later Adams, “a reserved New Englander,” himself called Baade in and confirmed the essentials of this unofficial offer.58 Baade had no intention of becoming a naturalized American then or later; he always intended to return to Germany some day, as he did fifteen years after Hitler committed suicide in his bunker, his Third Reich in ruins, swept away by the victorious Russian, American, and British armies and air armadas. But in 1937 the German

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astronomer realized clearly just how badly Adams wanted to keep him at Mount Wilson. Nevertheless, Baade felt a strong emotional attraction to his homeland. In January and again in March he cabled Schorr that he would accept the Hamburg directorship if the government would build an 80-cm (32-inch) Schmidt telescope for him and provide adequate funds to operate it effectively. With it, he said, he could take the lead away from the Americans, and show that the land where the Schmidt camera was born could forge ahead of them without spending millions. As Baade reported, the Mount Wilson astronomers, led by Hubble (he carefully wrote, although he himself was a full participant in the project) were planning a large Schmidt themselves, which would certainly be built at Palomar. Baade was genuinely attracted by the idea of going back to Hamburg and having his own telescope there.59 Finally in late June the Hamburg authorities, acting on the advice of the German government, formally offered Baade the directorship. He immediately took the letter to Adams. The canny Mount Wilson director was expecting it and had a plan ready. Baade’s salary had just been raised to $4,000 on January 1; although the Hamburg offer was for the equivalent of $7,500, he agreed that he would stay at Mount Wilson if it were raised to $6,000 a year. To find the necessary additional money, Adams shifted the salary of Sinclair Smith, a young Mount Wilson staff member who was assigned to work almost entirely on the Palomar project, from half-time at each institution to full-time on the Rockefeller funds. This freed $2,000 a year for Baade’s raise from the Carnegie Institution money. Mason, the former Rockefeller Foundation president now in place at Caltech in charge of completing the 200-inch, agreed readily by telegram from his vacation in Rhode Island, while Merriam, who was at Yosemite en route to another vacation spot in northern California, raised a few petty objections but soon agreed to the deal too, under Adams’s careful handling. The ascetic Yankee was the only one of the three actually at work that summer.60 Baade cabled Schorr, “With heavy heart I have decided for Mount Wilson, hoping the homeland will understand”; the old director could only reply “Schade, Schade” (“What a shame!”).61 In a consoling letter to Schorr, Adams diplomatically spelled out the reason

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Baade had finally decided to stay. At Mount Wilson he already had the 100-inch telescope, with the 18-inch Schmidt at Palomar; without a doubt he would soon have the 200-inch and the 48-inch Schmidt, all in the fine observing climate of California. In Germany he had been promised a 32-inch Schmidt in a climate he knew was much poorer, and although Adams did not mention it, if war came (as it did, two years later), Baade might never use the telescope, even if it was built. The attraction of his science had won out over the tug of his homeland.62 Baade himself wrote a long, unconvincing letter of explanation to the Hamburg Education Board which had formally offered him the directorship, trying to explain his decision. He claimed it would have been easier for him to return to Germany, but that he was staying in Pasadena “in the German interest,” to make sure his native land was represented on the staff of the 200-inch. According to him, this had been the desire of “the official German side,” probably meaning that one of his friends among the officials at the consulate in Los Angeles had urged him to stay. Baade’s chief aim in writing this letter was to keep the plan for the Hamburg 32-inch Schmidt alive for his colleagues, and he apparently signed it under the required official closing, “Heil Hitler!” Baade never was a Nazi, but like Otto Heckmann (eventually appointed Hamburg director in January 1942) and every other German astronomer who stayed in the Third Reich, he realized that he would have to follow their forms to be heard and survive. In a long, emotional letter to Schorr, Baade wrote that his time at Hamburg had been the most beautiful years of his life, that he had not sold out for American dollars, and that he would work with the next director, whoever he might be, to make the 32-inch a reality (as he did after World War II). Just a few days later, Baade and Muschi joined in an even more heartfelt message of congratulation to Schorr on his seventieth birthday, as “we two married Bergedorfers in distant Pasadena.” Baade could be extremely sentimental in German, but equally realistic in his decisions. Apparently he never again considered leaving the big California telescopes until after his retirement. By the spring of 1938 he could see tremendous progress on the 200-inch; the dome was nearly finished “like a gigantic cathedral,” and the mounting was

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approaching completion at the steelworks in Philadelphia. Construction of the Palomar 48-inch Schmidt telescope was to begin that summer. He received an informal offer of the directorship of the Potsdam Observatory, and turned it down without hesitation. Baade knew his earlier decision had been the right one, although his heart was with the German troops invading Austria.63

Photography with Red Light In the 1930s all the big photographic companies, like Agfa in Germany and Eastman Kodak in the United States, were developing and improving their orthochromatic (green- and yellow-sensitive) and panchromatic (red-sensitive) films, and plates as well. Baade kept in close touch with these developments, and with the prestige of Mount Wilson Observatory behind him often succeeded in getting some of the most sensitive emulsions, coated on glass, directly from the manufacturers’ laboratories. They were tricky, requiring hypersensitization with just the right combinations of water, ammonia, and alcohol immediately before use, and all too likely to fog on the slightest provocation, but his careful, experimental approach made him an expert with them. Once he received a shipment of especially good plates, Baade hoarded them carefully, using them for the most important problems he was working on and never wasting them.64 He also used panchromatic film (and a red filter) with the 18-inch Schmidt telescope at Palomar to survey a large region around the galactic center. His approach paid off. The great advantage of red light is its space penetration; interstellar dust absorbs and scatters blue light much more effectively than red. Hence Baade’s direct photographs penetrated the nearby dust clouds in the galactic plane, and revealed the gaseous nebulae and globular clusters behind them. The nebulae themselves emitted the red Hα λ6563 emission line especially strongly, making that wavelength region especially well suited for detecting them and photographing their structure. With these panchromatic plates, especially an experimental emulsion called “Hα Special” which C. E. Kenneth Mees, the head of the Eastman Kodak Research Laboratory, sent him, Baade found that

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he could record many more stars in globular clusters near the galactic center, and outside the clusters in the field itself. He systematically covered this whole region with long-exposure photographs, and discovered the least heavily obscured region at galactic longitude 329° (in the coordinates of that time, since relabeled 1°), galactic longitude −4°, now known as “Baade’s window.” On the other hand, in the region about the globular cluster NGC 6553, which he described as “situated close to the center of our [G]alaxy” his plates showed graphically the contrast between the few stars visible in the blue, and the many more in the red, resulting from strong extinction by dust. Baade announced these new results in a paper read for him at the meeting of the American Astronomical Society held at Williams College in Massachusetts in September 1937. He did not go himself (September is the best observing month at Mount Wilson), but sent slides which Duncan, the astronomy professor at Wellesley College and a frequent visitor to Mount Wilson as a guest observer, showed as he described these results. It was at this meeting that Baade first became a member of the society, six years after he had joined the Mount Wilson staff.65 Then, in the summer of 1938, Baade was able to return to Europe, to attend the triennial General Assembly of the International Astronomical Union held at Stockholm that August. Now enjoying a higher salary, he could afford to take his wife with him. Germany had still not adhered to the union, founded by the victors of World War I soon after its close, but Baade was invited as a “consultant” from America, and eight other German astronomers attended as “guests.” Baade and Muschi spent a month in Germany before going on to Sweden; he visited his mother and brother in Minden for two weeks, then joined his wife in Hamburg where she had been visiting her family. He met Schorr at the observatory in nearby Bergedorf, and had several talks with him and his other old friends there. Then he went on to Sweden and the week-long General Assembly. There at a large “Discussion of Galactic Structure” he gave an invited talk on his work with red-sensitive plates at Mount Wilson. This time he showed the slides and did the talking (in English—it and French were the two official languages of the union) himself.66 Undoubtedly Baade discussed this and many

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Walter Baade (left) and Jan H. Oort on an excursion boat at the Stockholm IAU General Assembly, 1938. (Courtesy of Leiden Observatory.)

other aspects of his research with Jan H. Oort, the brilliant younger Dutch theorist with whom he was to begin collaborating closely a decade later. After the General Assembly Baade and Muschi returned to Germany and continued their vacation, “visiting old friends and talking politics.” “Politics” meant the crisis over Czechoslovakia. Hitler was determined to take it over, as he had Austria in the spring, and was willing to go to the brink of war to do so. In September the tension reached new heights. War seemed about to break out. Baade abandoned his plans to attend a big German scientific meeting and to visit the Zeiss works in Jena, to discuss glass disks for a corrector

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plate for the planned 48-inch Schmidt. Instead he and his wife departed two weeks early on the Europa just days before British prime minister Neville Chamberlain flew to Munich and handed Czechoslovakia over to Hitler’s tender mercies in exchange for “peace in our time,” which was to last for less than a year.67 Shapley, who had been at Stockholm, had been impressed by Baade’s work on galactic structure with red-sensitive plates, as everyone was who had seen the slides. He pumped Baade for advice, and turned it over to his underlings in South Africa and in Cambridge, but they were not nearly as successful in their photographic efforts as the master observer himself.68 A few years later those panchromatic emulsions, and the direct photographs he took with them, were to play a key role in Baade’s recognition of the two stellar populations.

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War and a Great Discovery MOUNT WILSON, 1939–1947

McDonald Observatory Dedication Soon after his return to Pasadena from Stockholm and Germany, Walter Baade received an invitation from Otto Struve, director of Yerkes Observatory, to take part in a symposium at the dedication of McDonald Observatory, to be held on its site in the mountains of west Texas the following year. Its 82-inch reflecting telescope, built under the auspices of the University of Texas thanks to a generous small-town banker who had amassed a fortune before he died, would be the second-largest telescope in the world when completed. Struve had brought about a cooperative arrangement under which he and the other Yerkes astronomers would operate McDonald Observatory, for Texas had no observing astronomers and very few research scientists of any kind. He had consulted several astronomers who were experienced in using large reflectors, including Baade, before he finalized the plans for the telescope.1 For the dedication Struve was bringing many of the top American astronomers to isolated Mount Locke for a symposium on current astrophysical research, then devoted mostly to stars, but including papers on galaxies by Edwin Hubble and Baade. Struve had hoped to bring several outstanding European astronomers as well, but Hitler’s continuing threats of war kept most of them at home. Baade gladly agreed to take part in the dedication, which he and Struve had discussed in Stockholm. They both wanted to make it possible for Albrecht Unso¨ld, the rising young Kiel astrophysicist who had come to Hamburg to work with Baade in 1930, to combine a trip to America for the dedication with a stay to use the big telescopes at Mount Wilson and McDonald Observatories. However, Walter S. Adams, the Mount Wilson director, was distinctly cool to

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this idea. He, most of the members of his staff, and all the Carnegie Institution of Washington officials and trustees were strongly antiHitler and pro-British by this time. Adams accepted Baade as a great astronomer, but he had no intention of making Carnegie funds or Mount Wilson telescopes available to a visitor from Germany. Thus Unso¨ ld came to America, attended the McDonald dedication and observed there with Struve, but did not get to Pasadena or Mount Wilson Observatory. The only other Europeans who took part in the Texas symposium were Edward A. Milne, the theoretical astrophysicist from England, and Jan H. Oort, the galactic structure and dynamics expert from Leiden.2 Baade’s talk at the McDonald dedication, delivered to an audience of some fifty or sixty astronomers, many of them the research leaders of that generation, including Harlow Shapley, Adams, Struve, and William H. Wright, the director of Lick Observatory, and others to be leaders of the next, was a combination of a detailed description of his photometric methods and an application of them to a specific galaxy. First he gave a long review of all the problems of photographic photometry, his ways for overcoming them, and the checks and comparisons he applied to be sure he had overcome them. He emphasized the difficulties in trying to measure the stars’ brightnesses down to magnitude 21, but explained his painstaking procedures for reducing the errors to the order of ±0.07 magnitude for a single plate over the whole range, and only ±0.03 near the faint limit. The application was to IC 1613, the dwarf galaxy within the Local Group which Baade himself had identified as a star system like the Magellanic Clouds at Hamburg a decade earlier. As soon as he joined the Mount Wilson staff he began observing it with the 100inch telescope, taking long-exposure photographs and searching them for variables. Baade had set up his magnitude scale in SA 68, midway between IC 1613 and NGC 6822, the similar dwarf galaxy which Hubble had studied even before M 31. One of the important checks Baade mentioned in his talk was the photoelectric measurement of the magnitudes of several of the stars in SA 68, which he had arranged for Joel Stebbins and Albert E. Whitford to make at the 60-inch reflector on one of their visits to Mount Wilson.

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Then, with the magnitude scale set up in SA 68, Baade transferred it to local standards in and near IC 1613, and measured the varying brightnesses of the Cepheid variables he had discovered on his long series of 100-inch plates, dating back to 1932. Thus he was able to derive and plot the period-luminosity relationship for the variables in this dwarf galaxy with periods ranging from 3 to 146 days. In his presentation he showed a slide of this plot, and compared it with published plots for the Cepheid variables in the Large and Small Magellanic Clouds (from Harvard), M 31 and NGC 6822. This comparison showed that his errors of measurement were smaller than in any previously published period-luminosity relation. The distance he derived for IC 1613 was practically the same as that of M 31, but the former was only one five-hundredth as luminous as the giant spiral. IC 1613 was a true dwarf galaxy. But it contained stars just like those in M 31, not only Cepheid variables but high-luminosity blue stars and diffuse emission nebulae, he emphasized. He was close to the population concept which he was to publish five years later.3 After the McDonald symposium Oort went on to Pasadena, where he took part in a conference on the structure of “nebulae” (galaxies), especially organized to take advantage of his and Swedish astronomer Bertil Lindblad’s presence in America that summer. The direction of rotation of spiral galaxies (arms trailing or leading), on which Hubble and Lindblad had opposite opinions, was the main topic of discussion, but Baade and Oort were both more interested in the analogies between our Galaxy and other spirals. This conference, which Nicholas U. Mayall also attended, gave Baade and Oort the chance to get to know each other better and to respect one another. They were to become the two world leaders in galaxy research after World War II.4

Paris Conference Soon after the McDonald Observatory dedication, Baade traveled to Europe again for his second visit in two years. This time he went to participate in a select conference on novae, supernovae, and white dwarfs, held in Paris in July 1939. Sponsored by the

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wealthy Princess de Polignac, attendance was by invitation only, with all expenses paid from the funds she provided. Baade naturally planned a stay in Germany after the week-long conference to visit his mother again, and to see the Hamburg astronomers. His wife did not accompany him; her travel would have been at their expense. There were only fifteen participants at the conference, headed by Arthur S. Eddington, Henry Norris Russell, Knut Lundmark, and F.J.M. Stratton, a longtime nova observational spectroscopist of the older generation. But most were the younger, upcoming theorists and observers, including Bengt Stro¨ mgren, Gerard P. Kuiper, Subrahmanyan Chandrasekhar, Cecilia Payne-Gaposchkin, Bengt Edle´ n, and Pol Swings. Only one French astronomer, Henri Mineur, was a participant, although several others were present and took part in the discussion. No other Germans but Baade were there. Walter Grotrian had originally been scheduled but the rising tension between Germany and France kept him at home. They had sessions every day for a week, sitting around one big table in a room at the Colle`ge de France, with one or two talks each morning, and another in the afternoon, each followed by general discussion of the topic. Russell, the chairman of the conference, gave a general introduction on Monday morning, and Eddington closed it on Saturday morning with his version of the theory of white dwarfs, which unfortunately was wrong, because he could not accept the upper limit to the mass of a white dwarf which Chandrasekhar had derived using the correct theory. Baade was at his best in this type of conference, a small group of top scientists discussing their results and theories in an informal setting.5 His talk was on Thursday morning, on supernovae. In it Baade emphasized that all the observational data which had been obtained since his and Fritz Zwicky’s first paper fully confirmed the uniqueness of supernovae—that they were quite different from ordinary novae. He gave full credit to Zwicky for his work with the 18-inch Palomar Schmidt. It had led to the discovery of nine supernovae, all followed up by Baade with the big telescopes at Mount Wilson. As a result there was a coherent observational picture of the frequency of supernovae (roughly one per “typical” spiral gal-

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Group at conference on novae, supernovae, and white dwarfs, Paris 1939. Left to right, front row: F.J.M. Stratton, Cecilia Payne-Gaposchkin, Henry Norris Russell, Amos J. Shaler (organizer of the conference), Arthur S. Eddington, Sergei Gaposchkin; rear row: Carleton S. Beals, Bengt Edle´ n, Pol Swings, Gerard P. Kuiper, Bengt Stro¨ mgren, Bertil Lindblad (in front), Subrahmanyan Chandrasekhar (behind), Walter Baade. (Courtesy of Yerkes Observatory.)

axy per six hundred years, from the preliminary data), their light curves, their average absolute magnitude at maximum light (Mpg = −14.3 ± 0.3 with a dispersion of 1.2 mag.), and their energy release. But their spectra, which showed a continuum with very broad, unidentified emission bands, had not yielded to analysis, so the physical nature of the supernovae remained unknown. Zwicky would continue the supernova search to the end of 1939, Baade said, and then rediscuss all the data, which would be much more complete. As a result, the parameters of supernova magnitudes and light curves would be more accurately known, but the fundamental problem of their nature would remain. Baade care-

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fully calculated the total energy released in each of the three supernovae which had been caught before maximum light, deriving numerical values comparable with the total “heat content” (potential, thermal, and ionization energy), indicating a “radical change in the constitution of the star,” he said. However, by itself the result could not confirm or deny the idea that a supernova represented the collapse of a gaseous star to a “degenerate state” (a white dwarf) as Milne and Chandrasekhar had suggested. Baade did not even mention Zwicky’s idea that a significant amount of rest mass disappeared in a supernova outburst; all his own estimates had been on the conservative side, while Zwicky’s had been in the opposite direction. At the end of his talk, Baade described the work he had begun on the Crab nebula, and how it might help to solve some of these riddles in the near future. Baade’s paper was a masterly observational summary of the supernova phenomenon as it was known then, largely as the result of his, Rudolph Minkowski’s, and Zwicky’s efforts.6 After the conference, Baade hurried to Minden. His mother was in failing health, and he knew each visit might be his last chance to see her. He wrote to Richard Schorr at Bergedorf to arrange to meet him there. However his former director was going to a meeting of the German Astronomical Society at Danzig, the “free city” which Hitler was demanding be “returned” to the Reich. Schorr urged Baade to come to the meeting of the society to which he had belonged for years, and discuss astronomy with his friends who would be there. Holding the meeting in Danzig was a provocative act; Baade knew that no English or American astronomer would attend, as some always had in previous years. He replied that he had to stay in Minden to take part in his aunt and uncle’s golden anniversary celebration; “they would kick me out of the family if I weren’t there for it,” he replied to Schorr in his picturesque style. Baade always could find a “reason” for avoiding a public stand that would get him in trouble in America or Germany. In a long letter to Schorr, he did not mention that he had in fact dropped out of the German Astronomical Society soon after joining the Mount Wilson staff. Baade did express his strong support for Otto Heckmann for the Hamburg directorship, as a true scientist, and his opposition to

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Johannes Hellerich, the Nazi education officials’ candidate for the post. It would be a catastrophe if Hellerich succeeded to the directorship, Baade wrote; there would be no fresh initiatives, and only lethargy at the observatory. Schorr agreed fully with Baade, and reassured him that the Hamburg faculty felt the same way; they would block Hellerich’s appointment. The two astronomers planned to meet at Bergedorf in late August, after Schorr’s return from Danzig. However, as the crisis escalated Baade, who had earlier thought there would be no war, now decided to get out of Europe while he still could. He moved up his departure date, hastened back to France, and managed to get to Le Havre and board the Ile de France before the French mobilized their army and closed their borders. An Austrian biology professor at Stanford whom Baade knew did not move quite fast enough; he was trapped inside France and “landed in a concentration camp.” The war began on September 1, two weeks after Baade sailed. Home safe in Pasadena, Baade inquired anxiously about Schorr’s three sons, who were all of military age, and railed against “the holy democracy” (America) and President Franklin D. Roosevelt’s policy of “cash-and-carry” neutrality. He made it very clear to Schorr, if not to Adams, that his sympathies lay with the German people. But Baade was never to see his former director again, nor his own mother.7

Nova and Supernova Shells The Paris conference and preparing his paper for it revitalized Baade’s interest in supernovae. He was convinced that the Crab nebula was a supernova remnant; hence examining closely every peculiar nebula might turn up another, previously unknown remnant. Baade’s skills with the telescope enabled him to obtain excellent direct images, showing the finest detail the atmospheric conditions would allow. Mount Wilson frequently has superb seeing, and he made the most of it. He had quickly grasped the fact that the redsensitive plates with a sharply defined red filter isolated a narrow spectral region around the strong nebular emission lines Hα, [N II] λλ 6548, 6583, ideal for photographing these objects.

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One of the objects to which he early applied this method was “Campbell’s hydrogen envelope star,” a nearly stellar object with strong emission lines. Heber D. Curtis, from his plates and spectra taken with the 36-inch Crossley reflector, had thought in 1918 that it was a tiny planetary nebula, but most astronomers called it a nebulous star, and it was included in both the Bonner Durchmusterung and the Henry Draper star catalogues. In 1940 Baade obtained direct photographs with the 100-inch reflector in fine seeing which clearly showed the object is a ring about 5″ in diameter, a small version of the much larger, more famous Ring nebula in Lyra, NGC 6720. The narrow wavelength band he used had suppressed the light of the star and kept its overexposed image from hiding the ring. Now there was no doubt it was a planetary nebula. Probably Baade was disappointed that it was not more abnormal, potentially a supernova, but he sent copies of his photographs to Struve and Wright. They had both worked on the “hydrogen envelope star,” and were suitably impressed. Baade never bothered to publish his result.8 Another, more interesting object was the shell around Nova Herculis 1934. A tiny nebulosity had first been glimpsed (by skilled observers with large telescopes) in 1938, and Baade succeeded in photographing it in Hα, [N II], and also in the green [O III] lines (using a yellow filter and the natural blue photographic plate sensitivity to isolate a narrow spectral region around them). The little nebula was ellipsoidal rather than circular in the sky; evidently the shell was elongated. In August 1940, when Baade obtained these images, the dimensions of the shell were 2.7″ < 3.5.″ Earlier, observing it visually at Yerkes Observatory, Kuiper had thought the nova had split and become a double star. As Baade realized from his direct plates, Kuiper had seen the two ends of the shell and interpreted them as two components of a binary; the position he had measured and reported was in fact the position of the major axis of the little nebula. The self-confident Kuiper found it hard to conceive that he could have been mistaken, but as the nebula expanded and became fainter, by 1942 he clearly saw the shell and admitted to Baade that “[y]our description is certainly closer than my own.” The Mount Wilson astronomer was always sympathetic, putting

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the best possible construction he could on earlier, less accurate observational results. He never gloated or boasted, and except for Zwicky, a notoriously difficult person, never made an enemy.9 Baade did publish his results for Nova Herculis in two short papers, including the distance he derived to it by comparing the angular expansion of the shell with the spectroscopically measured radial expansion velocity. It was a “normal” nova, as he knew it would be, with absolute magnitude at maximum light Mpg ≈ −6.5. His direct plates showed a different distribution of [N II] emission over the shell from [O III], indicating that the ionization conditions, like the shape, were not circularly symmetric, but cylindrically.10 The most interesting object for Baade, and the one for which his direct exposures in various wavelength regions showed the most structure, was the Crab nebula. He knew it was a supernova remnant, and had encouraged Mayall to publish his results on it. When Oort had come to America for the McDonald dedication in 1939, and afterward to Mount Wilson to work with the telescope and discuss research, Baade learned that he too was extremely interested in nova shells and in this supernova remnant. Oort and Mayall were still not quite as certain as Baade was that it was one. They were concerned about uncertainties in the Chinese accounts of just where in Taurus the “guest star” had appeared, and the difference between Mayall’s measured radial velocity of expansion (1300 km/ sec) and the widths of the broad emission lines in observed supernovae (given as 6000 km/sec in some early papers). Baade encouraged them to go on trying to learn more from the early records, but reasoned that the Crab nebula was such a unique object that it must be the supernova remnant.11 After he got back to Holland in early September 1939, just a few days after the outbreak of World War II, Oort encouraged two of his fellow Leiden faculty members, professors of Chinese and Japanese respectively, to dig further into the old writings, and tried to look into Arabic, Jewish, and Byzantine sources himself. The mails between America and the continent of Europe were often delayed for months, and after the Germans occupied Holland in April 1940, Oort knew that it might be completely cut off at any time. He suggested that Mayall publish the Chinese and Japanese translations

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in the United States. The young astronomer discussed the idea with Wright and the other senior astronomers at Lick Observatory and with Baade; they all agreed he should publish what he could in a joint paper with Oort. The Dutch astronomer agreed and finally in June 1941 sent the completed manuscript of a paper by J.J.L. Duyvendak, the scholar of ancient Chinese, containing the relevant translations and their interpretation. It reached California in early August, and all the astronomers who read it now agreed that the Crab nebula was certainly the supernova remnant. Mayall wrote a companion paper, analyzing the astronomical evidence from Duyvendak’s translations as well as the corroborative material from Japanese records. He sent a copy of the manuscript, naming Oort and himself as coauthors, to Holland, but it never got through. The Leiden astronomer had authorized him to publish their paper if he did not hear from him, and Mayall did so in 1942. Oort never saw it until after the Germans had been driven out of Holland and the war had ended; he was very pleased with it. Duyvendak was still alive too, and was pleased with his paper also; a true scholar, he nevertheless immediately sent Mayall a list of corrections which the latter had not caught in the proofs.12 Baade was holding up his own paper on the Crab nebula, waiting until after he knew Duyvendak, Oort, and Mayall’s work would be published. He was particularly enthusiastic that Duyvendak’s translations revealed to him for the first time how very bright the supernova in A.D.1054 had been. The Chinese had compared it with various planets and bright stars, making possible a reconstruction of its light curve. From this, Baade classified the supernova of nine hundred years ago as type I. His first direct photographs of the Crab nebula with narrow-band red filters revealed clearly its true structure, a network of filaments emitting Hα, [N II], [O III], and other typical nebular forbidden lines, surrounding and interpenetrating a diffuse, amorphous nebulous region which emitted only a continuous spectrum, with no emission lines. Baade had displayed his first versions of these photographs, showing the structures graphically, at the Paris symposium in 1939, but its proceedings were not published until 1941. In the interim he had published them in Die Himmelswelt, a German popular astronomy magazine,

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in 1940. Probably he had left the prints with a friend before his hurried departure for America, just before the war broke out in Europe the previous summer.13 Minkowski obtained spectra of the Crab nebula with the slit of the fast spectrograph on the 100-inch placed across some of its brightest filaments; they confirmed Baade’s statements beautifully. He and Minkowski searched for an “exciting star,” which they thought must be emitting high-energy ultraviolet photons which ionized the nebula. There was a close pair of sixteenth-magnitude stars near the center of the Crab nebula, but Baade found one too red, and Minkowski confirmed that it was an F or G star, too cool to be a candidate. The other was somewhat bluer, and on Minkowski’s very low-dispersion, faintly exposed spectrogram showed no absorption lines (meaning that any actually present must be very weak), indicating it could be a hot star. Minkowski tried a spectrophotometric approach, in which he derived the “color temperature” of the diffuse, amorphous nebulosity as approximately 36,000 K. The difficulty was that it should have shown an easily observable Balmer discontinuity, but did not. Minkowski tried to explain this away and thought he could by stretching all the observational errors and unknown interstellar reddening to their maximum values. He ended up with the blue star as an object with a fantastically high temperature, T = 500,000 K, and a correspondingly small radius, R = 0.02 R(. This fitted well with the idea which Chandrasekhar had put forward, that a supernova outburst represented the collapse of a “normal” star to a white dwarf.14 However, this picture left completely unexplained the absence of any emission lines from the diffuse nebulosity, and the source of ionization and excitation of the gas in the filaments. Baade was rightly skeptical about this analysis but discussed fully the published proper-motion measurements of the expansion, which he now clearly identified with the filamentary system. The 120-year discrepancy between when the Chinese observed the bright supernova and the year when the present rate of expansion, projected backward as constant, indicated the remnant would first have appeared, Baade stated, must have resulted from an initial rapid acceleration, followed by coasting at constant velocity. He fully con-

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firmed Mayall’s value for the distance to the Crab nebula and the supernova’s magnitude at maximum light. The older German was generous in his praise of the young Lick spectroscopist, and continued to suggest further work he could do on the Crab nebula and other supernova remnants.15 Baade actively continued his investigations of historic supernovae. He put his own knowledge of Latin, Italian, and German to use to comb through old records of the observations of Tycho Brahe’s bright “nova” of 1572 and Johannes Kepler’s of 1604, confirming his earlier published statements that they both were supernovae and reconstructing their light curves. From these he classified them both as type I supernovae. At the position of Kepler’s supernova he found, with very long narrow-band red exposures, wisps of nebulosity which he identified as its gaseous remnant. It was in a very heavily reddened region in Ophiuchus and the nebulosity did not show at all in the blue. Minkowski obtained two spectrograms, each a four-night exposure totaling sixteen hours, which showed Hα, [N II], [S II], [O I], and [O III], quite similar to the line spectrum of the Crab nebula filaments and confirming the identification. Baade also searched very hard for a remnant of Tycho’s supernova, but did not find one. From the light curve he was convinced that a remnant must be present, just a little too faint to detect with the 60-inch, which he had to use since this object is at declination +64°, too far north for the 100-inch to reach. At the end of his paper Baade concluded that there was every reason to hope that a remnant would be found when the 200-inch reflector went into operation.16 Baade also sought the remnant of the first supernova which Curtis and Lundmark had recognized back when he was a student himself, S Andromedae, the “bright nova” of 1885 in M 31. He knew just the right place to search for the remnant because the supernova’s position was very accurately known; it had flared up near the nucleus of the galaxy and several visual double-star observers of that era had measured the position angle and separation of this temporary “pair.” Baade had tried to find the remnant on ordinary blue-sensitive plates he took with the 100-inch reflector in the late 1930s, hoping that some nebular emission might be bright

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enough to be detected against the bright background of stars in the central bulge of M 31, but his search was unsuccessful. Just a few years later he realized that interstellar extinction might be very heavy there, and he intended to try again on red-sensitive plates in the fall of 1944, but he had not done so by 1946. Almost certainly he did try again later, for he took many direct plates of the central region of M 31 with various filter and plate combinations, but if so he never found the remnant and never published the facts of his search.17 Indeed, just a few years later, Baade and Minkowski were to begin observing supernova remnants with the 200-inch telescope and to become the world leaders in identifying newly discovered radio sources with supernova remnants. They had the jump on everyone, not only in telescope aperture but in observing skills and knowledge of supernovae and their remnants. Though their idea about photoionization of the remnant nebulosity by the remnant blue star was wrong, they were to grasp the new concepts of synchrotron radiation quickly, and Baade and Oort were to confirm them in the Crab nebula.

Sculptor and Fornax Dwarf Galaxies Baade had long been interested in dwarf galaxies. His aim was to learn all the properties of all the types of star systems in the universe, rather than simply assuming that their absolute magnitudes had a relatively small dispersion about a well-defined mean, as Hubble did. At Hamburg, Baade had thoroughly investigated IC 1613, which he proved was a dwarf irregular galaxy of the same general type as the Large Magellanic Cloud and NGC 6822. Since these dwarfs by definition have relatively faint absolute magnitudes, they are difficult to find at large distances. Encouraged by Baade, Zwicky had begun inspecting all the films he took with the 18-inch Schmidt for dwarf-galaxy candidates. Soon he started taking high-latitude fields in a systematic search for such objects. The arrangement was that Baade would use the 100-inch to investigate the candidates Zwicky found. By 1940 when Hubble wrote a semitechnical article on galaxies, Baade had checked six of the best can-

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didates, and three of them had turned out to be dwarf irregular galaxies. Unfortunately, although Hubble described the arrangement correctly in the text, in the captions of the photographs he called one of them “Baade’s system in Sextans,” and another “Baade’s system in Leo.” This infuriated Zwicky. He demanded, and got, a handsome retraction. This incident, together no doubt with the fact that Baade had been an invited star of the Paris colloquium on supernovae, while he himself had been excluded from it, brought the growing tension between the two into the open. Zwicky, a beginner in astronomy who had not been very successful in any branch of physics he had tried earlier, was highly insecure. Hungry for recognition, he struck out against the “high priests and sycophants” who had achieved it. Baade was rapidly becoming a prime target; Zwicky, an independent, strongly anti-Hitler Swiss, could not hurt the dictator but seemed to regard Baade as a convenient proxy for him. They never collaborated after 1940, and Zwicky afterward constantly criticized Baade, as well as Hubble, Minkowski, the “high pope of American astronomy Henry Norris Russell,” Allan Sandage, Maarten Schmidt, and many other famous astronomers.18 Because of his interest in globular clusters, dwarf galaxies, and related objects, Baade undoubtedly read two papers by Shapley, published in 1938, with keen attention. Both described stellar systems “of a new type,” discovered on plates taken with telescopes at Harvard’s Boyden Station in the Southern Hemisphere and shipped to Cambridge for Shapley and his assistants to inspect. He called the first “stellar system,” discovered in the constellation Sculptor, a “large rich cluster” of stars, quite extended, with its brightest stars about nineteenth magnitude, numbering roughly a thousand, many more only slightly fainter than that. On the one hand the Sculptor system had no emission nebulae, no supergiant stars, no “open [galactic] clusters,” but in form and size it was like the Magellanic Clouds. On the other hand the luminosity function of the stars it contained resembled that of a globular cluster, with brightest stars Mpg ≈ −1.5 (Shapley had no red or yellow plates of it, from which to measure color indices), but its size and low surface brightness were quite unlike those of any globular cluster he knew.

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The second system, in Fornax, was similar, but the brightest stars were even fainter, indicating it was more distant than the Sculptor “cluster.” He planned to have more plates taken with the 60-inch reflector at the Boyden Station to look for variable stars, Shapley wrote. Both these systems, he stated, had some properties in common with globular clusters, some with elliptical galaxies, and still others with the Magellanic Clouds. At the end of his paper he mentioned that the much more distant Virgo cluster of galaxies was known to contain a number of seventeenth-magnitude, small, lowsurface-brightness objects, which he speculated might possibly be “clusters of the Sculptor-Fornax type” much further away.19 As Baade wrote Shapley in February 1939, as soon as he and Hubble had read his paper they had begun “play[ing] around a little” with the two new systems.20 They probably each decided independently to find out more about the two systems, and then agreed to share the project. The 100-inch was a larger and much better telescope than the Boyden 60-inch, and although they could only get at these far southern objects (both at declination δ −34°) for a few hours each night, they could take good direct photographs which went considerably fainter than Shapley’s. Baade and Hubble quickly confirmed Shapley’s descriptions, but went far beyond them. They found some forty variable stars in the Sculptor system, nearly all of them RR Lyrae stars, as shown by the fact that their periods were shorter than a day. With an accurate magnitude system established by Baade by comparison with SA 68, the distance modulus turned out to be m − M = 19.6, corresponding to a distance of 8.4 < 104 pc, an absolute magnitude −10.6, and linear dimensions 1000 < 1100 pc. Two variable stars had longer periods, of the order of a week, and were brighter, corresponding to Mpg = −1.4. Though Baade and Hubble could not determine light curves for these two variables from such a short series of observations, all their properties were consistent with their being Cepheid variables, and tended to confirm the distance derived from the RR Lyrae variables. There were no supergiants, very blue, or very red stars (they had taken direct exposures in yellow light), but there were large numbers of yellow giants fainter than Mpg = −1.4 Neither Sculptor nor Fornax

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had a well-defined nucleus, or much structure; they were both diffuse, with a uniform distribution of stars. The Fornax system was more distant, and Baade and Hubble’s plates showed no RR Lyrae variables. But it contained two globular clusters, both of which were partly resolved into stars in their outer regions. The brighter globular cluster had even been catalogued earlier, as NGC 1049. Baade and Hubble could see that its brightest stars were essentially the same apparent magnitude as the brightest “field” stars in the Fornax system, indicating that the cluster was in it. From these brightest stars, Baade could determine the distance to NGC 1049 using the same standard methods originated by Shapley, which he used to measure the distances to globular clusters. Its distance modulus was m − M = 21.4, corresponding to a distance of 1.9 < 105 pc; this was thus the distance of the Fornax system in which it lay. Its size then came out to be 1900 < 2800 pc, and its absolute magnitude Mpg = −11.9. These systems were comparable in luminosity and size with IC 1613, true dwarf extragalactic systems, Baade and Hubble wrote. Yet unlike it they contained no supergiants, no highly luminous blue or red stars. But, they emphasized at the end of their paper, this lack of supergiants was not unique to the Sculptor or Fornax system. M 32 and NGC 205, the round and elliptical dwarf galaxy companions of M 31, shared it, as was shown by the fact that they were not resolved into stars, though high-luminosity ones brighter than Mpg = −1.5 would have already been detected, if present, with the 100inch. A long footnote at the end of their paper stated that “as a working hypothesis it could be assumed that . . . supergiants are lacking in the [galaxies] usually described as ‘elliptical,’ [and] in the central regions of ‘early’-type spirals.” But, they abruptly declared, “discussion of the data now available would be largely speculative, and hence of little permanent value.” That last sentence undoubtedly had been written by Hubble; his idea of science was to report data and check theories. The previous description of elliptical galaxies containing exactly the same types of stars as globular clusters must have been Baade’s. His goal was always to draw new insights from observational data, guided by current theories, and to go be-

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yond them.21 Just five years later, in the midst of World War II, left on his own without Hubble to hold him back, Baade was to prove that this same supergiant-free population of stars was present in M 32 and NGC 205, and to describe it as population II, his greatest discovery.

World War II and the Two Stellar Populations America entered World War II on December 7, 1941, while Baade was completing his paper on the Crab nebula. After the Japanese attack on Pearl Harbor, the United States was suddenly at war with all the Axis nations, including Baade’s homeland. Now there was no hope of his communicating with his family in Germany or with any of his astronomer friends there, such as Unso¨ ld, whom he had helped get to America briefly in the spring of 1939, or Karl Wurm and Erich Schoenberg, who had hoped to see him in their country that fateful August.22 With the coming of the war the German consulate in Los Angeles was closed, and the professional diplomats were sent home. German citizens like Baade (and there were many more in Hollywood) had to register and were restricted in their movements, but were not sent to concentration camps in the interior of the country, like the Japanese and their American-born children. Even before the United States entered the war, Mount Wilson Observatory, like many university and other research centers, had begun an in-house government-supported weapons development program. Most of the older astronomers, including Minkowski, who had become a naturalized citizen in 1939, just as soon as he could, knew enough physics to do optical design and testing of range finders, gun sights, and the like, and to calculate fields of fire of American and German bombers, and how best to defend or attack them, respectively. Baade, an enemy alien, could not participate in any of this work, and was not supposed to know what was going on. Three of the younger astronomers on the Mount Wilson staff went on leave to work on rocket development at a large project centered at Caltech, which had test-firing ranges in a canyon near Pasadena, and at Inyokern in the Mojave Desert.

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Mount Wilson Observatory astronomers outside their offices in Pasadena, 1939. Left to right, front row: Joseph O. Hickox, Adriaan van Maanen, Gustaf Stromberg, Harold D. Babcock, Frederick H. Seares, John A. Anderson, Walter S. Adams, Paul W. Merrill, Alfred H. Joy; rear row: Ralph E. Wilson, Milton L. Humason, Robert S. Richardson, Seth B. Nicholson, William H. Christie, Arthur S. King, Theodore H. Dunham, Edwin Hubble, Robert B. King, Rudolph Minkowski, Walter Baade, Edison Pettit, Olin C. Wilson, Edison Hoge, Roscoe F. Sanford. (Courtesy of the Huntington Library, San Marino, California.)

In April 1942 military security briefly caught up with Baade. The army district issued a curfew order requiring all enemy aliens to be in their homes between the hours of 8 P.M. and 6 A.M. Literally interpreted, it meant that Baade could not go to Mount Wilson to use the telescope, and thus was barred from observing. Adams, who was to stay at his post of director past retirement age until the war ended, sent a letter to Vannevar Bush, who had recently succeeded Merriam as head of the Carnegie Institution of Washington. Bush was the czar of American science during World War II,

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the head of the Office of Scientific Research and Development, so his advice counted. Adams wrote that Baade had taken out his first papers in 1939, waiting that long he said partly because he wanted to protect his relatives in Germany, and partly because he was an impractical scientist! One of the options Adams considered was sending Baade to McDonald Observatory for the duration; it was far enough from the coast that these army security restrictions did not apply. But Adams wanted to keep him at work at Mount Wilson if he possibly could, because he was so productive. Baade, who no doubt had his observing plans completely mapped out for the 100inch, certainly did not want to make this move, although Kuiper, who as one of the main users of the McDonald telescope visualized himself as being “traded” to Mount Wilson for Baade, was eager to go ahead with it. Bush, after a lot of good advice about making very sure that Baade was completely isolated from the war project (and that everyone knew that he was, to keep Mount Wilson’s and Carnegie’s reputations spotless), advised Adams simply to wait and let the matter work itself out, “take the inconvenience, and look for the proper time to raise the question.” The Mount Wilson director, eager to push matters along, was already pulling strings long before he received Bush’s advice by airmail. Adams and Frederick H. Seares, both with good old American names and widely known in Pasadena, wrote letters strongly vouching for Baade. Adams went so far as to say that he knew “definitely” that Baade’s feelings were “strongly anti-Nazi” (which was undoubtedly true by then, especially when he was talking with the director), “and that was one of his reasons for coming to the United States and remaining here” (which was a strange reading of his motives in 1931 at least). Most important, Adams sent Milton L. Humason, a practical, down-to-earth, lodge-brother type who knew how to win friends and influence people very well indeed, together with Baade, a personable and attractive individual, to see the provost marshal in charge of preserving security in Pasadena. The colonel wilted, and by May the army command issued an exception allowing Baade to spend “regular observing periods” at Mount Wilson “as part of his professional work.” He had missed only one month, April 1942,

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when the weather is often bad at Mount Wilson, and when M 31 and its companions, his primary targets, were not well placed in the sky anyhow.23 Then in the summer of 1942 Hubble departed, on leave for the duration of the war, to head the exterior ballistics work at Aberdeen Proving Ground. His former astronomy professor at the University of Chicago, Forest R. Moulton, had had much the same job during World War I. To an astronomer an artillery shell’s trajectory is simply an orbit in the earth’s gravitational field, with the added complication of a resisting medium, the air, but amenable to both calculation and highly accurate measurement. Hubble himself did not know much about the scientific problem, but his great prestige, his presence, his service as an officer during the previous war, and his authoritative way of talking made him ideal for the job. His departure freed his telescope time, which Baade, the only person on the staff not immersed in the war project, gladly took over. It also left Baade free to work on his own, uninhibited by a senior, aging legend in his own time who abhorred “speculation.” Baade had decided to turn his full attention to using yellow- and red-sensitive plates, to see if he could not photograph individual bright yellow and red giants in the central region of the “bulge” of M 31, and its companion galaxies. He had already drawn the analogy with the Sculptor and Fornax systems, in which he knew from his own careful photometric measurements, made after his joint paper with Hubble, that the brightest stars had the colors of K giants. Hence he believed that he had a real chance of “resolving” the apparent amorphous regions in M 31 into individual bright stars, seen against a background of the fainter, more numerous, still unresolved members of the system. This achievement required all of his skill and experience, together with the beautifully clear, stable late summer and early fall nights at Mount Wilson. The skies were darkened by the wartime “brownout” (a partial blackout) still in force in Los Angeles and the San Gabriel Valley. Baade nursed the 100-inch telescope’s mirror, arranging the ventilation to keep its temperature constant all night long, and hence its form rigorously paraboloidal. He could even

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adjust the focus slightly during the four-hour long exposures by eye, watching the coma pattern of the star he was “guiding” on with a high-power eyepiece rigidly attached to the plateholder. In the autumn of 1942 Baade tried very hard with yellow-sensitive plates and almost achieved the resolution into stars he was seeking, but not quite. Just a few more tenths of a magnitude were all he needed, and this he achieved with red-sensitive plates in August, September, and October 1943. M 32, the “inner” amorphous region of M 31 itself, NGC 205, its fainter elliptical companion, and NGC 185, one of its still fainter, more distant elliptical companions, each yielded to this treatment. In all of them the brightest stars were yellow giants with absolute magnitudes Mpv ≈ −2.5, Mpg ≈ −1.2, and color indices +1.3. There were no supergiants, no highly luminous blue or red stars in any of these galaxies, just as in the Sculptor and Fornax systems, and evidently just as in more distant giant elliptical galaxies, he now realized. None of them, even the nearest, showed any individual stars, but some of them, like M 87 in the Virgo cluster of galaxies, did contain globular clusters. Evidently all these elliptical galaxies formed one family. In his paper on M 32, NGC 205, and M 31 Baade dubbed these types of stars, which occur in great numbers in elliptical galaxies, globular clusters, and the central regions of spiral galaxies, “population II,” to contrast them with “population I,” the “ordinary” stars (familiar in our Galaxy near the sun), whose brightest stars are highly luminous O and B stars and red supergiants. In his paper Baade published a schematic Hertzsprung-Russell diagram, which graphically differentiated the two populations; it was to become the keystone for future work in galactic structure, just as Hubble’s redshift-distance plot had become the chart for ongoing research on galaxies. Baade recognized that population II was spread throughout our Galaxy; the RR Lyrae variables and globular clusters he had studied for so many years were its markers. The RR Lyrae stars were also one of the types of “high-velocity stars” which Oort had recognized and singled out years before. In contrast the O and B stars of our Galaxy were known to be the opposite extreme, “low-velocity stars.” Thus working back and forth between Sculptor and Fornax,

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M 31 and its companions, and our Galaxy, Baade had put together a new picture which linked many facets of galactic structure and kinematics, stellar spectroscopy and motions, elliptical and spiral galaxies. Other astronomers, like Oort and Lindblad, William W. Morgan, and Philip C. Keenan, had earlier caught glimpses of it, but Baade, with his wide knowledge of published research and his own well-planned and skillfully executed observing program with the biggest telescope in the world, had discovered it.24 When he began this program in 1942, Baade had written Shapley that he was working on magnitude systems in M 31 and M 33, but did not reveal any of his ideas about the stars in elliptical galaxies. He urged Shapley to carry out several specific programs in the southern hemisphere, in order to measure accurately the bright ends of the luminosity functions of the two Magellanic Clouds. This suggestion was closely related to Baade’s population concept, but he kept that part to himself. Shapley, however, wrote mostly about why he could not carry out such programs (most of his staff was also engaged in war work, not galaxy research, and a fast spectrograph had been lost en route to South Africa, in a ship torpedoed by the Germans) and tried to find out what Baade was doing. Humason finally revealed the news to him in 1944, shortly before Baade published his results.25 In contrast, Baade’s relations with the Lick Observatory astronomers, whose work he admired, were open and friendly. Early in 1942 Mayall had gone on leave to the Massachusetts Institute of Technology Radiation Laboratory to work on radar, but neither the cold Eastern climate nor his assignments there suited him very well. In the summer of 1943 he managed to transfer to the Mount Wilson Observatory war project in Pasadena, and there he, Baade, and Minkowski ate lunch together on a daily basis. They always discussed astronomy, and presumably the two scientific warriors never revealed any military secrets to the enemy alien in their midst! Mayall borrowed filters from Lick for Baade, had photographs of M 31 and its companions sent to him, and saw the Mount Wilson astronomer’s results as he got them. Wright, now retired but continuing his research, and Joseph H. Moore, nearly as old and now the interim wartime director, learned all about Baade’s

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Nicholas U. Mayall at the Crossley reflector, Lick Observatory, c. 1938. (Courtesy of the Mary Lea Shane Archives of the Lick Observatory.)

resolution of the brightest population II stars in M 31 and its companions within a month of his accomplishing it. They applauded his work.26 The few other American astronomers who still were not completely occupied in wartime laboratories learned of Baade’s feat and the two-population concept in August 1944, when his two papers appeared in the Astrophysical Journal. He knew that many of the readers especially “doubting Thomases, [o]ne of them . . . Shapley,” would be skeptical, and he wanted to reproduce one of his plates as proof which could not be denied. Struve, the director of Yerkes Observatory and editor of the Journal, agreed. He had been impressed himself by seeing high-quality prints of Baade’s di-

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rect photographs, and he arranged to have enough copies made at Yerkes Observatory so that one could be bound into every copy of the issue containing the two papers. Two young assistants printed all seven hundred of them (the total combined circulation of the Astrophysical Journal and the Contributions of the Mount Wilson Observatory) in the basement of the observatory as part of their summer jobs. Baade was very pleased with the result, and extant copies still show the resolution well today. But most astronomers, in America and abroad, only learned about this great discovery, and the new concept which came from it, after Germany and Japan had been defeated and they came home.27 One theorist who did learn earlier was Fred Hoyle, just beginning his extremely productive astrophysical career. During the war he was a young member of the British wartime radar-development program, and in the fall of 1944 he was sent on a brief visit to the United States to consult with American scientists working on parallel projects there. Hoyle’s travels to several research centers took him from Washington to Los Angeles and San Diego, and then back to Boston and home to England via Montreal. On successive weekends Hoyle managed to meet, each for the first time, Henry Norris Russell in Princeton, Baade in Pasadena, and Shapley at Harvard. Hoyle discussed astronomy with all three of them, and Baade gave him several reprints, two of which must have been his epoch-making papers reporting the observational evidence for his recognition of population II. Their discussion that Monday afternoon, and the preprints Baade gave him stimulated Hoyle to begin thinking about thermonuclear reactions in stars and nucleogenesis as soon as he got back to Cambridge, in the wartime winter of 1944–45. Hoyle remained a great believer in Baade’s research all the rest of his life, and visited him in Pasadena every chance that he had, to learn about his latest insights and discoveries.28

Baade’s Window and the Nucleus of Our Galaxy When Baade published his two papers on population II in August 1944, the Allied armies, securely lodged on the continent of Europe, had just broken out of Normandy and were sweeping through

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France. The Russian army groups were closing in from the east. The Germans held out and that winter achieved a brief tactical victory in the Battle of the Bulge, but as soon as the skies cleared and the American and British bombers could get back in the air, the Allies regained control and Hitler’s days were numbered. He committed suicide in April 1945 and in May the German armed forces surrendered. As these events were unfolding, Baade was working up his paper on two distant globular clusters in our Galaxy, now recognized by him as representatives of population II. There were no new insights in the paper; he could have written it ten years earlier. But he was mulling over the implications of his discovery.29 Baade had long considered that our Galaxy was most probably an Sb, intermediate spiral galaxy like M 31. Then, like M 31, it should have a huge central bulge, composed of population II stars, strongly concentrated to its center. RR Lyrae variables were the invariable markers of that population. Hence in our Galaxy there should be a strong concentration of RR Lyrae variables to its center. He decided to look for it. The best place to look was along a line that passed as close to the center of the Galaxy as possible. From his previous surveys made in red light with the 18-inch Schmidt and the 100-inch reflector, Baade knew that the only relatively clear region was the field centered at l = 1°, b = −4° (in our present galactic coordinates), which we call Baade’s window. Thus in September 1945, just two weeks after the Japanese surrender on the USS Missouri, he began taking direct exposures of this far southern field with the 100-inch telescope, several on each night he had it to use. On them he found a very large number of RR Lyrae variables, 152 from his intercomparisons of the first ten plates. This corresponded to roughly 400 of these stars per square degree, more than thirty times as many as in any Milky Way field anyone had previously surveyed. Obviously, there were even more RR Lyrae variables in this field which had not yet been discovered (as subsequent searches have abundantly confirmed).30 The great bulk of these RR Lyrae variables had apparent magnitudes between 16.5 and 18, the number decreasing at fainter magnitudes. Thus there was a maximum number per unit magnitude interval between mpg = 17.0 and 17.5. Since all the RR Lyrae variables

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have nearly the same absolute magnitude (Mpg = 0.0 was the value Baade used), this would give the distance modulus to the concentration of population II along this line 4° from the galactic center, or very closely, to the galactic center. Because of interstellar extinction, which though relatively small in this field was certainly not zero, it was an upper limit to the distance to the center. Rather than use the whole field, Baade determined the distance modulus to the globular cluster NGC 6522 (now frequently called “Baade’s cluster”), which lies in the center of the field. From its brightest stars its apparent distance modulus was 17.3, which indicated to Baade that it lay close to the region of greatest density of RR Lyrae variables. Stebbins and Whitford, the photoelectric experts from the University of Wisconsin, had measured the color index of this cluster at Baade’s request. Comparing it with the “normal” color of an unreddened globular cluster, he derived the “color excess” due to interstellar extinction, and from it the amount of this extinction. In this way he derived the distance modulus to the galactic center m − M = 14.7, corresponding to a distance of 8.7 kpc. He was to improve the accuracy of this determination several times in the next decade, but the final value remained much the same. Today we can see all kinds of improvements to make, but the actual numerical result has changed remarkably little in the intervening half century. Apparently Baade was not only intelligent, dedicated, hard working, and skillful but lucky as well.31 Baade presented this paper orally in 1946, at the first meeting of the Astronomical Society of the Pacific since 1941. He had joined the ASP in 1934, three years after becoming a Mount Wilson staff member. Travel restrictions, particularly severe along the West Coast, had prevented the society from holding even a single meeting while America was at war. The 1946 meeting took place on the campus of the University of Nevada in Reno in June, and as he gave his paper and participated in the scientific discussions in the physics building, lounged in the spacious dormitory alongside an artificial lake in this pleasant, shaded oasis in the desert, or ate his hearty meals at the cafeteria, Baade must have thanked his lucky stars that he had left Germany in 1931, and decided not to go back in 1937. He had been in correspondence with fellow German as-

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tronomers since the summer of 1945. He knew they not only were not observing with a 100-inch telescope while waiting for a 200inch to be completed as he was, but that they were spending almost all their time trying to find food and shelter for themselves and their families, while they wondered whether or when they would lose their jobs because they had cooperated with their Nazi rulers.32

After the War In 1946 and 1947 Baade was waiting impatiently for the 200-inch telescope to be finished, but he never stopped working with the 100-inch. Ira S. Bowen, the Caltech laboratory spectroscopist who had solved the riddle of the nebular emission lines in the late 1920s, proving they arose in forbidden transitions of ions of O, N, Ne, and S, became the new director, replacing Adams, who retired on December 31, 1945. Bowen supported Baade and his research very strongly, appointing him to the new program committee he set up to plan the transition to doing research with both telescopes, and saw to it that the discoverer of population II was assigned plenty of observing time with the 100-inch in the interim.33 Baade now realized that diffuse emission nebulae, or H II regions, as they were later called, must be considered population I objects. He began a systematic program of photographing the entire body of M 31 in red light, which he knew would best show the emission nebulae in it, just as in our Galaxy. He used the narrow band around Hα and [N II] λλ6548, 6583, and for comparison, a band in the near infrared, defined by 103-U plates with a deep red filter which suppressed these same lines, and thus recorded only the continuum light from the stars. He soon saw that there were many emission nebulae in M 31, many more than Hubble had seen in his earlier studies. He had used unfiltered blue light, recording chiefly Hβ, [O III] λλ4959, 5007, and [O II] λ3727, all but the last considerably weaker than the lines in the red, and all, especially λ3727, subject to much stronger interstellar extinction. Baade got Humason to take spectra of some of the doubtful nebulae, but as he was devoting most of his time to working on redshifts of galaxies for Hubble, progress was slow. So Baade sent the charts of his

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nebulae in M 31 to Mayall at Lick, who was happy to obtain their spectra with his very fast spectrograph on the Crossley reflector. From his own direct photographs and Mayall’s spectra, Baade concluded that emission nebulae, extinction by dust, and high-luminosity O and B stars were always associated. They were “pure” population I objects or features. And the spiral arms were features seen in these emission nebulae, not concentrations of all types of stars, as previous observers had thought and as many theorists had tried to explain. The spiral arms almost disappeared in photographs taken in the deep-red continuum, Baade discovered in 1947. As he put it in a letter to Mayall, “the pop[ulation] II is the backbone of the whole [galaxy] and the spiral pattern is a flashy but rather inconsequential adornment.” It was another outstanding discovery.34 Only a minority of American astronomers had read Baade’s first two papers on stellar populations when he published them. Most of them were out of touch with astronomy, at wartime weapons development projects or teaching mathematics, physics, and navigation to student soldiers, sailors, and cadets. After the war ended, as the astronomers came home to their observatories and colleges, they caught up on their own specialties, but Baade’s papers with titles about resolving galaxies were out of the mainstream. At that time most astronomers did research on stars. Baade’s work first received wide attention at the American Astronomical Society (AAS) meeting held on the campus of Ohio State University in Columbus, at the end of 1947. The organizers scheduled a symposium on “the relations between spectral characteristics and motions of stars,” led off with a 45-minute invited lecture by Baade on a “survey of the problem of the two stellar populations.” He was very well prepared for this, his first major invited talk in English. In the two years 1946 and 1947 he had given four oral reports on his work on the two populations and the comparison of the structure of our Galaxy with that of M 31 in the Journal Club, the informal research discussion group made up of all the astronomers on the Mount Wilson staff, together with visiting scientists and Caltech physicists interested in astronomical research. Baade’s notes for these talks show the growth of his understanding of the

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various manifestations of the two populations, applied to M 31 and our Galaxy. All four talks were detailed, high-level scientific presentations. Whether or not the other Pasadena astronomers, all of them specialized in their own quite different fields, understood them or not, these talks provided an impetus for Baade to work out his ideas.35 Some 150 astronomers were present at the meeting in Ohio at the end of 1947. Baade was among friends; on the first evening of the meeting Adams presented the second Henry Norris Russell Lecture on his very high dispersion spectroscopic work on interstellar absorption lines, revealing clearly the cloud structure of the interstellar medium. (Russell himself had given the first lecture the previous year.) The next day all the astronomers went by bus to Perkins Observatory, some twenty miles away near Delaware, Ohio, for the symposium. Baade’s lecture, as he wrote it out beforehand in English on fifteen large sheets, was excellent. He presented all the ideas of the two stellar populations in clear, logical form, going back to his early work on RR Lyrae variables in the field, Oort’s analysis of the high-velocity stars, and his and Hubble’s study of Sculptor and Fornax. Baade continued up to his discovery of population II in M 31 and its companions, and his observations of the concentration of RR Lyrae variables near the center of our Galaxy. He did not include (at least in his extant notes) his latest results on the spiral arms as features in the interstellar gas.36 After Baade’s lecture, six shorter invited papers were presented, including one by Minkowski, who spoke on planetary nebulae and concluded from their radial velocities and their space distribution that they were population II objects. Cecilia Payne-Gaposchkin spoke on variable stars, and introduced the idea of treating population as a continuous variable, with RR Lyrae variables being extreme population II, Cepheid variables extreme population I, and various types of long-period variables between them. That evening they all went back to Columbus for the society dinner, held at the Ohio State University faculty club, with many speeches, presided over by Stebbins, a noted raconteur. The next morning, December 31, was devoted to more papers, and then the meeting broke up.

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Baade, Minkowski, and Adams boarded a train and started back to California on New Year’s Eve.37 Baade’s lecture had been a tremendous success. He inspired many who were there to apply their own particular research skills to the two-population concept. All those who were present recognized that it was important. They all remembered Baade and his expressive way of speaking. He had described the two populations and their defining characteristics well. But he had not explained the physical reason for the difference between the two populations, because he did not understand it himself. In fact, several theoretical astrophysicists had already recognized that it was stellar evolution; population I was composed of young, recently formed stars, and population II of old, evolved ones. One theorist had even recognized it several years before Baade discovered the two populations. But the great observer who always sought physical understanding had rejected this explanation as late as June 1947. He was still puzzled at the time of his Columbus lecture. But soon afterward he became convinced of his error, and in the last twelve years of his life he was a leader in unlocking the secrets of stellar evolution. He inspired a whole generation of astronomers and astrophysicists to work along many different lines on these problems through his frequent invited lectures, delivered at meetings, symposia, summer schools, and courses.38

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Young Stars and Old PA L O M A R A N D P R I N C E T O N , 1948–1953

Introduction Walter Baade’s education, training, and early research experience in Germany, followed by his years working with the 100-inch Mount Wilson reflector, led him to the discovery of the two stellar populations during World War II. His two 1944 papers and the invited lecture he had given at the end of 1947 at the American Astronomical Society meeting in Ohio made him a well-known, highly respected figure among research astronomers. Baade was then waiting impatiently for the completion of the Palomar 200-inch, so that he could exploit the ideas and concepts he was rapidly developing on the basis of his discovery. The rest of his life was to be a series of triumphs as he led the way in pushing our knowledge of the universe out in space and back in time with this largest telescope in the world. Astronomers had finally begun to learn how stars change as they age, or “evolve.” That is the reason that Baade’s scientific career is so important to us today. From the end of World War II until he died, Baade’s most important colleague in the scientific world was Jan H. Oort. This reserved, intense, ascetic Dutch astronomer, some seven years younger than Baade, was in many ways a striking contrast to the older, outgoing, colorful German, who did not mind a few drinks and a good cigar on occasion and who preferred long vacations whenever he could afford them, but they shared a consuming interest in understanding galaxies. Oort, the son of a physician, had studied astronomy at Groningen University under the outstanding Dutch astronomer of the previous generation, Jacobus C. Kapteyn. After finishing his academic courses and assisting in the Kapteyn

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Jan H. Oort in his office at Leiden Observatory, 1939. (Courtesy of Leiden Observatory.)

Laboratory for one year, Oort had traveled to America and worked at Yale University with Frank Schlesinger in 1922–1924, becoming an expert in astrometric research. In 1924 he was appointed a staff member at Leiden Observatory, then directed by Willem de Sitter, and spent the rest of his life there. Oort’s doctoral thesis, completed in 1926, was on the high-velocity stars, which Baade later recognized as important members of his population II. Almost all of Oort’s research was in galactic structure. Baade had first met him in Europe, and came to know him well in 1939 at the McDonald Observatory dedication and during the middle-aged Dutchman’s subsequent visit to Pasadena. After it they had corresponded, especially about supernovae, until in the fall of 1941 the mails between America and German-occupied Holland were interrupted. Oort’s last letter to Baade about the Crab nebula and distant Cepheid variables was returned to him.1 That same year, Oort, one of the Dutch professors who publicly protested the Nazis’ dismissal of their Jewish colleagues, many of whom were banished to concentration camps, had to leave Leiden and go underground in the countryside. There he was able to con-

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tinue theoretical research, although in the harsh winter of 1944–45 he, along with most of his compatriots, came close to starving as the Allied armies approached and fought their way through the Netherlands, and the retreating Wehrmacht left a wake of destruction behind it. In the spring of 1945 Oort was rescued by Gerard P. Kuiper, the Yerkes astronomer who returned to his native country as a naturalized American citizen, close behind the lines, a member of the ALSOS scientific and technical intelligence mission. Kuiper arranged for Oort to spend a few weeks in England, where food was more plentiful, before he returned to Leiden as full professor and director of the observatory.2 Through Kuiper and Bertil Lindblad, in Sweden (which had sat out the war as a neutral, poised uneasily between Germany and the Soviet Union), the Dutch astronomer had obtained copies of Baade’s papers on the Crab nebula and on his identification of population II in the Andromeda galaxy and its companions. At the end of August 1945, as soon as civilian mail between Europe and America started getting through again, Oort sent Baade a long letter, congratulating him on this work and asking him many questions about it. He commented on the ideas Baade had expressed for further research, particularly on trying to identify the spiral arms of our Galaxy as concentrations of population I stars. Oort told of his own theoretical work, groping toward understanding the internal structure and kinematics of gas shells thrown off by novae and supernovae, as they expanded and interacted with interstellar gas and “dust” particles. His greatest joy, he said, was that at long last he was again receiving astronomical research publications from America. Baade, greatly interested in Oort’s analysis, replied with a long report on the observational results he had obtained on several nova shells, mostly still unpublished. He described the interactions in very physical terms, although completely qualitatively. He knew Oort could generalize them, and work them out mathematically. Baade also briefed Oort on his latest findings on the distant Cepheid variables in the Milky Way, and told him of the difference between the population I “classical” Cepheids, and the population II W Virginis variables, which mimicked them but had different lu-

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minosities. With the help of the spectroscopists Alfred H. Joy and Philip C. Keenan, Baade was beginning to distinguish between them; within a very few years they would lead him to doubling the size of the universe. Baade and Oort were already deep into research discussions, supplementing each other’s areas of expertise. They both referred obliquely and gingerly to the war in their first exchange, then dropped the subject; their common scientific interests overcame whatever resentment Oort may have felt toward Baade as a representative of German “culture.”3 Oort had been awarded the Royal Astronomical Society’s George Darwin Medal, and invited to come to London to receive it and present the Darwin Lecture in the spring of 1946. It dealt mostly with the interactions between gas, stars, and dust about which he had written Baade. Oort borrowed direct photographs from the Mount Wilson astronomer and obtained some of his unpublished data to use in the lecture. He sent Baade a written copy of his paper soon after presenting it; the latter was entranced as he read it. He congratulated Oort, saying the findings would have a tremendous influence in guiding future research, but also sent him detailed comments, suggesting additional studies and offering to cooperate with him.4 Baade told him all his unpublished results on the concentration of RR Lyrae variables at the center of our Galaxy. By then they were on a “Dear Oort” and “Dear Baade” basis, as close as senior scientists of their time allowed themselves to get with their contemporaries, and for the rest of Baade’s life they remained intimate friends who inspired each other’s research.5 Toward the end of 1947, Oort came to America for the first time since the war. He arranged to visit Pasadena for several long discussions with Baade. No doubt they went over Baade’s manuscript for his invited lecture at the Ohio AAS meeting. Then Oort left for McDonald Observatory for a session with the 82-inch reflector, but after the meeting Baade wrote him about the discussions at Columbus and the new information he had gleaned there. Oort, the leading figure in Dutch postwar astronomy, spent considerable effort on sending the best young prospects from his country to the United States for seasoning, making sure that they would meet Baade and discuss science with him there. The Leiden director always made it

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clear that he wanted them back in a year or less; Baade, who favored longer stays, in those early years was particularly impressed by the young Adriaan Blaauw and Hendrik van de Hulst and encouraged them to seek the best research opportunities they could, whether that meant remaining in America as he had, or not. Oort believed in strict control; Baade always favored what he called “rein Spass” (“pure fun,” though in English he often wrote “a pure thrill”), by which he meant following one’s scientific ideas wherever they led.6

The 200-inch Hale Telescope On December 30, 1947, Baade’s invited lecture on the two stellar populations had alerted the members of the American Astronomical Society to an exciting new idea. His talk inspired many of them to follow it up in their own subfields of stellar and nebular research, but he did not rest on his laurels. He continued observing with the 100-inch telescope, exploring M 31, the Andromeda galaxy, the nearest representative of a spiral galaxy, to learn as much as he could about the distribution of population I and II objects within it. It covers a large area on the sky, which he carefully divided up on his finding charts into fields, each of which would fit onto a single exposure, and systematically photographed them with various emulsion and filter combinations designed to reveal hot blue stars, diffuse emission nebulae (soon to become known as H II regions), and “dust” (recognized by its extinction of stars behind it). A highly skilled observer, he used only the nights of best seeing (which at Mount Wilson occur most often in the fall, when M 31 is well placed in the sky), when the images are smallest and the starlight most concentrated. A consummate diplomat, he skillfully enlisted C. E. Kenneth Mees, head of the Eastman Kodak Research Laboratory, and his assistant W. F. Swann in his quest for the most sensitive, stable red-sensitive photographic plates that were essential for his program. Two years later Baade was even willing to fly across the country to speak at a big national camera convention at Rochester, New York, Kodak headquarters, no doubt at the request of Mees, to forecast succinctly the research he and other Palomar observers

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would do with the 200-inch reflector. Speaking from a three-page script provided by Bowen, he said that their program would be to “continue where we had to leave off with the 100-inch telescope,” penetrating deeper into space and backward in time. Baade spoke in generalities of observational cosmology, stellar evolution, and the abundances of the elements. “In a very real sense the stars ha[d] . . . become the cosmical laboratories of the nuclear physicist,” he said, and the 200-inch would “prove a powerful tool in these explorations.” It was the director’s language, not his own. Mees and the other applied physicists had wanted to see Baade in person, and exhibit him to their colleagues. No doubt in conversations he told them more vividly of his own plans to investigate every facet of the populations in M 31 and our Galaxy.7 By the fall of 1948 Baade knew he was close to finishing what he could do with the 100-inch on M 31. As he inspected and analyzed the plates, he realized more and more clearly that the outstanding feature of a spiral galaxy, its spiral arms, were primarily an “organization” or structure of dust, and that the supergiant stars always occurred in close association with the dust.8 Baade summarized this concept and the evidence for it in detail at the symposium at the University of Michigan in 1950, held in connection with the dedication of its new 24-inch Schmidt telescope. By 1948 he had also already worked out which populations novae and supernovae belonged to, by going back over all the data he had on these briefly brilliant types of stars. Novae, which occur frequently only in the central region of M 31, our Galaxy, and other nearby spirals, but infrequently in the Magellanic Clouds, which Baade thought “consist of an almost pure population I,” were clearly population II objects. Rudolph Minkowski had earlier divided supernovae into two types on the basis of their spectra, and Baade had found that their light curves could also be classified to these same two types; now he found that supernovae of type I occur in all types of galaxies from elliptical to late-type spirals, and therefore belong to population II, while supernovae of type II occur only in spirals, and hence are population I. He was using the population concept to bring order to apparently unrelated observational results, but he still did not grasp the physical reason behind it.9

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As he worked with the 100-inch, Baade was simultaneously planning for the 200-inch, which he knew would soon double his effective observing range by quadrupling his light-gathering power. He would devote himself to direct photographic work on galaxies, but he knew he would need help with spectroscopy and the quickly advancing photoelectric photometry. Milton L. Humason, the outstanding spectroscopic observer of faint nebulae and galaxies, would devote himself to measuring redshifts for Edwin Hubble’s cosmological program, while Minkowski preferred his own more arcane nebular investigations to observing at Baade’s behest. Hence Baade encouraged and stimulated Nicholas U. Mayall, at Lick Observatory, to continue and expand his spectroscopic measurements of the radial velocities of globular clusters (pure population II objects) in our Galaxy, and of H II regions (pure population I) in M 31. Mayall had only the ancient 36-inch Crossley reflector on Mount Hamilton at his disposal, but his very fast spectrograph, optimized for his work, his exceptionally keen eyesight (making it possible for him to find and observe these faint objects), and his unbounded enthusiasm made him a valuable ally. Besides, Lick Observatory had the go-ahead to build a 120-inch reflector, which would be second only to the 200-inch when completed. This was then expected to occur only a few years in the future, although in the event it did not come on line until the year before Baade’s death. Baade kept Mayall fully informed on all his latest results, diplomatically suggested program after program he could do, and helped him over whatever rough spots came up in them.10 The Baades were childless, but the astronomer, in his visits to Germany, had loved to play the role of the friendly uncle to his brother’s and sister’s children. Now he reenacted the part, becoming a sort of surrogate older brother to Mayall and his wife, and uncle to their children, especially when they took vacations together at Oceanside, on the Southern California coast not far from Palomar and San Diego. There Baade also formed an unlikely friendship with a Catholic priest, Lawrence R. Schmieder, who was a chaplain at the nearby Camp Pendleton Marine Base. “The Padre” was fluent in German as well as English, and a confirmed astronomy buff; he loved to sip martinis and grill steaks for Baade, May-

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Group on the observing floor in the 200-inch telescope dome, Palomar, c. 1951. Left to right: Nicholas U. Mayall, Lawrence R. Schmieder, S. J., Walter Baade, Muschi Baade, Bruce Mayall, and Pamela Mayall, with the Baades’ dog Li in front. (From the author’s collection.)

all, and their wives on the beach as he pumped them for their latest insights on the universe to supplement his own wide reading in popular scientific and news magazines. Baade, a “hard boiled [P]rotestant” in his own words, liked to claim he had a Lutheran theological background and evidently enjoyed friendly disputations with the worldly priest. Baade’s dog Li (“Lee” to Americans), a powerful, threatening-looking chow which responded only to German, was an important part of these vacations, always firmly under control when the children were present.11 Baade and his wife had named the dog Li Tai Bei, after an ancient Chinese poet and philosopher who was a cult figure in the Germany of the Weimar Republic days. Years later George Gamow remembered a visit he made to Pasadena, probably in the early 1950s, to talk astronomy with Baade. Gamow stayed with the Baades in their home, where they spoke English with him, but Li was quite unfriendly. They had to keep the dog under restraint. After a day

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or two Baade asked Gamow if he could not speak German with them, and the Russian-born physicist, who boasted he could speak fifteen languages, “all in the Gamowian dialect,” switched to his own ungrammatical version of the language of Goethe and Schiller. According to Gamow, Li became friendly almost instantly, so he walked over to the chow, loosened his “chain” (probably a leash), petted him on the head, and said in German, “Ach, du dumes bo¨ se Hu¨ nchen,” which might be translated into the equivalent broken, misspelled English, “Oh, you dum bad pupp!” As Gamow remembered it, Li licked his hand and remained friendly for the remainder of his visit, during which he learned a lot of astronomy and a little better German.12 After World War II, Joel Stebbins, the great pioneer of photoelectric photometry, who had done so much of his observing at Mount Wilson Observatory, was close to retirement. Baade wanted to bring in one of the next generation of his students and prote´ ge´ s, Albert E. Whitford, Gerald E. Kron, Olin J. Eggen, or Harold L. Johnson, to make the accurate measurements of the magnitudes of faint stars that the 200-inch would make possible. Ira S. Bowen, the new director of Mount Wilson and Palomar Observatories, was less enthusiastic about them. Baade respected Bowen as an expert in applied optics, the perfect director to get the big telescope into operation, but felt that his one shortcoming was that he would always think a Caltech Ph.D. the best man for any job, even better than outsiders who had already proved themselves.13 Baade helped all these former University of Wisconsin students and young faculty members gain access to the 100-inch, schooled them in his type of galactic structure problems, and promised to help them get observing time with the 200-inch when it was ready. He knew that with it they could measure the faint magnitudes that would be necessary to solve those problems.14 The equally new director at Lick, where Kron and Eggen became staff members along with Mayall, and where Johnson had been a graduate student, was C. Donald Shane. A longtime teacher on the Berkeley campus, at Lick he took over the new, 20-inch wide-field “astrograph” or Ross camera and its research program. It was designed to measure the proper motions of faint stars with respect to

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the reference system fixed in the universe provided by the many faint, distant galaxies, a program that would take years to pay off. Baade encouraged Shane to count galaxies on the plates taken for it, and thus gain some quick results on the patchy distribution of interstellar dust in our Galaxy, and on more distant clusters of galaxies in the universe. Baade flattered Shane with his attention, promoted informal meetings of the astronomers working on nebular and galactic research at the two observatories, gave him excellent research advice, and gained the Lick director’s cooperation and support.15 By November 1947 Bowen and the members of the Observatory Council judged the 200-inch mirror close enough to its correct figure to be sent from the optical shop on the Caltech campus to Palomar. There Don O. Hendrix, the skilled young Mount Wilson optician who had taken over the work on the mirror, touched up its surface and aluminized it. Then it was put in the telescope, and on December 21 Hendrix and Bowen made the first visual tests with it on stars, the “first light” for the 200-inch. The following night they began photographic tests with a full-size Hartmann screen, a procedure involving detailed measurements to determine the exact shape of the reflecting surface, which Hendrix could then further improve. Baade was anxious to use the telescope, but predicted it would not be ready until the following fall, when the best seeing would be available for the final critical tests.16 The Carnegie Institution of Washington and Caltech bigwigs decided the telescope, which was to be named for George Ellery Hale, would be dedicated in the summer of 1948. It was the fourth successive largest telescope in the world for which he had secured the necessary funds, but he had died before it was completed. The AAS and the Astronomical Society of the Pacific (ASP) were to hold a joint meeting in Pasadena in June, and then move on to Palomar for the one-day dedication ceremony. Astronomers all over America were anxious to see the telescope, and the meeting would be a big one. Otto Struve, president of the AAS, and Harlow Shapley, its recent past president, first planned to schedule a symposium of three invited lectures on galactic structure and dynamics for the Pasadena meeting. Hubble was to speak on the structure of galaxies from the

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observational point of view, Mayall on their internal kinematics from his spectroscopic radial-velocity measurements, especially of H II regions in M 31 and M 33, and Subrahmanyan Chandrasekhar, of Yerkes Observatory, on their dynamics, from the theoretical point of view. However, the head of the local organizing committee for the ASP, Seth B. Nicholson, a Mount Wilson solar astronomer and planetary-orbit expert, feared that Hubble, a master at attracting personal publicity, would steal the show. Probably aided by covert pressure from Walter S. Adams, the former Mount Wilson director, who by then no longer respected Hubble as a scientist and warned Bowen against him, and from the new director himself, who was president of the ASP and wanted the men who had built the telescope to be recognized, that symposium disappeared from the program.17 Instead John A. Anderson, the head of the 200-inch project from the beginning, spoke on the optics of the 200-inch; Bruce Rule, the Caltech engineer, described the engineering aspects of the telescope; and Baade spoke on a “program of extragalactic research” for it. They gave their talks in the 200-inch telescope dome on July 1, 1948, before some 350 members of the two astronomical societies and their guests, seated on folding chairs. Bowen opened the session by welcoming the visiting astronomers, and then demonstrated the motions of the giant telescope, which he controlled with push buttons from the night assistant’s console. Then he presided as Anderson, Rule, and Baade gave their talks. Baade began by emphasizing Hubble’s crucial part in starting “the great attack on the structure of the universe” twenty-five years earlier, but then got down to brass tacks. He stressed the important work that would now have to be done to make real progress, setting up accurate magnitude scales down to the faintest limit to which the Hale telescope could be pushed, measuring Cepheid variables to this limit, using them to calibrate the absolute magnitudes of the brightest stars which could then be used to extend distance measurements to more distant galaxies, calibrating RR Lyrae variables and population II Cepheid variables, comparing them with population I Cepheids and thus “check[ing] . . . if our basic assumptions are correct.” It was very forward looking and made a deep impression on

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the many astronomers who had traveled across the country to see the 200-inch and hear about the problems it would solve.18 Sadly, Hubble had never caught up on research after his return from Aberdeen Proving Ground at the end of World War II and after his disappointment in not being named director to succeed Adams.19 He was so out of touch, or possibly so self-centered, that in a popular talk he gave in 1946 in Los Angeles before the Sunset Club, an establishment dining and discussion group, he began by saying, “There was little or no real progress in science during the war (except perhaps in the fields of medicine and surgery). The research men are returning to find the same old problems just about as they were left.” He evidently had not read Baade’s two papers, published during the war, on the discovery of the two populations, or if he had read them, he had not understood them. Every astronomer even slightly interested in galaxies or galactic structure, and every astronomer at Lick, Mount Wilson, and Yerkes Observatories (except Hubble) was well aware by then that Baade’s two papers had fundamentally changed their science.20 Getting the 200-inch into full operation took even longer than Baade had predicted. The tests confirmed Bowen’s and Rule’s fears that the support system, designed in 1935 to keep the figure of the huge mirror from “flexing,” or distorting slightly under its own weight, suffered from too much friction to fulfill this task completely. They updated the design, tested a pilot model, and put the new system into operation in the fall of 1948. It greatly improved the performance of the mirror, but the tests were then sensitive enough to confirm that its shape deviated from a perfect paraboloid, with a “turned-up edge.” Also they found that the mirror figure suffered from thermal distortions when the night-time temperature changed. Hendrix touched up the figure, working in the dome and testing each improvement on stars, a tedious but necessary process that ate up time. Bowen and Rule added thermal insulation at the sides and back of the mirror, plus electrical fans to help equalize the temperature within it, and by January 1949 the figure was reported to be “remarkably fine”—but still needing improvement. Tests continued all through the summer, as the atmospheric conditions and seeing gradually improved and the measurements thus

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became more precise. Finally in September 1949 Bowen judged the mirror as nearly perfect as it was possible to make it, Hendrix aluminized it in October, and on November 12, 1949, regularly scheduled observations began and continued in each dark of the moon. The big coude´ spectrograph that could be used in bright moonlight was not yet ready, so the period near full moon was devoted to further testing. The winter weather revealed that thermal distortion still occurred, though at a lower level, but more insulation solved that problem too by March 1950.21 Bowen, a quiet, diplomatic leader, had involved Hubble strongly in the testing process. As the senior astronomer, he went to Palomar to test the telescope in actual use, obtaining direct photographs of many of the most famous, spectacular nebulae, star clusters, and galaxies, as well as critical test plates of stars, beginning in January 1949. The newspapers were clamoring for information, and he enjoyed the attention. In the spring he prepared a short paper in which he published the “first photographs with the 200-inch Hale telescope.” He gave it his seal of approval, writing that these pictures “confirm[ed] the most optimistic predictions of its designers” and that they had demonstrated that it could reach stars 1.5 magnitudes fainter than the limit of the 100-inch. Further improvements in definition could be expected, he stated, but the telescope had already proved itself a great success. The spectacular reproductions of six of his photographs must surely have convinced any skeptical readers.22 Unfortunately the great observational cosmologist was never to get the chance to do real research with the 200-inch. In July of that same year, while on a fishing vacation in Colorado, Hubble suffered a heart attack and nearly died. He pulled through, but when he was finally able to return to his office in Pasadena, Baade reported privately that he looked “awful,” tired, drawn, and easily depressed. A year later Hubble was able to get to Palomar and observe with the 200-inch again, but his creative juices were gone, and he had too little time left to regain them. On September 27, 1953, he collapsed and died of a massive stroke as his wife was driving him home from a morning at the office. In an emotional letter to Hubble’s widow, Mayall wrote that her husband, more than anyone

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else, had stimulated and counseled him in the field of research he had made so much his own. In truth, however, after about 1939 Baade, not Hubble, had played that role not only for him but also for American astronomy in general.23 After Hubble’s death, Baade was named to take his place on the Observatory Council, the small group of senior Mount Wilson and Caltech scientists which Bowen chaired and who were charged with advising him. Happily, in the years since his return to Pasadena after his wartime service at Aberdeen, Hubble had had time to read scientific journals and to discuss research with his colleagues again. Two years before his death, in an invited lecture to the American Philosophical Society on the “cosmological program” for the 200-inch telescope, he gave a masterly summary of his planned explorations into deep space with it. By then he understood fully the importance of the two stellar populations, especially in distinguishing the different kinds of Cepheid variables in them. Otherwise the distance scales used within our Galaxy would be different from those used between galaxies. Hubble told of Baade’s program to check and improve the distance scale, and of the discrepancies he had begun uncovering in it. Hubble gave full credit to his German colleague for this breakthrough, made during World War II, and thus atoned for his 1946 statement that there had been no progress in science during the conflict.24 Baade had been taking an active part in the 200-inch tests from the beginning. He had provided the only quantitative data in Hubble’s 1949 paper, the actual magnitude limit his plates had reached. Baade had drawn up a simple plan for Hubble to take a series of exposures of Selected Area 57 with the 200-inch mirror diaphragmed to various apertures, and then had used the photoelectric magnitudes he had arranged for Stebbins and Whitford to measure with the 100-inch to calibrate Hubble’s plate. Baade’s reduction, analysis, and extrapolation gave the magnitude limit 22.6 or 22.7 which Hubble had implicitly stated.25 After Hubble’s first heart attack, Baade took all the most important test exposures with the 200-inch. These gradually tapered off, but months after the telescope went into full research operation in January 1950, he still had occasional spurts of obtaining test plates for Bowen to analyze. By

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January 1951 they were both convinced that the big reflector was in tip-top shape. Baade found it “wonderful to work with,” and was vigorously pushing his program on local-group galaxies to the new faint-magnitude limit that they had all worked so hard to achieve.26 With more data coming in every month, Baade clearly needed help with reductions, particularly in measuring the magnitudes of the stars in these galaxies. Mount Wilson Observatory had a long tradition of being badly understaffed with assistants. Young visiting astronomers, many of them from abroad, had done some of the reduction work, usually only for a year or two until they had gained enough experience to get a better job or return to a higherlevel post in their native countries. A few women assistants were on the permanent staff, dividing their time between helping various astronomers. Bowen recognized Baade as the most important research worker at the observatory, and particularly after Hubble’s activity slackened, tried to give him all the support he needed. In 1951 Baade briefly had his first full-time assistant, Sister Mary Therese, a nun who taught astronomy at Mundelein College in Chicago. Supported by her order, she came as a volunteer, but because she had little previous experience and stayed only a few months, her help was largely symbolic. However, the work she did for Baade apparently awakened him to how useful another pair of hands and eyes could be. Before long he had found Henrietta H. Swope in New York City, a much more skilled, experienced worker, and by January of the following year she was driving across the country with her sister-in-law, the Hollywood actress Dorothy McGuire, to begin work down the hall from Baade. She was to be his trusted assistant, friend, and confidante until the day he died, and to continue his work after his death.27 “Miss Swope,” as she was universally known in those bad old sexist days, had done her undergraduate work in astronomy at Columbia University, graduating in 1925 and going on to earn a master’s degree at Radcliffe in 1928. Then she began work as an assistant at Harvard College Observatory (HCO). It was commonly reputed to have an even lower salary scale for assistants than Mount Wilson, but whatever she was paid could hardly have mat-

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tered to her, for her father, Gerald Swope, the president of General Electric, was one of the richest men in America. In January 1931 he and his wife took Henrietta on a trip to Los Angeles, and arranged for a weekend visit to Mount Wilson. Henrietta was quiet and somewhat shy; her father was not. He ordered his Los Angeles manager, who organized the visit, to emphasize that he would prefer to go up the mountain to see the telescope with Albert Einstein (then visiting Pasadena), who, he said, “had entertained him on one of his recent trips to Berlin.” He had to be content with Adams, then director of the observatory, instead. At the HCO, Henrietta Swope worked chiefly on measuring photographic magnitudes for Shapley, just as she would in Pasadena for Baade years later. Shapley encouraged her and made her feel a member of the observatory family. He did the same for all the many women assistants, though he probably devoted extra attention to Swope, for her father was contributing several thousand dollars a year to Harvard research. She lived unostentatiously and did not flaunt her wealth, but her relatives and her ability to finance whatever trips she wanted to make, such as to the 1942 dedication of Tonanzintla Observatory in Mexico, could not escape notice. During World War II Swope left her Harvard job for war work, first at the Radiation Laboratory, the radar development center on the MIT campus, and then for four years at the Navy Hydrographic Office in Washington, working on LORAN, a radio navigation system. At the end of the war she longed to get back to Harvard but Shapley, preoccupied with national and international humanitarian projects, was not particularly attentive or positive about the future of the variable-star research Swope loved. In 1947 she accepted a teaching and research job in astronomy at Barnard College, the women’s sister-institution of Columbia. She tried to do various variable-star projects on her own there, but she missed the family atmosphere of the HCO and her many friends on its staff.28 Baade had certainly read Swope’s variable-star papers, especially as many of them dealt with the galactic-center region. No doubt he had met her on visits to Harvard and at astronomical meetings. Somehow, one or the other of them got the idea that she might work with him, and by November 1951 she had definitely decided to

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leave Barnard for the assistantship with Baade. Shapley was surprised and hurt; he liked to think the HCO was the ideal place for anyone. Before long he realized that Swope would not come back; then he began trying to ferret out from her what Baade was doing. But she did not answer his queries; she was now a loyal Mount Wilson and Palomar Observatories staff member, and had all the variable-star work she could handle, at the very forefront of galactic-structure research. With her assistance, Baade was plunging on with the largest telescope in the world.29

The 48-inch Schmidt Telescope The “Big Schmidt” for Palomar, though like the 200-inch twice as large as the largest telescope of the same type previously built, was a much more straightforward project. Its primary mirror, 72 inches in diameter, was small enough so that telescope designers and engineers had encountered any problems it might have provided years earlier, and had solved them. The young Caltech engineer Rule had drawn up the final design for the 48-inch Schmidt, taking full advantage of all the previous studies made for it and the 200inch. The mounting was completed and assembled in its dome on Palomar in the first half of 1948, while Hendrix, the optician, was working on the 48-inch corrector plate in Pasadena. He had made many smaller correctors in the Mount Wilson Observatory optical shop during World War II for long-range aerial and ground-based military surveillance cameras. He completed the 48-inch corrector, installed it in the Palomar Schmidt, and took the first test plates with it himself in September 1948. Baade was very favorably impressed by the small star images, which gave excellent definition over the whole field, better than he had expected. He praised Rule and Hendrix fulsomely, and began using the 48-inch Schmidt himself in January 1949.30 Years earlier, shortly after he declined the proffered Hamburg directorship, Baade had requested a photograph of the optician Bernhard Schmidt (who had died in 1935) from Richard Schorr, to be hung in an honored place at Palomar. But much had changed since those prewar days; Baade knew that the Carnegie Institution and

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most Americans did not like to be reminded that he himself was a German, and even though Schmidt had in reality been Estonian, his picture never went up on the wall in the 48-inch Schmidt dome.31 However, though in Germany the telescopic system he had invented had been called “the coma-free reflector,” Baade from the moment he came to America in 1931 consistently referred to it as the “Schmidt telescope” or “Schmidt camera.” That name stuck, and after a few years it came into use in Germany as well as in English-speaking countries. Baade had made his friend’s name a household word in astronomy, far more enduring than a picture on a wall would have been.32 Baade was soon repeating his earlier search for the nucleus of our Galaxy, this time with the 48-inch Schmidt and red-sensitive plates. Although they showed much fainter stars than his earlier 18-inch Schmidt films of the same region, these new exposures still did not penetrate the dense, optically thick layer of dust between us and the center. That would have to await longer-wavelength infrared detectors of the future.33 In the course of this survey, Baade noticed the long trail of an unusual asteroid on one of his wide-field Schmidt plates. Always alert to minor planets, which he liked to joke about but never ignored, he could see that it had moved rapidly in the sky during his hour-long exposure. His interest awakened, he took a second plate two nights later, and a third two nights after that, capturing the quickly moving asteroid on each one of them. Baade turned the plates over to Nicholson, the Mount Wilson Observatory planetary expert, for measurement. He, with Robert S. Richardson, computed its orbit from these measurements; the asteroid had a highly eccentric orbit which took it from closer to the sun than Mercury to farther than Mars. Soon named Icarus, it gave Baade the distinction of being the discoverer of both the most distant asteroid then known, Hidalgo, and this, the nearest.34 His find brought forth a revealing exchange between Bowen and Shapley, director of HCO, who by now was busily engaged in world-betterment activities and had little time or energy left for science. He wrote to congratulate Bowen on “the first big discovery from Palomar Mountain,” made some inane remarks about the new

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Milton L. Humason, Edwin Hubble, Walter Baade, and Rudolph Minkowski (left to right) in the library of the Mount Wilson and Palomar Observatories in Pasadena, examining prints of photographs of nebulae, galaxies, and star fields taken with the 48-inch Schmidt telescope, 1950. (Courtesy of Hamburg Observatory.)

object’s orbit, and urged the Palomar director to get out a “proper and full news release” on it. Shapley went on with some wordplay about “Eros and other Erotic minor planets” and “amorous godlets,” referring to the names used for asteroids with small perihelion distances and large eccentricities. He suggested an “angle” for the news release, that the new object was smaller than Palomar Mountain, an estimate based on its brightness. Bowen replied in a letter fifty-seven words long, thanking Shapley, saying that so far as he knew Baade had not yet “given any thought to the name,” and ending that he suspected “that we better wait and see if we can hold on to it before we worry too much about a name for it.” No better contrast could be made between the long-winded, publicityseeking Shapley and the terse, dry, factual, publicity-shy Bowen.35 That same summer Minkowski published some of the first direct photographs taken with the 48-inch Schmidt. They were exposures, taken in red and blue light, of a large diffuse nebula in Monoceros, surrounding the galactic cluster NGC 2244. The “Big Schmidt” re-

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vealed, as no telescope had before, intricate structure in the ionized gas and in the dust clouds and condensations imbedded in it. They were spectacular photographs, just as the 200-inch photographs published two months earlier had been.36 Under an agreement with the National Geographic Society, nearly all the observing time with the 48-inch Schmidt telescope was set aside for a survey of the entire sky north of -27° declination. The results, in the form of photographic prints, were to be made available to observatories everywhere. Once the survey got fully under way in November 1949, following a delay caused by damage in transit to a shipment of the large glass photographic plates used in the telescope, Baade and the other regular observers could only use the 48-inch very occasionally, but he had had a first look at most of his important fields by then. In a symposium on Schmidt telescopes at the AAS meeting in Tucson in December, Baade presented an invited paper on the 48-inch, displaying and discussing some of the first photographs taken with it. The one of the entire Andromeda galaxy, five degrees in diameter, was particularly impressive, and his paper aroused great attention.37 Earlier that year, Oort, just as serious a director and astronomical statesman as Bowen, was organizing his first postwar international conference and wanted Baade to take part in it. It was to be held in Europe that summer. The subject was to be the motions and interactions of interstellar gas clouds, nova and supernova shells, and other “gaseous masses of cosmical dimensions,” the topics of Oort’s 1946 Darwin Lecture. He tried again and again to persuade Baade to come, and to bring some of the new, tantalizing 48-inch Schmidt and 200-inch photographs of gaseous nebulae and dust clouds he had heard so much about. It was to be a joint meeting of astronomers, most of whom would bring their data, and theoretical hydrodynamicists, who would try to understand and interpret it. At first Oort planned to hold the meeting in Oslo, but that idea fell through and he changed the venue to Paris. Initially Baade expressed interest, but then said he could not come, blaming his switch on lack of support, presumably from Bowen. More probably one of the real reasons was that after Hubble’s heart attack (which was kept confidential for several weeks), the director wanted Baade to stay to

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make the crucial observational tests on the 200-inch (as he did). This was compounded by Baade’s skepticism that hydrodynamicists could provide real insights into complicated astrophysical situations, and his now much deeper interest in stellar populations than in nova shells.38 However, at Baade’s urging Mayall went to Paris instead, and presented a joint paper, mostly on his own radialvelocity measurements of gaseous nebulae in M 31, but including some of Baade’s direct photographs too. Mayall showed several of Baade’s pairs of matched 100-inch photographs of regions in M 31, one taken in the light of Hα in which the diffuse nebulae stood out clearly in the spiral arms, the other in a comparison band which suppressed the nebular light, to help eliminate stars and star clusters. These reproductions, published in their paper, demonstrated that the spiral arms were outlined by interstellar gas, not by stars in general, as a whole previous generation of observers and theoreticians had imagined.39 One other paper at the conference was based completely on Baade’s observational material. It was by Van de Hulst, who as a postdoc had visited Pasadena in early 1948. Baade had been impressed with his versatility in applying physics to a wide range of astronomical problems, and had shown him all his own and other Mount Wilson observers’ plates of the expanding shell around Nova Aquilae 1918. From them, and from spectrograms taken at Lick Observatory, Baade had deduced that the shell had a cylindrically symmetric structure, a series of rings about a common axis, all expanding radially from the central star. Van de Hulst carried out the analysis quantitatively, and Baade encouraged him to give the paper at the Paris conference.40

Stellar Evolution Baade’s discovery of the two stellar populations in 1944 was a completely empirical result. He had based his work on his previous observational knowledge of the properties of stars in globular clusters, star clouds, the Milky Way, and other galaxies. He was not following up on a theory, nor did he have any physical explanation for what he had found, although he was actively seeking one.

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Actually, however, five years earlier, a young theorist, Lyman Spitzer, Jr., then a postdoctoral fellow at HCO, had predicted the very same concept on theoretical grounds, though Baade was not aware of it. Very few astronomers were, for Spitzer had not published it. In 1939 Hans Bethe had convincingly proved that energy radiated by stars at their surfaces is “produced” (or liberated) by nuclear reactions deep in their interiors. Earlier theorists, going back to Arthur S. Eddington, had argued that nuclear energy must be released somehow in a conversion of mass (rest energy) to heat, and in 1919 Henry Norris Russell, a great problem-solver who with his contemporaries knew nothing at all about the physics of thermonuclear reactions, had deduced that the “unknown process” depended strongly on temperature, two decades before Bethe. Russell knew then that “induced radioactivity” (nuclear reactions in our terminology) were the only possible energy source, and in 1920 Eddington speculated that “subatomic [nuclear] energy” was released either by some mass-annihilation process or by hydrogen combining to form “more complex” (heavier) elements. But nuclear physics was in its infancy then, and it was impossible to proceed further. Very soon after Russell had recognized his early form of what we call the Hertzsprung-Russell diagram, he had realized that it would be an important tool for studying stellar evolution, and that the existence of the two classes of stars, “giants” and main-sequence stars or “dwarfs,” would prove a key to understanding it. At first Russell had thought that the giants might be the earliest stage of a star’s life, and that after it had reached the dwarf stage it would evolve along the main sequence to fainter, cooler, “later” types. However, advances in theoretical understanding of stellar interiors soon ruled out that idea. Later he suggested several more complicated, tentative, partial theories. Russell’s physical reasoning was excellent, but as he emphasized, without knowledge of the unknown process or processes of energy liberation, it was impossible to trace the evolutionary path a star would follow or to make any quantitative predictions about its evolution. By 1937 his qualitative “theory” had become very complicated, but still incomplete. But then two years later, Bethe, a refugee from Hitler Germany, had shown quantitatively that this process was the carbon cycle, a chain of nuclear reac-

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tions converting hydrogen to helium through interactions with carbon and nitrogen isotopes.41 Bethe himself, Spitzer, Martin Schwarzschild, then at Harvard with Spitzer, and a few other theoretical astrophysicists immediately realized that this new knowledge set an upper limit to the lifetime of any star; it could not radiate longer than it would take to convert all its hydrogen to helium. From the roughly known masses of a few representative stars, the conclusion was inescapable that the most luminous supergiants and hot O stars, even if they were originally composed of pure hydrogen, could not have been shining for as long as the age of the Galaxy, which had to be at least several billion years old, its accepted age in the 1940s and 1950s. Thus these luminous stars must be young, recently formed objects. The only thing they could have formed from was material that was not stars, that is, interstellar material. Spitzer recognized that supergiant stars are found only in spiral galaxies, rich in interstellar matter, and now he understood why. High-luminosity stars were always associated with interstellar matter because they had formed recently from it. In a rotating system, namely a galaxy, interstellar matter would tend to sink to a plane, and it was there that gas and high-luminosity young stars would be found. Such high-luminosity stars, interstellar gas, and dust are most prevalent in Sc spiral galaxies, which Spitzer therefore regarded as one extreme type of stellar system. The other extreme was systems without interstellar matter, namely elliptical galaxies, which Spitzer knew do not contain high-luminosity stars. Baade’s great discovery, five years later, was that the upper limit to the luminosity in such systems is about Mv = −3; Spitzer, with only a rough estimate of the luminosities of the brightest stars in elliptical galaxies (based on the fact that they had not been observed individually), realized that they could be as old as the earth. His two extreme types of star systems, “Sc systems” and “Globular systems,” rich in interstellar gas and dust or free of it, containing young stars or not containing such stars, closely paralleled Baade’s later observationally discovered populations I and II. Spitzer included these ideas in the draft of his first paper on the dynamics of the interstellar medium, which he prepared in 1940,

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after he had become a young faculty member at Yale. He intended them as the introduction to it, to explain why his dynamical study was important. But two senior colleagues to whom he sent the draft paper both advised him that these ideas were too speculative, too “philosophical,” and that he should leave them out and stick to the quantitative, mathematical hard science that made up the rest of his paper. He did so, and Baade discovered the two stellar populations on the basis of the hard evidence on his plates of M 31 and its elliptical companion galaxies, and his years of globular cluster and RR Lyrae variables research.42 Russell, keenly interested in every facet of stellar astronomy, undoubtedly discussed this early work with Spitzer, his former student, prote´ ge´ , and confidant. The older theoretical astrophysicist pushed these ideas further himself, particularly for the specific example of Y Cygni, a spectroscopic, eclipsing binary supergiant, with known mass and luminosity. In a paper he presented at the dedication of Tonanzintla Observatory in Mexico in 1942, Russell concluded that this star must have formed recently from interstellar matter. In his paper he referred to Spitzer’s work, and to related investigations by Fred L. Whipple, who had been at Harvard and had also heard Spitzer describe his ideas.43 Cecilia PayneGaposchkin, who had been at the Tonanzintla dedication, grasped the implications of Russell’s paper immediately and combined them with the observational results on variable stars and novae, which she knew well. She wrote a popular article on “problems of stellar evolution,” published in Sky and Telescope a year before Baade’s discovery, in which she summarized Russell’s, Spitzer’s, Bethe’s, and Whipple’s theoretical ideas. If Baade read it, which is unlikely, he did not believe it, and parts of it were wrong (following Russell’s early ideas, she thought that giants and supergiants were “infants,” younger than main-sequence stars), but parts of it were right.44 In 1944, when Baade wrote his two papers on the stellar populations, he sent his final drafts to Russell for his advice and comments before submitting them for publication. The Princeton astrophysicist, a Carnegie Institution research fellow who visited Pasadena frequently and dispensed theoretical advice, approved highly of

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Baade’s papers. In his letter to the Mount Wilson astronomer, he called “the two types of relation between color and magnitude” (the two populations) “a major discovery . . . which . . . will probably have important consequences in the future.” Russell wrote Baade that his draft papers tied up with the ideas the older man had expressed at the Tonanzintla dedication “to the effect that super-giant stars are relatively short-lived, and are still forming but only in specialized regions” (interstellar matter). Thus Russell had immediately grasped that the two populations were young stars and old, but Baade evidently did not understand what he meant or follow up on it.45 Gamow, the Russian-born theorist of nuclear reactions, also realized, even before Baade’s papers appeared, that nuclear burning would have important consequences on the aging, or “evolution,” of high-luminosity stars. He was hampered by his lack of knowledge of the empirical data on the properties of stars, but in 1944 wrote Adams to express some of his ideas on the Hertzsprung-Russell diagram and its relationship to stellar evolution. Adams took Gamow’s theoretical ideas seriously, but could not make much out of them, expressed in the emigre´ physicist’s ungrammatical, badly misspelled version of English. The Mount Wilson director did not realize that they were very closely related to Baade’s two stellar populations, and evidently never passed the letter on to him.46 Thus by 1947, after World War II had ended and the physicists and astrophysicists had come home from their wartime laboratories, several theorists realized that Baade’s populations I and II were almost certainly young and old stars respectively. But Baade himself initially resisted this interpretation. Whipple and Cecilia Payne-Gaposchkin both suggested it to him in 1947, but he rejected it. He scoffed at the idea that stellar evolution was the reason for the difference between the two populations, writing that “all the astronomical Darwinism of the last decades had led only into blind alleys.” He saw on his direct photographs of M 31 that the highluminosity stars were always associated with interstellar gas and dust, but he could not grasp that was because they had recently been born from that same gas and dust. Baade saw the “globules,” small, dense masses of dust, which Bart J. Bok had postulated as

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the sites of star formation in nebulae, but did not believe they were that important, even though he wrote in the same letter his own observational conclusion, “[n]o dust, no stars.”47 In 1948 Russell published a short note explicitly stating that the high-luminosity stars of population I could not survive for the age of the Galaxy and must be young objects, which had formed recently from the interstellar matter in which they were always involved. Still Baade did not accept it.48 He thought the Russell-Vogt theorem (an existence theorem) implied that a star’s surface temperature (essentially its spectral type or color index) and luminosity are completely determined by its mass and composition, but this was a misconception; actually Russell had carefully pointed out years before that the run of the abundances of all the elements through the star could also play an important role. But nearly all astronomers were so used to thinking of the abundances as fixed quantities which never changed with time, or from place to place, that this had seemed an unimportant quibble, which was soon forgotten. They had not realized that the very energy-producing reactions which make a star shine, also change the relative abundances of hydrogen and helium within it.49 Baade was converted to recognizing age as the main difference between the two populations in the spring of 1950, when he spent a month at Princeton University, giving a series of ten lectures to the small group of faculty members and graduate students in its astronomy department. This was the first of many similar lecture series Baade was to give in the ensuing decade, which were far more influential in spreading his concepts and ideas than his relatively few published papers. By 1950 Spitzer and Schwarzschild were the key Princeton astronomy professors, and Russell, though retired, attended all the lectures and participated vigorously in the discussions. Afterward Baade remembered the weeks in Princeton as one of the golden periods of his life, with “an atmosphere as stimulating as Go¨ ttingen during my student days.” Spitzer and Schwarzschild were “a wonderful combination.” They explained the theoretical concepts of nuclear reactions and how they must affect stellar evolution to him in terms he could understand, and discussed his data with him from fresh, new angles.50

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L. C. Eichner (left), Martin Schwarzschild (seated at an Eichner iris photometer), and Walter Baade (looking on), probably in New Jersey, 1950. (Courtesy of Leiden Observatory.)

Baade especially related to Schwarzschild, whose father Karl had been the leading German theoretical astrophysicist of the previous generation, until his untimely death during World War I while serving as an officer in the German army. According to Hitler’s racial edicts, Martin like his father was a “Non-Aryan”; he had earned his Ph.D. at Go¨ ttingen in 1935, escaped to a fellowship in Norway, and emigrated to the United States. He served in the American army during World War II, and longed to get into the shooting war, but ended up interrogating German prisoners and seeking technical intelligence just behind the lines. A small, intense, talkative, sensitive, hard-working, and fantastically considerate individual, he had already resolved to make studying stellar evolution his life work. Schwarzschild and Baade hit it off very well together, talking science constantly (always in English), and by the end of March 1950 Baade was convinced. No doubt a paper by the “good Ger-

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man” theoretical physicist, Carl F. von Weizsa¨ cker, who had worked on Werner Heisenberg’s atomic-bomb project but then visited America in 1949–50, also influenced Baade’s thinking. He respected the theorist and discussed his ideas on stellar evolution, which were similar to Bethe’s, Gamow’s, Spitzer’s, and Schwarzschild’s, with him.51 One result of Baade’s visit to Princeton was a joint paper in which he and Spitzer suggested that S0 galaxies in rich clusters such as Coma had lost their gas and dust in high-velocity “collisions” between pairs of galaxies, in which the stars passed right through but the interstellar clouds collided and were left behind. In this paper they explicitly stated that the most luminous population I stars were young, while the population II stars could be as old as the galaxies themselves. From then on Baade was to use and exploit this concept frequently.52 Caltech had begun its new graduate program in astronomy and astrophysics under the leadership of Jesse L. Greenstein in 1948. Two of its very first graduate students were Allan Sandage, who was assigned to work as Hubble’s research assistant, and Halton Arp, who did the same for Baade. Both of them began theses under Baade’s supervision, measuring color-magnitude diagrams of globular clusters to provide the quantitative data on population II which Baade needed. His primary aim was to fit their measurements of the main-sequence stars in these clusters (which could be reached on direct plates taken with the new 200-inch telescope) to the main sequence of nearby stars with known distances, thus determining the absolute magnitudes of all the population II stars in the globular clusters. Baade was particularly anxious to make an independent determination of the absolute magnitudes of RR Lyrae variables, the most abundant type of population II variable stars, in this way. Arp and Sandage used the methods of photographic photometry Baade had taught them; William A. Baum, a recent Caltech Ph.D. in physics whom Bowen added to the Mount Wilson staff as its photoelectric expert, set up the standard sequences around each cluster which they used to calibrate their measurements.

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The first results of this program, presented by Arp, Baum, and Sandage at a symposium at an AAS meeting in Cleveland at the end of 1951, showed that the cluster main sequences could be made to match the nearby stars up to absolute magnitude Mv ≈ 3.5, but then deviated into the subgiant region, just as Baade had sketched it schematically in his famous diagram in his 1944 paper. The most straightforward interpretation was that all the stars in a globular cluster had formed at essentially the same time, initially populating the main sequence up to high luminosity, but the more luminous stars exhausted some or all of their fuel and moved away from it. The color-magnitude (or Hertzsprung-Russell) diagram itself demonstrated stellar evolution.53 By that time theorists including Gamow, Chandrasekhar, Mario Scho¨ nberg, and Schwarzschild had begun exploring stellar evolution, and realized that main-sequence stars would evolve slowly at first, remaining close to their initial luminosities until they had exhausted all the hydrogen in their hot central convective cores, where the nuclear reactions were fastest. But when this central fuel supply was used up, the stars would expand and thus evolve rapidly to the subgiant and giant stages. Schwarzschild had the right combination of physical insight, mathematical skills, astronomical knowledge, analytic reasoning powers, and determination to reach the solution first. While Schwarzschild was visiting Pasadena in early 1951, Baade had Sandage show him his preliminary results for M 3, which seemed to reveal the main sequence beginning at visual magnitude mV ≈ 19. Schwarzschild at once suspected that the turn-off immediately above it marked the Scho¨ nberg-Chandrasekhar limit, where the rapid stellar evolution would begin.54 After the 1951 symposium, at which Schwarzschild gave a preliminary theoretical interpretation of the observational data, he, Baade, and Spitzer arranged for Sandage, still a Caltech graduate student, to come east to Princeton and work with him. There Sandage helped numerically integrate the stellar-interiors equations to model the evolution of stars just as they burned the last fuel in their convective cores, which then contracted until the remaining hydrogen just outside the core became hot enough to ignite. These models reproduced remarkably well the observed “turn-off” from the

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main sequence in the globular cluster, thus confirming the entire picture. Finally, they made it possible to determine the initial parts of the evolutionary tracks of stars in the color-magnitude diagrams, and to calculate the time the stars took to reach any point on these tracks. It was a most important result, achieved by two of Baade’s favorite prote´ ge´ s, and from then on he never doubted the physical meaning of the two populations.55 Throughout Sandage’s stay in Princeton, Baade kept him informed of progress in Pasadena and Palomar, including the excellent series of plates he obtained of M 3 with the 200-inch, to enable his student, on his return, to push the measurements down to as faint a magnitude limit as possible. Again and again in his letters, Baade expressed his great respect for Schwarzschild’s theoretical work and his excitement at the new results Sandage was reporting to him.56

Baade’s Disciples Throughout his career, and particularly after he announced his concept of the two stellar populations, Baade played an important role in inspiring and guiding some of the most productive astronomical research workers in America. Not only Sandage and Arp in Pasadena, and Mayall and Shane at Lick, but numerous other top-notch observational astronomers sought his guidance and delighted in showing or sending him their most recent results. Baade had read and seemed to remember almost every published paper on galactic structure and dynamics, and always had time for serious discussions with real scientists who visited him in his office, or for writing long letters to them. But he had no sympathy for the “weak pets” of scientific “politicians,” as he called unproductive poseurs and superannuated relics of the past. One highly productive contemporary of Baade’s, who depended heavily on him for advice, particularly after Hubble’s death, was Humason. A highly skilled observer with practically no scientific education, he, Mayall, and Sandage published a most important paper in 1956 on the redshift-distance relation, full of new data and giving an updated value of the Hubble constant. This paper owed much to Baade’s behind-the-scenes influence on all three of them, particularly his continued insistence

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on the importance of accurately measured apparent magnitudes on a well-defined photometric system.57 With Stebbins retired and moved to California, Whitford, his successor as director at Wisconsin, made frequent trips to Mount Wilson to continue precision photoelectric photometry with the 60-inch and 100-inch telescopes. Baade encouraged him to measure distant galaxies, RR Lyrae variables, and other stars in and around the globular cluster in “Baade’s window” near the galactic center, and Whitford responded with data.58 At Lick, Kron and Eggen, both Stebbins’s former students, were now applying much improved versions of his photoelectric techniques to population questions, and Harold F. Weaver, who had worked with Baade at Mount Wilson for a year as a postdoctoral fellow, was applying photographic photometry to similar problems. They all sought Baade’s guidance, and he dispensed it freely, sometimes intervening tactfully between them when their competition became too intense.59 Arthur D. Code and Johnson were two other photoelectric observers whom Baade frequently advised and encouraged. He was glad to get these young observers’ data and results, for they were all engaged in population research, four of them using the best modern detectors, much more sensitive and accurate than the older photographic plates. One result they all confirmed was what Baade had seen qualitatively on the early 48-inch Schmidt plates. There was obscuring dust near the galactic plane in all directions; it had a very spotty irregular distribution; but there were no extinction-free paths or real “windows” to distant clusters and high-luminosity stars. All the old ideas about star counts and ignoring extinction corrections were wrong. Baade seemed to relish stating these facts, so contrary to the old Kapteyn assumptions, matter-of-factly but forcefully in a report to the International Astronomical Union in 1950.60 At Harvard, Payne-Gaposchkin had quickly grasped Baade’s population concept, and applied and extended it in her own research. She had an excellent English education in physics and astronomy, a Radcliffe Ph.D., and a deep knowledge of stellar spectra and variable stars. As a woman astronomer at Harvard in those

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days, she was condemned to a poorly paid assistant’s position, in spite of her doctoral degree and her keen, creative research mind. Shapley had assigned her to work on photographic photometry, though she greatly preferred stellar spectroscopy, so much more meaningful in gaining physical understanding of variable stars. In early 1948, after the AAS meeting in Ohio at which Baade was the main speaker and she had presented her ideas on the space distribution of variable stars in the Galaxy, related to the two populations, he had urged her to come to Pasadena and work with him there, as a visitor. However, both Bowen and Shapley saw problems with that plan, and it never came about.61 Payne-Gaposchkin was more highly valued in her native England than by the Harvard director. In 1952 the University of London invited her to give a series of lectures, which she converted into a book, Variable Stars and Galactic Structure, published in 1954. In it Payne-Gaposchkin adopted the population idea as the key to understanding stellar evolution. Baade had explained his own thoughts on long-period variables, Cepheid variables, and RR Lyrae variables to her at length; in general he believed that there were two discrete populations, and every star belonged to one or the other. To her it was more natural to think of the age of a star as a continuous variable, with two extreme values, the oldest and youngest, but with a continuum of ages between them. Baade’s resistance to this thought was probably the subconscious reason he had held out against the stellar evolution interpretation so long. But he encouraged her to write the book, and considered her as the most knowledgeable research worker on variable stars and stellar populations he knew.62 Payne-Gaposchkin’s husband, Sergei, was a different type of scientist altogether. He had practically no training in astronomy, and no creative ideas whatsoever. Nevertheless he had a long-term job at Harvard, estimating magnitudes on photographic plates others had taken, no doubt with the hope that his wife would keep him working productively. Baade, highly sympathetic to her, and badly needing an assistant to measure the light curves of the RR Lyrae stars and other variables he had found on his 100-inch plates of

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“Baade’s window,” taken in 1945, invited Sergei to collaborate with him on that project. It was a long, tedious job. Gaposchkin ended with many incorrect periods for the RR Lyrae variables, an understandable error because the plates of the galactic center region, necessarily taken at intervals of almost exactly one day, could all too easily yield an “aliased” or false period. Baade was well aware of this problem and had warned him against it; Gaposchkin insisted he had avoided this mistake, but he had not. Baade remained suspicious and evidently did not trust Gaposchkin’s statement, for the light curves appeared in a paper signed by the Harvard astronomer alone. After that Baade gave him as little raw data as he could. Doing the best you could was fine, but saying that you had done better than you had was not.63 One of Baade’s closest friends in American astronomy was Jason J. Nassau, whom he had first met at the dedication of McDonald Observatory in 1939. Nassau, an ethnic Greek born in Smyrna, Turkey, was the same age as Baade. He had come to America with his family as a youth, had studied civil engineering at Syracuse University, become a U.S. citizen, and served in the American Army in France in World War I. After the war ended he studied for six months in an army program at the University of Edinburgh, and then returned to Syracuse, where he completed his Ph.D. in mathematics in 1920. Almost immediately he joined the faculty of Case Institute of Technology in Cleveland, where he became a full-time teacher of mathematics and astronomy (for civil engineers, basically surveying), and director of its little Warner and Swasey Observatory.64 Before long Nassau was a well-known popularizer of science in Cleveland, but he longed to do research, and in 1927 spent a sabbatical year at Cambridge, working on theory and astronomical statistics, guided (loosely) by Eddington.65 In 1939, after the death of A. P. Burrell, the Warner and Swasey Company’s chief engineer, his widow, no doubt inspired by Nassau, decided to contribute, with friends among the company’s executives, the money needed to provide a new, larger telescope for the observatory. Warner and Swasey had built the McDonald Observatory 82-inch reflector, and Nassau was one of the astronomers

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invited to its dedication, along with many of the stars of the American research community, including Baade. The two of them happened to be assigned to stay in the same dude ranch quarters in little Fort Davis, Texas, and hit it off very well. Within a few days they were fast friends.66 Baade advised Nassau not to use the precious funds on a conventional telescope, which could only be a medium-sized one, but instead to have a Schmidt telescope built with which he could really do something. Nassau leaped at the idea. Robert Lundin, the optician who had made the 82-inch mirror, produced the optics for a 24-inch Schmidt which, when it went into operation in 1942, was briefly the largest telescope of its kind in existence. Baade had provided a great deal of practical advice, including the best focal ratio to adopt, taking account of the bright sky in the Cleveland suburb where the observatory was located, measured by Nassau’s test exposures with a small Ross camera. The Burrell Schmidt, as it was named, was outfitted with an objective prism at Baade’s suggestion, and Nassau and a series of younger faculty members were soon using it very productively in population research on M stars.67 They observed in the red and near infrared, found the M giants with their objective prism, and measured their magnitudes photographically by comparison with sequences which they set up in Selected Areas to meet Baade’s specifications. Nassau, with his German friend’s help, had switched from being a minor mathematical theorist to an important galactic-structure observer. Baade constantly guided and encouraged Nassau, who responded with important research results. Nassau was hard working, dedicated to astronomy, highly intelligent, diplomatic, and personable, and after World War II ended he became a power in the American Astronomical Society. He was elected and re-elected its treasurer for several terms, making him a long-term member of its council, and he was also chairman of the U.S. National Committee for the International Astronomical Union for eight years. This gave Nassau an important voice in the IAU’s affairs as well, and he strongly supported Baade in it and in the AAS. The two men had the same general build, height, hair style

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and color, prominent hawk nose, and liked to dress alike at scientific meetings which they both attended. Frequently in group pictures they stood at opposite ends of the front row, often striking very similar poses, which they must have rehearsed or at least discussed in advance. Baade, his friends, and his colleagues, from Oort and Mayall, Minkowski and Humason, Payne-Gaposchkin and Nassau to his thesis students Sandage and Arp, were to accomplish much in astronomy in his lifetime and after it.

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Radio Astronomy and the Size of the Universe PA L O M A R A N D PA S A D E N A , 1948 –1958

Radio Astronomy and Spiral Arms Although Karl Jansky had detected radio-frequency continuum radiation from the Milky Way as early as 1933, and Grote Reber had confirmed it in 1940, radio astronomy did not really get under way as a scientific field until after World War II. As the war ended, several groups of British and Australian radar “boffins” shifted their research interests to the peaceful heavens. They easily detected the sun, and it was natural for them to begin building larger, more directional antennas, and more highly sensitive receivers, to search for other “radio stars.”1 One set of observations, however, was planned from the beginning for detecting interstellar gas in our Galaxy. Jan H. Oort, quietly sitting out the war and pondering galactic structure at his hideaway in eastern Holland, realized that although Jansky’s and Reber’s continuum radiation was important, if there was just one radio-frequency spectral line, it would be even more important. Such a line would provide the possibility of measuring radial velocities of interstellar gas throughout the Galaxy. Oort had confirmed the main ideas of galactic rotation in the late 1920s, and he knew that there was a well-defined relationship between radial velocities, distances, and angles. Hence measured systematic radial velocities could be converted to approximate distances in a straightforward way. Sometime during the war he asked Hendrik van de Hulst to investigate theoretically whether there was a suitable line. The brilliant

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young Utrecht student reported in 1944 that the hyperfine-structure transition of atomic hydrogen, the H I 21-cm line, should be measurable. Oort filed this problem and its solution away for immediate postwar attention.2 Once he began corresponding with Baade, after the liberation of the Netherlands, Oort soon realized that mapping the interstellar gas in the galactic plane was even more important than he had earlier realized. Baade’s letters and papers made clear that the spiral arms in the Andromeda galaxy and other nearby galaxies were primarily concentrations of gas, dust, and O stars, not of average stars as all previous theorists had assumed. Locating concentrations of 21-cm emitting interstellar atomic hydrogen in our Galaxy would mean discovering its spiral arms. In 1947, before Oort’s visit to Pasadena at the end of that year, Baade and Nicholas U. Mayall had both written to him about their joint work on M 31, which included determining its rotation curve from radial velocities of emission nebulae. Undoubtedly Oort saw all Baade’s direct photographs and other evidence during his stay.3 Back in Holland the Leiden director, organizing support for a Dutch radio-astronomy observatory, planned to find and measure H I 21-cm line radiation. This program was much better suited to the low, flat, cloudy Netherlands than optical astronomy. Occupied by the Germans through the war, Holland had no experienced radar scientists or equipment of its own, but it did have a strong technological base at the Phillips Eindhoven Laboratory, and more than one liberated German “Wu¨ rzburg dish” antenna to use as a radio telescope. Oort used all his prestige as the leading Dutch astronomer and friend of Baade to make 21-cm astronomy the key observational project in the country, and personally pushed it through. He called on Baade for help in persuading Netherlands Foundation for Radio Astronomy board members who visited California how important this research was. In June 1951 Oort and C. A. Muller, his chief radio engineer, were able to announce actual detection of 21-cm line radiation from interstellar hydrogen, only seven weeks after Edward M. Purcell and H. I. Ewen, both veterans of the highly successful American wartime radar crash program, had detected it at Harvard.4

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Oort converted his accomplishment into more government support for a bigger and better 21-cm observatory. Many of the very best young Dutch astronomers of this period worked on reducing, analyzing, and interpreting H I interstellar line radiation. Oort himself described the aims and methods of the program, and showed some of the earliest data from it in his Henry Norris Russell Lecture at the AAS meeting in Cleveland at the end of December 1951. In it he mentioned that if our Galaxy were a spiral (which he must have known it was), the arms would be recognizable in the 21-cm line contours.5 However, Oort and his group were not to be the first to discover the spiral arms. At Yerkes Observatory William W. Morgan, who had read Baade’s 1944 papers soon after they were submitted for publication, who had supervised the assistants who prepared the photographic prints which were inserted into every issue of the Astrophysical Journal as illustrations for one of the papers, and who had heard Baade’s invited lectures at Perkins Observatory in 1947 and at Palomar in 1948, had been inspired by them. His research on spectral classification enabled him to jump right into the problem of finding the spiral arms himself. He recognized that the emission nebulae (or H II regions) in our Galaxy must lie in (or define) its spiral arms. Morgan and his students identified as many large, bright H II regions as they could; then he determined the spectral types and luminosity classes of the hot O and B stars in them. These led directly to their absolute magnitudes; photoelectric color indices (mostly previously measured by Joel Stebbins, Albert E. Whitford, and C. Morse Huffer), combined with the “normal” or intrinsic color indices Morgan had determined for each type, gave the distance to the star. Most of the H II regions contained several hot, high-luminosity stars, each yielding an independent distance determination; the average of all the stars in one H II region thus gave a good value of its distance. Plotting these H II regions revealed portions of two spiral arms, one through the sun, the other approximately one kiloparsec further out from the galactic center. Morgan presented this result in a symposium at the same Cleveland meeting, in a session at which Oort himself presided. He congratulated Morgan, and the audience gave him an unprecedented standing

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ovation. Baade was not present, but enthusiastically endorsed the published map of the fragments of spiral arms.6 Although Morgan, Whitford, and Arthur D. Code soon extended this work and found a part of the next spiral arm further in toward the center, this optical method is fundamentally limited to a range of a few kiloparsecs by strong interstellar dust extinction close to the galactic plane. The H I measurements were better, because the long-wavelength radio-frequency radiation was completely unattenuated; however the assumption that all gas moved in exactly circular orbits, with a rotational velocity which depended only on distance, was an oversimplification that introduced unknown, presumably small, errors into the resulting maps. Oort sent Baade some of the first results in July 1952, and he now became even more enthusiastic over them, because they showed so much more of the spiral pattern than “the pedestrian [optical] methods.”7 Van de Hulst, Muller, and Oort published a long paper in 1954, in which they derived the positions of more spiral arms at larger distances from the sun and the galactic center. Another one of Oort’s best graduate students at Leiden, Maarten Schmidt, used the rotational velocities derived in this survey for the inner parts of the Galaxy, together with stellar-velocity data closer to the sun’s distance, to derive an improved model for the overall distribution of mass in the Galaxy. He had been working on this project for two years, after publishing earlier first-class shorter research papers on comets (collecting the photometric data on the differences between “new” and “old” comets), which helped define the concept of the Oort Cloud, and on the solar corona.8 Oort wanted to send him to Pasadena for further seasoning, and in the fall of 1954, fully two years before Schmidt was to come, Baade “reserved” one of the Carnegie Postdoctoral Fellowships which Ira S. Bowen controlled for him. “His later application will be a mere formality,” Baade assured Oort, but Schmidt should be sure to submit the necessary form in early 1955. It was up to Oort whether to send Schmidt for one or two years, Baade wrote the Dutch director; two years would give him a better chance to tackle a major observing program, but if he could not be spared that long, one year would be enough. Furthermore, Baade wrote, Bowen had promised “to respect [Oort’s]

Speakers at the Symposium on the Structure of the Galaxy, at the dedication of the Curtis Schmidt telescope, Ann Arbor, 1950. Left to right, front row: Freeman D. Miller, Karl Henize, Harlow Shapley, Walter Baade, Jason J. Nassau, Leo Goldberg; rear: Giorgio Abetti, Bertil Lindblad, William W. Morgan, A. N. Vyssotsky, Nicholas U. Mayall, Rudolph Minkowski, Joel Stebbins, Sidney W. McCuskey. (Courtesy of the Mary Lea Shane Archives of the Lick Observatory.)

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plan” and not offer Schmidt a job to stay at Mount Wilson and Palomar Observatories as a staff member. But Oort had better speak to Jesse L. Greenstein “since it is his dream to have a group of bright young men around him at Caltech.” Oort agreed he would favor Schmidt’s staying in Pasadena for two years on general principles, but could not yet estimate “whether we could really spare him during so long a time.” In those bad old days, directors were accustomed to thinking they could move eager young astronomers around like blocks of wood, but in the end Baade always sided with the youngsters and advised them to go their own way. Schmidt did send in an application, proposing a number of excellent research problems he could carry out with the big Mount Wilson optical telescopes, and he got the fellowship.9 In California he made a good start on several of them, but spent part of his time working out a mass model for M 31, based on new 21-cm data on it, which the Dutch radio astronomers had obtained with their big new dish at Dwingeloo. Baade loved this work, paralleling so closely his own optical program of comparing our Galaxy and the Andromeda galaxy, using insights gained from each to help understand the other. No doubt at Oort’s insistence, Schmidt’s institutional affiliation was stated in this paper as “Temporary Carnegie Fellow at Mount Wilson and Palomar Observatories.” Before he left, Schmidt began a new, semitheoretical study of the rate of star formation in the Galaxy; the unknown or unrecognized concept of just a decade ago had become his hot new research topic. Bowen did abide by his promise and Schmidt did return to Leiden in 1958, after two years in Pasadena. But Greenstein lured him back to Caltech in 1959, where he quickly became an outstanding observer with the 200-inch Hale telescope and an authority on galaxies, radio galaxies, and quasars, in many ways a latter-day amalgam of Oort, Baade, and Rudolph Minkowski.10

Radio Sources and Supernova Remnants While early 21-cm line research was dominated by traditional astronomers, especially Oort and his students in the immediate postwar years, radio-continuum research was led by “radiophysicists,”

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experts on receivers, antennas, and electronic techniques, who were quite unfamiliar with the astronomical literature. Baade, with his lively, friendly personality, his relatively open mind, and his diplomatic way of imparting information, got along well with both groups. Early postwar radio astronomy, carried out at long wavelengths by today’s standards, had very poor angular resolution. Some measured positions were in error by several degrees, as later, better-calibrated programs revealed. Nevertheless, it soon became evident that radio sources were not bright stars, or even any type of stars. The brightest radio source in the sky, Cygnus A, long remained unidentified. Taurus A was one of the very first sources to be identified, by John C. Bolton, Gordon J. Stanley, and O. B. Slee at the CSIRO Radiophysics Laboratory in Australia. It was NGC 1952, the Crab nebula, as they reported in a 1949 paper. In tracking down this identification, Bolton had written Oort and Minkowski, among others. He had chosen between Baade and Minkowski by flipping a coin. Oort replied with a five-page disquisition on the Crab nebula; Baade answered instead of Minkowski. Bolton addressed his next letter to Baade; Minkowski replied to it. They were both highly interested in identifying radio sources, especially as one of them had turned out to be their favorite supernova remnant. The other two radio sources Bolton, Stanley, and Slee identified, Virgo A and Centaurus A, were the bright galaxies M 87 and NGC 5128 the two astronomers also knew well. Baade and Minkowski plunged into the game and quickly became the acknowledged world leaders in the search for further identifications. With the largest telescope in the world at their disposal, large amounts of observing time with it, and their unrivaled knowledge of supernova remnants, nebulae, and galaxies, they were an ideal team. Baade took most of the direct exposures; Minkowski concentrated on spectroscopy but also obtained a few direct photographs. Radio astronomers throughout the world sent them positions of new sources they discovered; the two Palomar astronomers emphasized over and over again the necessity of improving their accuracy. Agreement with an optical position to a few seconds of arc was convincing evidence; agreement to a degree left open the possibility of identification with many other objects. Angular-size measurements also helped.11

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In 1951 Francis Graham Smith, using a large radio interferometer at Cambridge, England, measured much better positions of Taurus A, Virgo A, Cygnus A, and Cassiopeia A. He published them and sent them to Baade. By then the Palomar astronomer regarded the identifications of Taurus A and Virgo A as certain, not only because of positional agreement with the optical objects but because they both were such unusual objects, a supernova remnant and a giant elliptical galaxy with a jet extending out of its nucleus. Baade did not know what the jet was, but he knew it was unique. With Smith’s new positions, Baade and Minkowski were soon able to identify Cygnus A with an unusual galaxy, the brightest in a cluster of galaxies, and Cassiopeia A with a faint, heavily reddened, intricate, filamentary nebula highly reminiscent of the Crab nebula. Minkowski obtained spectra of the brightest filaments which showed unusually strong forbidden [N II] and [O III] emission lines. Similarly, Baade’s direct photographs of Cygnus A showed a most unusual structure, which he interpreted as two galaxies in collision. Its spectrum, taken by Minkowski, showed rare high-ionization lines, especially [Ne V], which were broadened by velocities of 1500 km/sec. Both these identifications as radio sources were thus almost certain, from positions and from near- uniqueness, but with similarities to the few previously identified radio sources. Baade and Minkowski, with the 200-inch, had broken the radiosource puzzle.12 They began writing two long papers, discussing critically all the optical identifications and adding as many more as they could. Meanwhile Baade informed Oort of the new results, gloating that he had won a bottle of scotch whiskey from Minkowski, who at first had not believed that Cygnus A was a pair of colliding galaxies, but now had to admit it was from the evidence in his own spectrograms. Oort congratulated Baade but expressed skepticism about the collision, and even more skepticism that there could be enough such interactions to account for many radio sources; Baade acknowledged that he was afraid it might be a fantastic coincidence, but he was trying to get more evidence at the telescope, one way or the other. He pointed out that Minkowski and Humason had found one other object they thought was a similar collision in prog-

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ress, NGC 1275 in the Perseus cluster of galaxies, and added that tidal distortions in close passages should be much more common than head-on collisions. All of these ideas became key issues in subsequent studies of Seyfert and radio galaxies in the ensuing half century.13 The news of Baade and Minkowski’s spectacular new results on radio-source identifications began to leak out as soon as they obtained them. In the fall of 1951 Harlow Shapley had heard rumors of their preliminary conclusions on Cygnus A and Cassiopeia A and wrote to ask Baade for more information, because he wanted to mention it “as one of the ten [astronomical] highlights of the year” in the annual list he was accustomed to releasing “to local astronomers” (and actually simultaneously to newspapers, magazines, and the Associated Press). Baade by this time regarded Shapley as more of a publicity hound than a serious scientist, and replied that it was far too early for public statements. He had only obtained the data at the telescope the previous month. Baade gave Shapley just enough of the facts to establish that he and Minkowski had made the identifications, but none of the details, and included a firm injunction that nothing should be published. However, when Donald H. Menzel, also at Harvard, asked for the same information, giving as his reason that he was working on a new theory of “radio stars,” which he hoped the new observational data would confirm, Baade promptly sent him their results. He was much more favorable to Menzel, a jaunty, younger admirer. Baade encouraged him to go on with his theoretical work on radio sources, and to present the results at a colloquium in Pasadena soon.14 Baade and Minkowski published their two long papers on the optical identifications of radio sources in 1953. Although they could find no optical remnant of B Cassiopeia, Tycho Brahe’s supernova of 1572, which Baade had searched for with the 100-inch in 1941 before radio sources had been discovered, by now one was known to be at its position, which they tentatively identified with it. Baade had hoped then that the 200-inch would reveal it, but the heavy dust extinction still proved impenetrable. They also identified Puppis A with a filamentary nebula similar to Cassiopeia A. The 48-inch Schmidt telescope was essential for this identification be-

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cause of the source’s far southern declination. These two joint papers established that “many” peculiar galaxies, such as M 87 and Cygnus A, supernova remnants, and filamentary nebulae (which Minkowski later established as older supernova remnants), were radio sources. They also estimated the radio-frequency luminosity of “normal” galaxies, and showed that they might account for many of the weaker sources. All subsequent work on identifications flowed from these two important papers.15 These identifications, together with the 21-cm emission-line work, linking atomic physics and the spiral arms in our Galaxy, suddenly made radio astronomy much more interesting to many physicists. Merle A. Tuve, director of the Department of Terrestrial Magnetism (DTM) in Washington, another branch of the Carnegie Institution of Washington, was anxious to get his organization into the subject in a big way. During World War II he had been a leader in the development of the proximity fuse, a tiny hard-wired radar set built into antiaircraft shells, which exploded them when they came close enough to a target enemy plane to damage it. It was a highly important weapon in winning the war, and Tuve had been decorated for it. He believed that physicists like himself could solve any problem in any “branch of physics,” but he thought that learning what a few of the best astronomers had been doing might be helpful to his team at DTM, and to others as well.16 Tuve therefore organized, on a crash program basis, the first international radio-astronomy symposium in the United States, held at the National Academy of Sciences in Washington in late April 1953. Most of the invited speakers were radiophysicists, including Martin Ryle of the Cavendish Laboratory at Cambridge University, England, A. J. Higgs from the Radio Physics Laboratory in Sydney, Australia, and Edward M. Purcell from Harvard. Another was Grote Reber, the electrical engineer and radio amateur who had been one of the earliest observers of galactic radio emission, from his backyard in Wheaton, Illinois. Tuve, who was much more free and easy with travel funds than the parsimonious Bowen, had invited Oort to come from Holland, but he begged off and sent van de Hulst instead. Baade was the other astronomer invited; he spoke on the optical identifications of the radio sources with supernova

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remnants, colliding galaxies, and peculiar nebulae which he and Minkowski were making and publishing.17 Traditionally, several other scientific societies and commissions met in Washington before, during, and after the National Academy meeting, and through a series of unlucky coincidences, on the very same Wednesday afternoon as the NAS symposium, another radioastronomy meeting was in progress at the National Bureau of Standards (NBS), a few miles away. This one was a strictly national affair. Minkowski and Greenstein, from Caltech, were the two optical astronomers invited to speak at it; they gave two joint papers, one of them on the Crab nebula. One speaker, John P. Hagen, head of the radio-astronomy group at the Naval Research Laboratory, spoke at both meetings, early on the program at the National Academy and then, probably after a hurried drive through downtown Washington, at the NBS meeting.18 Luckily Baade and Minkowski did not have to do that; they had split their appearances. They had not been in at the beginning of radio astronomy, but they were there from the scientific beginning of understanding what the “radio stars” were. The unknown mechanism by which the radio-frequency continuum is emitted in galaxies and supernova remnants turned out to be synchrotron radiation by relativistic electrons in a magnetic field. This solution had been suggested by Hannes Alfve´ n, Nicolai Herlofson, and Karl-Otto Kiepenheuer in 1950, and worked out quantitatively in great detail by Vitaly L. Ginzburg and other Soviet theorists within the next few years. This theory predicted that continuum radiation should be polarized perpendicular to the magnetic field. The brilliant Soviet astrophysicist Iosif S. Shklovsky then suggested that this synchrotron continuum was the source of the continuous spectrum of the Crab nebula in both the radio and the optical spectral regions. Viktor A. Dombrovsky at Byurakan Observatory and Mikhail A. Vashikadze at Abastumani confirmed Shklovsky’s suggestion by measuring the strong polarization in the optical region which he had predicted. However, because of the Cold War, communication between the two blocs was difficult; few Western astronomers were aware of his result, and those who did know about his paper were skeptical.

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These doubts began to change in 1954 when the Soviet Union held a symposium to celebrate the dedication of their rebuilt Pulkovo Observatory, which had been nearly destroyed during the siege of Leningrad during World War II. Two American astronomers attended the event, Dirk Brouwer and Jason J. Nassau. Baade had been invited but could not have gone even if he had wished to (which seems unlikely), because he had earlier agreed to give a two-week series of lectures at that very time in Berkeley, where his friend Otto Struve was now head of the University of California campus astronomy department. Oort was invited and did take part in the meeting. There he met the bright, alert Shklovsky in person, and learned in fascinating detail of his prediction and its observational verification.19 Oort was at least half convinced, for on his return to Holland he asked his photoelectric expert, Thedor Walraven, to try to measure the optical surface brightness of the Crab nebula in the continuum and to check to see if it were polarized. In 1955, in spite of the cloudy Dutch skies Walraven was able to get a few nights’ measurements, and discovered to his surprise that the polarization was quite high, 18% averaged over the “amorphous mass” which emitted the continuum. Measurements with a smaller diaphragm showed the amount and direction of the polarization varied systematically within it. Oort quickly notified Baade and suggested that he try to take direct exposures through a polaroid filter with the 200-inch, to obtain higher angular-resolution measurements of the polarization at each point in the nebula. He also reminded Baade of the “light ripples” which he had reported moving through the amorphous mass from a series of direct photographs he had taken in 1944 and 1945. Oort asked if they might possibly have resulted from bursts of high-energy electrons, released from the central star, exciting disturbances in the magnetic field which propagated outward.20 Baade was greatly excited by the letter; he had Alexander Pogo, the librarian at the Mount Wilson Observatory office, translate Shklovsky’s paper, and realized from it that the synchrotron mechanism would explain the observed features of the optical continuum of the Crab nebula, unlike all previous attempted interpretations in terms of “normal” thermal emission at any as-

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sumed temperature. Baade informed Oort of a paper by Carl O. Lampland, at Lowell Observatory, who had also noticed the light ripples years earlier.21 Meanwhile Oort, who was to see Baade at meetings in Hamburg and Dublin that summer, arranged to give brief reports of the exciting new result at a radio-astronomy meeting in Manchester in August and at a cosmic-ray conference in Mexico in September.22 After the cosmic-ray meeting, Oort stopped in Pasadena, where Baade had already borrowed Lampland’s plates for him to inspect, as well as others from Lick. They went to Palomar together, and Baade took a set of direct photographs of the Crab nebula with the 200-inch telescope, using a polaroid filter and another filter which isolated a band of wavelengths in the continuum, to measure the polarization. Oort took these plates back to Leiden with him; he had already chosen Lo Woltjer as the graduate student who would measure them, and soon began badgering Baade to take more of these polaroid-filter photographs of the Crab nebula as additional data. Meanwhile Walraven had gone with his photoelectric photometer to the larger telescope and clearer skies of Haute Provence Observatory in France, and was repeating his polarization measurements there. Oort was already thinking about mapping the magnetic lines of force in the nebula from the polarization, and discussing the concepts with Woltjer. Theorists everywhere had heard about the new results, and were bombarding Baade and Oort with requests for photographs and data. Finally in December 1955 Baade got another excellent series of polaroid photographs for Woltjer to measure. Oort was anxious to publish a paper quickly, because everyone in astronomy knew something about their results by hearsay. However, he and Walraven held up their publication, based on the Leiden photometry, so they could include Baade’s qualitative description of the 200-inch photographs, illustrated by the stunning pictures themselves, in the same issue of the journal. It was delayed because Baade was giving his main attention to his research on galaxies, and was also teaching a graduate course at Caltech.23 Baade sent prints for the illustrations, as well as the exciting news that he had detected polarization in the jet in M 87 with the same

Mount Wilson and Palomar Observatories astronomers in its library in Pasadena, 1955. Left to right, seated: William A. Baum, Fritz Zwicky, Milton L. Humason, Ira S. Bowen, Jesse L. Greenstein, Walter Baade, Armin J. Deutsch; standing: Guido Mu¨ nch, Allan Sandage, Edison Pettit, Horace W. Babcock, Rudolph Minkowski, Seth B. Nicholson, Robert S. Richardson, Donald E. Osterbrock, Olin C. Wilson. (From the author’s collection.)

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polaroid filters, proving that the synchrotron process was operative in more than one radio source. But he did not send his note on the Crab nebula until Oort threatened to write it for him and publish it.24 Thus Fritz Zwicky, who by now was on very bad terms with nearly all the other Pasadena astronomers, very nearly published his 200-inch polaroid photographs, also taken with the Hale telescope, before Oort, Baade, and Walraven got into print. Zwicky may have taken them soon after Oort had given his colloquium on the first results on the polarization in the Crab nebula in September 1955, when he stopped in California between Mexico and Holland, although Baade, who was in a position to know from the night assistants at Palomar, believed it had been as recently as March 1956. Zwicky had not said a word to Oort about this work before they learned it was in print, and it is hard to imagine any motive for his publishing his brief note except to try to “beat” his now hated rival Baade. Certainly Zwicky’s paper had no scientific content or quantitative measurements to report, and he did not even state the date of his exposures. He had taken the plates with the axis of the polaroid in only two perpendicular directions, so that it would have been impossible to determine the degree or axis of polarization if he had measured them. Baade, now anxious to establish his priority for discovering the polarization in the jet of M 87, quickly submitted a brief announcement, published as a fast-track “note” in the Astrophysical Journal.25 Oort and Walraven’s paper, with Baade’s note, finally appeared in the Bulletin of the Astronomical Institutes of the Netherlands just a week or two before Zwicky’s publication. Their long paper awakened great interest, especially among several Caltech theoretical physicists, who saw it as opening a fresh new field for them. The Dutch paper was packed with Walraven’s fully reduced data and, even more important, Oort’s quantitative discussion and analysis of their physical meaning. He clearly stated the idea that the light ripples were magnetohydrodynamic disturbances, excited by outbursts of particles from the stellar remnant of the supernova, propagating through the inner part of the nebular remnant. Baade’s illustrations were magnificent; and his very brief discussion of the magnetic field outlined in simple words the main idea of Oort’s

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analysis. In his note Baade stated that he had turned over his 200inch plates to Woltjer, who would analyze them in detail.26 Woltjer continued his reductions, and Oort “enticed” Mayall to obtain a new set of spectrograms of the filaments in the Crab nebula, designed to reveal their spatial distribution. Walraven reduced all his improved polarization measurements from Haute Provence, and Woltjer used this material in a very complete, far-reaching study of the Crab nebula. He combined all the data, analyzed the ionization, radial velocities, the central amorphous mass, the filamentary system, the nature of the magnetic field, and even the central star. After completing this thesis near the end of 1957, Woltjer departed for America with a one-year travel grant and a research position at Yerkes Observatory, both arranged by Oort. There the young Dutch postdoc spent several months with Chandrasekhar, working on the theory of “force-free” magnetic fields, applicable to the Crab nebula. Then Woltjer went on to Pasadena for two months, where Baade, just after he retired at the end of June 1958, inspired him to begin studying Seyfert galaxies and the physical processes operative in them, which result in emission-line spectra similar in many ways to that of the Crab nebula. Baade had long been interested in these rare, then mysterious galaxies, and Woltjer’s resulting paper, written back at Yerkes where he worked several more months with Chandrasekhar, was a landmark in beginning to understand their nature.27

Distance Scale Baade’s most famous result, for the readers of the popular scientific magazines and newspaper articles of his time, was “doubling the size of the universe.” It was especially newsworthy because in the 1940s and 1950s the age of the earth, based on geological evidence and radioactive dating, was believed to be 3 to 3.5 billion years, while the age of the universe, derived from the velocity-distance relation (the “Hubble constant” of today), was thought to be only 2 billion years. Doubling the size of the universe brought the two ages into much closer agreement. Probably in absolute terms Baade’s new distance scale was not as intrinsically important as his

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population concept, which opened the way to the study of stellar and galactic evolution, as his far-ranging work on supernovae, or as his leading the way in the identification of the radio sources, but “the universe” is always newsworthy, and doubling its size is a catchy thought. To him it was just part of his long-term project of trying to understand everything in the universe in quantitative terms, which included getting the numbers right. By 1947 Baade realized from his observational data that there was something wrong with the period-luminosity relation which was used to determine the absolute magnitudes of Cepheid variables from their periods. He had found that the “W Virginis Cepheids,” those with periods between fourteen and nineteen days and whose light curves had different forms from the “classical Cepheids” like δ Cephei and η Aquilae, were members of population II. The W Vir variables were all either members of globular clusters, high-velocity stars, or in a few cases located at high galactic latitudes. Yet they were the supposed link between the RR Lyrae variables, the “cluster-type variables” with periods less than a day, which were so numerous in globular clusters, and the classical Cepheids which until then had always been considered to lie on the same period-luminosity relation with the W Vir Cepheids. Baade realized that since the W Vir and classical Cepheids belonged to two different populations, there was no good reason to suppose that they were related to each other at all, or fell on a common period-luminosity relation. The measured distances to galaxies depended on the absolute magnitudes of classical Cepheids within them, while the measured distances within our Galaxy, especially to globular clusters and the galactic center, depended on the absolute magnitudes of the RR Lyrae variables. Baade was suspicious because the globular clusters in M 31 (whose distance depended upon classical Cepheids) as a group seemed to be considerably fainter (in absolute magnitude) than the globular clusters in our Galaxy. The period-luminosity relation had originally been determined by Henrietta Leavitt decades earlier, in terms of apparent magnitudes of classical Cepheids in one galaxy, the LMC, and it had been confirmed, extended, and strengthened by later measurements in both Clouds and in M 31, M 33, NGC 6822, and IC 1613 by Edwin Hubble and by Baade. In

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each galaxy the apparent magnitudes were measured; they were then shifted in magnitude together and did all fall on a common period-luminosity relation, determining the relative distances of the various galaxies. The absolute values of the distances depended on the “zero-point” adopted; in other words, on the distance of one galaxy or of one Cepheid variable. Baade’s careful analysis revealed to him that the zero-point then in use came largely from assuming that the RR Lyrae variables and W Vir variables lay on that same relation. Baade found that the discrepancy between the absolute magnitudes of the globular clusters could be removed if the zero-point of the period-luminosity relation was shifted by 0.6 or 0.7 magnitudes; he did not regard this as proof, because there was no reason to believe the globular clusters in the two galaxies should have the same absolute magnitude. However, it was suggestive, and various other, more indirect comparisons he could make tended to agree with that figure. In his 100-inch telescope survey, Baade had found four W Vir variables in the central, population II bulge of M 31, and tried to compare them directly with the classical Cepheids in it, but the region was too crowded for accurate photographic photometry. He planned to return to the problem as soon as the 200-inch was ready for operation.28 In the meantime, Baade wanted desperately to go to the Southern Hemisphere, to search for RR Lyrae variables in the Magellanic Clouds and compare them directly with the classical Cepheids in those same, closest galaxies. The Harvard College Observatory experts had not found a single RR Lyrae variable in the Clouds; that was an interesting clue, Baade knew, but perhaps they had not searched hard enough.29 He thought he had a chance to do so himself with the 60-inch reflector of the new Bosque Alegre station of Co´ rdoba Observatory in Argentina. Its director, Enrique Gaviola, was an astrophysicist who had studied and gained research experience in Germany and the United States, and had worked for a time on the 200-inch project as an optical physicist. When Bosque Alegre was dedicated in 1942, he had invited Adams, as the Mount Wilson Observatory director, to come to Argentina to take part in the ceremonies, or at least to send a paper. Adams had sent the paper, clearly based almost entirely on discussions with Baade, outlining

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possible observing programs which would take full advantage of the telescope’s location in the Southern Hemisphere. They were studies of the Magellanic Clouds, of the bright, relatively nearby globular cluster ω Cen, and of the center of our Galaxy. The most important program, Baade declared through Adams, would be to find RR Lyrae variables and long-period variables, both supposed to have absolute magnitudes Mpg ≈ 0, and thus well within reach of the Bosque Alegre 60-inch, which should go as faint as +2.5 at the distance of the Clouds. Actually that was a very optimistic estimate, based on assuming near-perfect conditions of the telescope, sky, and observer. Gaviola not only translated and published the paper, but wrote Adams that he would put some of his staff members to work on the program. He asked for data on Baade’s magnitude sequence in SA 57 to use in this study.30 However, the Argentinean astronomers were inexperienced, their telescope was new, and it was almost impossible to buy fast photographic plates during World War II; as a result, there was no progress. With peace restored, in 1946 Gaviola, at the suggestion of Baade, invited Bowen, now director in Pasadena, to send “an astronomer” to Bosque Alegre, to use the 60-inch and train the Argentineans “in the study of variables in the Magellanic Clouds.” Clearly he had Baade in mind, and the expert observer jumped at the chance. He was prepared to go and work with Martin Dartayet, the Argentine astronomer Gaviola had assigned to this project. However, Baade’s German citizenship, which had kept him out of the American weapons development programs and thus allowed him to use the Mount Wilson 100-inch telescope so effectively during the war, now kept him from using the Bosque Alegre 60-inch after the war. The United States still had not signed a peace treaty with Germany, and as a result Baade’s passport was not valid. He could remain in the country, but if he left for Argentina or anywhere else, he could not return legally. Hence the idea of his working visit to the Southern Hemisphere fell through and although Dartayet continued observing the Magellanic Clouds, he did not succeed in photographing RR Lyrae variables in them.31 The only other possibility was Harvard’s 60-inch in South Africa. Baade pressed Harlow Shapley to have his observers there mount

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an all-out search for RR Lyrae variables in the LMC, but the Harvard director could not get enthusiastic about it. He did not believe that there was any problem. When he did write in 1949 that his assistants could not find any RR Lyrae variables, even on the longest exposures, Baade knew that statement was highly significant. At that time he thought the LMC might be a pure population I system, without any population II, because no “real” globular clusters (in his terms) had been discovered in it. That would agree with the absence of RR Lyrae variables, but so would a fainter absolute magnitude for them; he was uncertain which explanation was correct.32 At Mount Wilson Alfred H. Joy had begun taking spectra of the known W Vir variables in our Galaxy, mostly in globular clusters, and was finding them different from the spectra of classical Cepheids. This cast considerable doubt on the idea that all these stars could form a one-parameter sequence with a common period-luminosity relation. Baade had discussed Joy’s results with him frequently, and they both agreed on this conclusion. Shapley would not hear of it; thirty years earlier he had been the great pioneer of pulsating-variable research, but he had not kept up to date and could not bear to think that any of his results needed revision in the light of more recent data. He argued rhetorically with Joy, insisting that W Vir variables were Cepheids and that therefore they all fit on one common period-luminosity relation. Likewise, after Baade called the idea that the RR Lyrae variables were on the same period-luminosity relation into question at a symposium on the structure of the Galaxy at Ann Arbor in 1950, Shapley sent him a plot of his data, which he believed ruled out this idea.33 By then the 200-inch was completed and tuned up, and Baade was obtaining direct photographs of M 31 with it every night he could. As the giant spiral was well placed for observing from Palomar, he could take several exposures each night to search for RR Lyrae variables, but he did not find any. Baade knew M 31 contained many population II objects; the RR Lyrae variables had to be too faint, not absent. At the then adopted distance of M 31, they should have been well above the faint magnitude limit of the 200inch, and hence observable. Clearly M 31 was more distant than earlier believed; the classical Cepheids were more luminous than

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previously assumed and the zero-point of the period-luminosity relation was wrong. Baade had hoped to measure the distance of M 31 from the W Vir variables in its central bulge, but found that even with the 200-inch the field was still much too crowded with stars for accurate photometry. Also, as we know now (but Baade did not realize then), there are only very few W Vir variables in the bulge (or halo) of our Galaxy, and thus presumably in M 31’s halo also. However by now Allan Sandage, using plates Baade had taken with the 200-inch, had measured the color-magnitude diagram of the globular cluster all the way down to the main sequence. He found it did fit well with the color-magnitude diagram of the nearby stars with known distances. Hence Baade had an absolutemagnitude calibration for population II, which he assumed held for all globular clusters and population II everywhere, including in the central region of M 31, the outer parts of it, and its dwarf elliptical companions. This assumption seemed valid as the color indices and apparent magnitudes of the brightest population II stars in these same regions were the same, and hence their absolute magnitudes were also. Baade’s figures gave Mpg = −1.5 for the brightest population II stars, and as he had measured their apparent magnitudes in M 31 as mpg ≈ 22.4, its distance modulus was m − M = 23.9, not the formerly accepted 22.4. The difference, 1.5 magnitudes, corresponds fairly closely to a factor of two in distance. Henrietta H. Swope was measuring the plates, and all the numbers would be refined slightly, but it was clear to Baade that the previous distance scale was wrong. Furthermore, there was an independent confirmation. At Lick Observatory, Joel Stebbins, now retired but still active, and his former student Gerald E. Kron had carefully measured photoelectrically the colors of the classical Cepheid η Aql and, using its well-determined radial-velocity curve, had applied the Baade-Wesselink method to measure its radius, and hence luminosity, as a function of phase. This was an absolute measurement of the luminosity of the star, independent of any distance scale, and it agreed with Baade’s new zero-point for the period-luminosity curve, not the old one.34 In September 1952 Baade took part in the IAU General Assembly in Rome, the first IAU meeting he had been able to attend since the

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Walter Baade, 1955. (Courtesy of The Observatories of the Carnegie Institution of Washington.)

war. He reported his new results on the distance scale, which he had not published, at a meeting of the commission on “extragalactic nebulae,” of which he was acting chairman, filling in for Hubble. When Baade finished his report, A. David Thackeray rose and announced that he and A. J. Wesselink had succeeded in finding a few RR Lyrae variables in NGC 121, one of the globular clusters in the SMC. They had observed with the 74-inch Radcliffe telescope, moved from England to South Africa after World War II, and with it they found the RR Lyrae variables not at mpg = 17.5, as the old distance scale had predicted, but at mpg ≈ 19. Thus they had confirmed Baade’s revision of the distance scale. Thackeray, trained in astronomical spectroscopy, had the largest telescope in the South-

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ern Hemisphere at Radcliffe Observatory, but had no spectrograph. He had wanted to do globular cluster research and recognized the special importance of his location, where he could observe the Magellanic Clouds, the Sculptor and Fornax dwarf galaxies, and the galactic-center region as no other sizable telescope could. The 74inch was equipped for direct photography and Thackeray had begun obtaining plates of several so-called globular clusters (as Shapley considered them) in the LMC. When Oort visited South Africa in 1949, Thackeray had discussed this work with him. The Dutch astronomer, always eager to encourage galactic research, urged him to write Baade for advice on some of the most interesting problems he might tackle. Thackeray did so, mentioning the clusters he had photographed, but asked whether this program would overlap too much with Shapley’s. Thackeray also asked Baade if he could work on Sculptor, and in addition mentioned a possible search for planetary nebulae. In a seven-page letter, Baade enthusiastically urged him to forget about spectroscopy and did not mention the planetary nebulae. He should push on to study the variable stars in the Sculptor and Fornax systems and, of “first-rate importance,” he should find out if those LMC clusters were “really” globular. Thackeray should ignore Shapley, who had a tendency “to consider the Magellanic Clouds as his personal property,” Baade wrote, adding that Hubble (much better known to the English astronomers in 1949) agreed with him on that. It was high time to tear down “the barbed wire fences and the warning signs, ‘Keep out. This means you!’ . . . Monopolies in science are intolerable and should never be respected.” Thackeray had already taken blue and red plates of NGC 1866, a large, bright “globular” cluster in the LMC, and sent prints of them to Baade. They showed it contained bright blue stars and therefore was in Baade’s terminology a population I object, and hence not a globular cluster. He explained his reasoning to Thackeray in complete detail, and as the South African astronomer obtained pairs of plates of several more clusters, he recognized himself that they were galactic clusters too, not globular. Baade suggested that they write a joint paper on NGC 1866, an idea to which Thackeray gladly agreed. But Baade had many other ongoing programs, and Thack-

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eray was busy with keeping the undermanned Radcliffe Observatory in productive operation. He did not mention in his correspondence that he had found the first RR Lyrae variables in NGC 121 in November 1950, and thus proved it was a “true” globular cluster. It and the other two RR Lyrae variables that Thackeray and Wesselink found were at the very faint limit of the 74-inch, and it took many more months of photographic observing to set up a photometric sequence, linked to the sequences in the Northern Hemisphere, to estimate their magnitudes. Thus Thackeray never mentioned these population II objects in his letters to Baade, who only learned of them immediately after he had given his paper at Rome, when the Radcliffe astronomer stood up and announced this confirmation of the new distance scale. Baade in turn had not written Thackeray to tell him of his recalibration of it, which he had only completed just before leaving for the IAU meeting. Thus neither of them knew in advance what the other was to say, but that probably made “doubling the size of the universe” even more convincing to nearly all the astronomers who were there to hear them both.35 However, Shapley, who was there, remained a skeptic. He asked for more details, which Baade provided at length. At that same Rome General Assembly, Baade also gave a long lecture on “basic facts of stellar evolution,” in which he described fully Halton Arp, William A. Baum, and Sandage’s measurements of the color-magnitude diagrams of M 3 and M 92 as samples of the “old” population II, the fitting procedures Sandage had used on M 3, and the resulting absolute magnitudes of the population II stars. Baade contrasted them with young population I O and B stars; he now fully understood and accepted stellar evolution as responsible for the difference between the two populations.36 After the Rome meeting, Baade stopped in Hamburg and revisited the observatory and telescope with which he had worked until 1931. He must have thought of Richard Schorr, his old director there, who had died the previous year at the age of eightyfour. Back in California that fall, Baade continued photographing three fields in M 31, plus two more centered on its companions, NGC 185 and NGC 205, and Swope tried to keep up with measur-

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ing the variable stars he had found on them. Although Baade had not yet published anything about the revised distance scale, most active research astronomers who were working on galaxies were well aware of it.37 Then suddenly a bombshell burst in Baade’s world. On January 5, 1953, the New York Times published an article reporting that Shapley, using data obtained at Harvard’s observatory in South Africa, had doubled the scale of the universe. The story was quickly picked up by newspapers everywhere and by Time and other masscirculation magazines. It did not mention Baade, and Shapley was quoted as saying that the data were sparse, so his own drastic revision should not be considered final until it had been tested appropriately by other investigations which he said were under way at Mount Wilson, Palomar, Leiden, and elsewhere. The story was easily traced to a press release from Science Service, written by Charles A. Federer, Jr., the editor of Sky and Telescope, whose offices were at HCO. Shapley had given him the story, based on a paper he had presented orally at the AAS meeting in Amherst, Massachusetts, just a few days before the new year. The old Harvard director had finally abandoned his resistance to the revision of the period-luminosity relation, and was trying to pretend that he was the one who had proved it wrong. According to the release, he based his reasoning almost entirely on the discrepancy between the absolute magnitudes of the globular clusters in M 31 and in our Galaxy, a difference which workers in the field, including himself, had long known but had not accepted as firm evidence.38 Baade was furious. He fired off a letter to Sergei Gaposchkin, denouncing Shapley’s performance at Amherst as “simply shameless.” He had “lifted wholesale” Baade’s “remarks at Rome, without any acknowledgements.” There the Harvard director “had no relevant data of his own and . . . was taken by complete surprise,” yet five months later, back in the United States, he had tried to pass off Baade’s conclusion as his own. In other letters the angry Palomar astronomer characterized Shapley as a “wind bag” and a “carnival barker” who had simply “repeat[ed] his old sales talk.” Learning that the Harvard director planned to publish this same paper

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in the Proceedings of the National Academy of Sciences (where it would not be refereed), Baade called it “a full discussion of his trash.” He wanted the IAU and the AAS to censure Shapley, and at least one important astronomer, Gerard P. Kuiper, agreed with him and wrote the IAU general secretary to say so.39 However, cooler heads prevailed. Shapley had just retired after thirty-two years as director of HCO, and no one but Baade wanted to attack him. Some believed that Shapley was worn down and losing his grip. Struve, then president of the IAU, and a former president of the AAS, thought the “whole matter [was] very delicate” and he did not “advise any action that would stir up excitement.” Bowen told Baade that when the minutes of the IAU commission meeting were published, everyone could read that he had revised the distance scale and that Shapley had asked questions showing that he either did not understand it or still doubted it after hearing the presentation four months before his “paper” at Amherst. Bowen did ask Donald H. Menzel, the acting director at Harvard, to make sure that no more popular articles or news releases were issued crediting the discovery to Shapley, and none were. However, Menzel did not have the courage to tell the old ex-director to his face that he had been revealed as a plagiarist, although he freely admitted it to Bowen and Baade. Cecilia Payne-Gaposchkin, who had idolized Shapley when she first came to Harvard but had soon learned to view him critically, now denounced him in a long letter to Baade. She had been at the meeting at Amherst but had deliberately not gone to hear Shapley’s paper, she said. She could not “endure listening to his unjustifiable boasting,” but neither could she bring herself to tell him that to his face. The one person who did beard the lion was Bart J. Bok. He had heard Shapley deliver the paper orally and “did not like it”; he thought at the time that “Shapley was trying to get on the band wagon” but said nothing. After the storm broke, however, he wrote Struve about it, then confronted Shapley, telling him that in his paper he should have referred to Baade’s prior announcement. The ex-director responded with a brief letter to Baade, saying he “hope[d] it was OK” to refer to the Palomar astronomer’s unpublished discussion at the Rome

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meeting. With it Shapley enclosed a revised abstract of his Amherst paper, briefly mentioning previous work by Baade, Joy, and PayneGaposchkin, but keeping most of the credit for himself. This revised version was published in the Astronomical Journal with other abstracts from the meeting. Baade did not reply and apparently never wrote to Shapley again.40 In the end, it all came out just as Bowen had said it would. Although Shapley published his paper with what Baade considered quite inadequate references and a slanted argument, no controversy erupted in print. Struve devoted two of his regular monthly articles in Sky and Telescope to “the distance scale of the universe,” the first describing Shapley’s important early work on the problem, the second giving Baade all the credit for the revision. Also, Struve’s quick letters to Federer had kept any story crediting Shapley with the discovery out of the Sky and Telescope postmeeting column, “American Astronomers Report.” Word of Shapley’s attempt to steal Baade’s idea, work, and results spread quickly through the little circle of senior American astronomers, largely by word of mouth, emanating chiefly from Harvard itself, where Menzel, Payne-Gaposchkin, Bok, and Fred L. Whipple all supported the Palomar observer. Shapley had held them all down for years, and had overbuilt and overstaffed the observatory; now they were reaping the consequences in the form of cutbacks ordered by Harvard University president James B. Conant. They felt they owed little loyalty to their former director. Shapley did not help matters when he presented another oral paper on the distance scale at the AAS meeting in Boulder in August of that same year. In it he went back to resting his case on the globular clusters, this time in the LMC, and on the brightest stars in them. Again he gave himself nearly all the credit, and this time he did not mention Baade at all. However, astronomers everywhere had accepted that the revised distance scale was Baade’s work, not Shapley’s, long before the IAU Transactions finally appeared in 1954, and anyone could read the report of the Commission 28 meeting.41 Before that, however, the public had to be told. The Rockefeller Foundation had paid for the 200-inch Hale telescope; its trustees

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expected it, not Harvard’s antiquated and rickety telescopes in South Africa, to reveal the true distance scale and the age of the universe. George W. Gray, the Rockefeller Foundation’s chief science writer, had first called Shapley’s story in the New York Times to Bowen’s attention, asking what was new in it that had not previously been known. Bowen tried to explain the point at issue, assuring him that this was one of the most important programs for the 200-inch, and one that Baade had been working on since the telescope went into operation. It was not completed, but Baade planned to published it “soon.” Because of the “difficult situation” created by Shapley’s release, Gray should write an article for the Scientific American, not attacking him, but giving “the truth,” which the astronomers already knew, to the public. This was just what Gray and the Rockefeller Foundation trustees were eager to do. Bowen briefed Gray, including Hubble’s role in the early planning of the 200-inch program. Gray then interviewed Baade in person, and followed up with written questions and a draft of an article. Their attempted collaboration quickly turned into a disaster; Gray was a fine popular writer but did not understand astronomy very well, and Baade refused to accept simplifications and demanded that every sentence in the article be scientifically correct. He crossed out most of what Gray had written, largely based on glowing material from Hubble, and substituted his own technical, precise verbiage. Baade hated publicity, and showed it; he and Gray were soon exchanging angry telegrams and letters. Bowen quickly stepped in, writing to Gray that “[u]nfortunately while Baade is a very brilliant astronomer, and has made great contributions to the field he does not always use the best judgement in these personal problems,” a correct but understated analysis if “personal problems” is taken to mean publicity. With Bowen’s help and sensitive suggestions, Gray was able to complete a well-written, understandable article which appeared in the June 1953 Scientific American, nearly a year before the IAU Transactions were actually published. All the major newspapers had carried long stories in May, no doubt based on an advance release of the article. Baade was the astronomer who had changed the size and age of the universe in the public mind.42

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Actually, even before Baade’s report to the IAU, Alfred Behr had published a paper on the distance scale of the galaxies in 1951, in Germany. It was based almost entirely on Baade’s two 1944 papers, incorporating his revisions of the magnitudes of the photometric standard stars he used for M 31, and the absolute magnitudes for the most luminous population II stars he used then, somewhat different from his later, better determinations with the 200-inch. Behr’s result was a change in the distance scale by 1.7 ± 1.1 magnitudes, corresponding roughly to a factor 2.2 in distance, which taken literally translated into changing the expansion age of the universe from 1.7 < 109 yr. (Hubble’s old value) to 3.8 < 109 yr Baade ignored this paper, at least in print, no doubt because by the time it appeared he had much better values of all the numbers Behr had used in it. Since Behr’s paper appeared in the Astronomische Nachrichten, few American astronomers of the time noticed it, but some theoreticians did, including George Gamow, who liked the larger value for the age of the universe and questioned Baade about it. Unfortunately, if Baade replied, his answer has not turned up.43 Dartayet and Jorge Landi Dessi published a paper announcing their discovery of twenty faint variable stars in the SMC, on plates they had taken with the Bosque Alegre 60-inch. Probably only two of them were RR Lyrae variables, right at its faint-magnitude limit, but two others evidently were related, somewhat longer-period population II Cepheids. Baade’s suggestion, transmitted through Adams’s 1942 paper, had paid off. In fact, Dartayet and Landi Dessi had completed their paper and submitted it in late 1951, but neither got to Rome in 1952. Thackeray and Wesselink published their discoveries of the three RR Lyrae variables in the SMC, and of twentythree more in two globular clusters in the LMC, in 1953. Baade’s revision had been confirmed in Argentina and in South Africa.44 Baade did not publish his own paper until 1956, as the written form of an invited lecture he gave in Pasadena the previous year, when he received the Catherine Wolfe Bruce Gold Medal of the Astronomical Society of the Pacific. In this paper he described his reasoning and methods very clearly. He showed that the previous attempts to calibrate the absolute magnitudes from proper motions

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and radial velocities of the nearest classical Cepheids in our Galaxy had gone wrong, both because of the smallness of their proper motions and because the extinction by interstellar dust had been badly underestimated. On the basis of all his data then available Baade gave m − M = 24.2 as the distance modulus of M 31; that figure, slightly modified by his later work to 24.25, corresponding to a distance of 590 kpc, was used for many years as the key ingredient of the distance scale of the universe.45

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✶

Telling the Good News AMERICA AND EUROPE, 1953–1959

Lectures and Courses Walter Baade never had a regular teaching position at a university, but he was an excellent lecturer, and after the success of his series of Princeton lectures in May 1950, he received many invitations to be a visiting professor. Most he declined or, more diplomatically, allowed his director, Ira S. Bowen, to kill, as when J. Allen Hynek, an astronomer who was temporarily a dean at Ohio State University, requested the great observer to be the chief speaker at a threeday “astronomical festival” there, his travel to be financed by Mount Wilson and Palomar Observatories. But one invitation Baade did accept was to spend two weeks in 1953 at Swarthmore College, an elite undergraduate school near Philadelphia. Peter van de Kamp, the professor of astronomy, did most of the teaching in the course, but had several guest speakers, including Bart J. Bok and Fred Hoyle, give one or two lectures each on their research specialties. Baade prepared his seven lectures carefully, and in addition gave a Sigma Xi talk on galaxies, intended for students and faculty members from all fields of science. It, like all his lectures and short courses, was on his one subject, the population and structure of galaxies. Each time he gave it, he updated it to include his latest results and interpretations, and the related work of others. Probably it was a little above the heads of most of the listeners who had not studied astronomy recently, but he surely gave them a glimpse of a dedicated scientist who was supremely interested in understanding the universe. On the way back to California Baade stopped for a visit with his longtime friend Jason J. Nassau at the Case Institute of Technology, where he repeated the galaxy lecture to the Cleveland

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Astronomical Society. Nassau also arranged a “neighborhood meeting,” an informal one-day (Saturday) conference of Midwestern astronomers, at which Baade spoke on his and Rudolph Minkowski’s ongoing identifications of radio sources. His presence attracted larger than normal attendance at both these events, and many engineers, astronomers, and other science buffs came away better informed and impressed by the speaker.1 The most productive series of lectures Baade gave was in a month-long symposium on astrophysics in the summer of 1953, at the University of Michigan. The recently appointed director, Leo Goldberg, modeled it on the highly successful “summer schools” in astronomy and astrophysics that Harlow Shapley had organized at Harvard from 1935 to 1942, and the famous prewar physics ones at Ann Arbor, at which such luminaries as Enrico Fermi, Wolfgang Pauli, and Arnold Sommerfeld had lectured. Goldberg was one of Baade’s favorite young theoretical astrophysicists, who, as a Harvard graduate student, had visited Mount Wilson and discussed research with him there. He persuaded Baade to come to Ann Arbor that summer to help put Michigan back on the map as an astronomical research power. The other visiting lecturers were George K. Batchelor, from Cambridge University, who spoke on the turbulence and magnetohydronamics which might be applicable in stars, nebulae, and galaxies; George Gamow; and Edwin E. Salpeter, whose lectures on nuclear physics and element formation were relevant to stellar evolution and the two populations. Baade himself had urged Goldberg to bring the physicists to the symposium.2 The National Science Foundation supported the summer school generously, and the cream of the crop of young postwar American graduate students and postdoctoral fellows were there. In addition to Gamow and Gerard P. Kuiper (who lectured on the solar system and attended some of Baade’s lectures), both then members of the National Academy of Sciences, eight of the attendees were later to be elected to it: Lawrence H. Aller, Margaret Burbidge, Goldberg, Donald E. Osterbrock, Eugene N. Parker, Vera C. Rubin, Salpeter, and Allan Sandage. They, and an exceptionally high percentage of

Participants at the Summer School on Astrophysics, University of Michigan, 1953. Left to right, front: John Kraus, Stanley Wyatt, Owen Gingerich, George Gamow, Walter Baade, E. Edwin Salpeter, George K. Batchelor, Leo Goldberg, Dean B. McLaughlin, Edward A. Spiegel, Carl K. Seyfert; middle: J. Beverly Oke, unknown, Nancy G. Roman, Eugene N. Parker, Arthur N. Cox, Vera C. Rubin, Paul Mutschlecner, unknown, John P. Cox, Nancy W. Boggess, Albert Boggess, Kenneth M. Yoss, Geoffrey Keller; third row: Geoffrey R. Burbidge, Allan Sandage, Margaret Burbidge, Thomas A. Matthews, Eugenio Mendoza, H. Lawrence Helfer, unknown, Robert Rubin, Karl Henize; rear: D. Nelson Limber, Freeman D. Miller, Edith Mueller, Donald E. Osterbrock, Irene H. Osterbrock, Marshall H. Wrubel, Frank N. Edmonds, unknown, Robert Brownlee, Lawrence H. Aller, John Waddell, Walter Fitch, James Milligan, Lowell R. Doherty, Bernard Pagel, Joyce Newkirk, Edward Dennison. (From the author’s collection.)

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the other forty-odd young scientists who were there, had long and fruitful research careers. In Ann Arbor Baade lived with most of the single students and postdocs in a fraternity house which the summer school leased for the month. He thrived in the atmosphere, talking and discussing astronomy almost constantly, drinking too much coffee, smoking too many cigarettes, and getting too little sleep, but it was wonderful for the attendees. All of the participants in the summer school, whether they lived in the house or elsewhere, were tremendously inspired by Baade’s lectures, and nearly all of their subsequent research was related in one way or another to his population concept.3 Baade updated and repeated this course as the Hitchcock Lectures, a prestigious endowed series at the University of California, in 1954. Although they were intended by the terms of a bequest for the “educated public,” after the first lecture Baade actually aimed them at the Berkeley astronomy graduate students and faculty. In addition he gave one popular lecture on the campus, and a colloquium on a two-day visit to Lick Observatory. At Berkeley Baade especially inspired Harold F. Weaver, by then a faculty member there, and Morton S. Roberts, a graduate student, and encouraged them to go on in galactic research.4 Back in Pasadena, Baade gave a graduate course at Caltech on “extragalactic nebulae,” the old-fashioned name for galaxies which never quite died out of his mind, in the winter quarter of 1956. With one quarter to lecture, more time than he had for any of his previous guest appearances, he began with a historical overview that went all the way back to 1750 and the primitive but perceptive ideas of Thomas Wright on the Milky Way as revealing the shape of the Galaxy. As always Baade described his own growing realization of the population concept, starting with his and Hubble’s work on the Sculptor and Fornax systems, and then his own resolution of M 31 and its companions with the 100-inch during World War II. But he got up to his most recent results on the distance to M 31, and to Martin Schwarzschild and Sandage’s interpretation of the color-magnitude diagrams of globular clusters in terms of stellar evolution. Several younger faculty members sat in on Baade’s

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course. They, and some of the students in the class, were led into galaxy and stellar evolution research by it or were strengthened in their resolve to go on in that subject. Baade was an excellent lecturer, knowledgeable, a lively speaker whom it was impossible to ignore (he once described his voice to me, quite correctly, as sounding “like a barking dog”). He always prepared carefully and presented his material logically, although sometimes in oversimplified form, to get his point across clearly. Baade was master of his subject, and loved to discuss any detail of it in answer to questions.5 Yet oddly, after only one lecture he thought he had failed to hold the interest of the Caltech students, and believed that most of them were “obviously bored” and that “there must be something about teaching which [he had] never grasped.” He could not have been more wrong, but he evidently expected more from the students than he saw them giving.6 Baade’s two Ph.D. thesis students, Sandage and Halton Arp, who both finished in 1953, were an altogether different story. He expected much of them, and they gave it, and more. Both of them were supremely interested in astronomy; he taught them to observe and the techniques they needed, and they did the rest. Baade was generous in his praise of their research, and they deserved it. Both of them admired him greatly, not only for what he had taught them but for his warm, friendly personality and fatherly interest in their careers. Sandage went directly on to the Mount Wilson and Palomar Observatories staff after earning his Ph.D.; Arp became a Carnegie postdoctoral fellow for two years, carrying out a search and statistical survey of novae in M 31, as a typical spiral galaxy. For this program he lived at Mount Wilson with his family for two years, so that he could obtain at least one hour-long exposure with the 60-inch each night M 31 was above the horizon and the sky was moonless long enough. In this period Arp took nearly a thousand plates on 290 nights out of the 490 assigned to him. He found the mean frequency of novae in M 31, 26 ± 4 per year, and derived an accurate value of their mean absolute magnitude at maximum light, Mpg = −7. Then, under an NSF grant administered by Indiana University, Arp went to South Africa and observed Southern Hemisphere Cepheid variables, globular clusters, and a region near the

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galactic center. In 1957 he returned to Pasadena as a member of the Mount Wilson and Palomar Observatories staff and, like Sandage, became a leader in applying Baade’s ideas to galactic and extragalactic research.7 They were Baade’s closest scientific heirs, but there were many others into whom he had breathed the excitement of his kind of research in his courses and lectures over the years.

Later Stellar Evolution In 1952 as Arp, William A. Baum, and Sandage were measuring and plotting the color-magnitude diagrams of several of the richest globular clusters, Bowen began testing the coude´ spectrograph of the 200-inch telescope on faint stars. He had designed this huge instrument, built around large reflection gratings especially ruled for it, and fast Schmidt cameras, all expertly constructed in the observatory’s shops in Pasadena. The fastest camera provided rela˚ /mm dispertively high spectral resolution for those days (38 A sion), and with it Bowen was able to obtain fairly good spectrograms of a few of the brightest stars in the globular clusters M 3 and M 92. These spectra were so interesting that Olin C. Wilson, Baade’s favorite among the younger spectroscopists as Alfred H. Joy was among the older ones, took spectrograms of several more of the brightest M 92 stars with the same camera. Their spectra, especially those of the M 92 stars, were quite different from the spectra of relatively bright stars (much nearer the sun) with similar colors. The globular cluster stars’ spectral types to a first approximation (the Mount Wilson classification system) were those of F and G bright giants or intermediate supergiants, “earlier” than predicted by their colors, which would make them G and K stars. Daniel M. Popper, who had taken much lower resolution spectrograms with the smaller McDonald Observatory 82-inch reflector of a number of stars (nearly thirty in all) in the globular clusters M 3 and M 13 several years before, had only been able to see a hint of this difference. He had described the cluster giants as having spectra like high-velocity giants.8 Such stars were known to have weaker absorption lines of the CN molecule than “normal” giants. Bertil Lindblad had discovered this subtle spectral difference years

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earlier, and William W. Morgan and Philip C. Keenan had illustrated it in their 1943 spectral atlas. Popper had been unlucky, not only in having a less powerful telescope and spectrograph but also in observing two clusters which were much closer to “normal” stars than M 92. It ultimately turned out to be one of the most deviant globular clusters in this spectral property. Wilson, Joy, and Jesse L. Greenstein at Caltech, the three spectroscopic experts whose judgment Baade trusted most, all agreed that the real systematic difference was that the globular cluster stars, especially those in M 92, had weaker metallic absorption lines, such as those of Fe, Ti, and Sr, than “normal” stars. The simplest interpretation was that the heavy elements were less abundant (with respect to hydrogen) in the globular-cluster stars, Baade’s defining type of population II, than in the “ordinary” stars near the sun, in the galactic plane and in fact in a spiral arm of our Galaxy. Baade assumed these latter stars to belong to population I. These spectral differences thus led to the first clear recognition of an abundance difference between the two populations. Earlier, astronomers had implicitly assumed that practically all stars had the same relative abundances of the elements, called in those days “the” abundance of the elements. Lower abundances of all the heavy elements show up particularly strongly in CN, which Popper had been able to recognize, because this diatomic molecule’s abundance is proportional to the square of the mean abundance of the “metals” (including C and N!), instead of to the first power, as in atoms or ions. Baade saw the difference as a simple dichotomy; population I consisted of young stars with “normal” (or relatively high) “metal” abundances, while population II stars had “low” heavy-element abundances. He tended to ignore the differences among globular clusters, exemplified by the quite different appearance of the spectra of the M 92 and M 3 intermediate supergiants. However, Baum and Sandage both noticed that the color-magnitude diagrams of these two clusters, though similar in form, were displaced in color (if matched in absolute magnitude by the RR Lyrae variables they contain). Both believed these differences might be correlated with their different metal abundances, as turned out to be the case. Sandage in particular noted that these two clusters were essentially

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equally old, that is, they had the same ages, although their heavyelement abundances were clearly quite different. Baade tended to overlook or deemphasize this complication to his simple picture, but Sandage was to become the leader in exploiting it.9 Although the stellar models which Schwarzschild and other members of his group had calculated matched the observed globular-cluster color-magnitude diagrams well up to the turn-off from the main sequence and the beginning stages of stellar evolution, after that the models deviated greatly from the real stars. The models predicted continuing expansion of the subgiants to large, lowtemperature giants, while the color-magnitude diagrams showed that real stars instead moved up the diagram in luminosity, at roughly constant effective temperature. Then in 1954 Schwarzschild and Hoyle, who was visiting Princeton to work with him on stellar evolution, realized that the problem was that convection had been left out of the previous calculations of red-giant models. Convection is the name given the transport of heat by large-scale motions of gas, currents flowing upward and downward, carrying hot gas outward and cooler gas inward, where it is reheated and started upward again. Convection was known to be effective in red-dwarf stars, and Schwarzschild and Hoyle demonstrated that it dominated the structure of red giants and supergiants as well. Even the first models they calculated taking convection into account showed much better agreement with the observed globular-cluster colormagnitude diagrams than any of the previous stellar-evolution models had. Furthermore, the later stages of the evolutionary tracks in the diagram depended strongly on the assumed heavy-element abundance, as Hoyle sketched in a letter to Baade in April 1954. It was clear that with this new understanding it would be possible to follow stellar evolution to much later stages, right up to the “tip” (highest-luminosity) end of the helium flash, where helium burning begins and leads to the still higher interior temperatures at which heavy elements can be produced by thermonuclear reactions. Hoyle, Schwarzschild, Baade, Sandage, and other astronomers all realized that these heavy elements would ultimately be fed back to interstellar matter somehow, leading to enrichment of the material from which later generations of stars formed, from the lowest

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abundances (as in M 92) to the highest (as in the sun and young galactic clusters).10 Sandage quickly became the world leader in quantitative studies of stellar evolution, comparing the color-magnitude diagrams of clusters he observed with Hoyle and Schwarzschild’s families of computed models. He chose the objects to observe to find their ages and metal contents, ranging from numerous globular clusters (all old), through the old galactic cluster M 67 (which showed population I metal abundances), to young OB associations with ages as short as 106 years, such as NGC 2362 and h and χ Persei. All Sandage’s new, more quantitative results flowed naturally from Baade’s 1944 population concept.11 Baade constantly tried to test the similarities between the nearby Andromeda galaxy and our Galaxy. He expected planetary nebulae to be present in the nearby spiral, and though they would be close to the limit of detection he set out to find them. Since they would certainly be too small to be resolved at the distance of M 31, they would appear as points and his only hope was to recognize them by their spectra. Their strongest emission lines in the optical region are [O III] λλ4959, 5007, so Baade took direct photographs with the 200-inch of a field in M 31 well away from its nucleus (and the bright, unresolved stellar background that would make the planetary nebulae undetectable) and relatively free of interstellar extinction. He used fast orthochromatic (green-yellow sensitive) plates with one filter that transmitted the nebular lines and thus would show the planetary nebulae, and with another filter which absorbed the lines but transmitted the nearby continuum. Comparison of these two images allowed stars (which appeared on both) to be eliminated, leaving only the nebulae which appeared on one image but not on the other. In this way Baade found five planetary nebulae in the field, which he showed were comparable to the more luminous planetary nebulae in our Galaxy. Baade presented these results orally in a paper he gave at the Princeton AAS meeting in April 1955, but he never published more than the abstract. At that same meeting Henrietta H. Swope read another oral paper, reporting the preliminary results of their joint work on Cepheid variables in M 31. It led to the distance modulus

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m − M = 24.25, corresponding to a distance of 7.1 < 105 pc, a value which was widely used for many years. After Baade’s death, Swope published a brief note listing the accurate positions of these planetary nebulae, with identification charts on which they were marked.12

Symposia and ESO Baade published considerably fewer papers than Jan H. Oort, Subrahmanyan Chandrasekhar, Otto Struve, or any of the other great research astronomers of his time. Yet he spread his ideas, concepts, and results very effectively in lectures, courses, and symposia. His heart was in Germany and in Europe; once he could travel freely after the war, he liked to go there and participate in international research conferences, and the Europeans, hungry to learn of the latest observational results from Palomar, were eager to have him come. Oort, who had been secretary-general of the IAU from 1935 to 1948, was a key figure in organizing most of the Continental astronomical meetings in the 1950s, and he naturally included Baade in all of them that involved galactic structure. As soon as Oort learned that the Palomar astronomer would accept an invitation to Holland in 1953, he began pulling strings to get the necessary travel funds for him. Baade’s visit quickly turned into the occasion for the first IAU symposium held separately from the triennial general assemblies. The subject for the symposium was to be coordination of galactic research, and the place Groningen. Baade, who believed in research conferences for experts only, wanted to keep this meeting as small as possible; Oort wanted to be sure many countries were represented, particularly European ones; and Bok, the most vocal member of the organizing committee, wanted everyone working on galactic structure to come. In the end they compromised on twenty-five scientists from ten countries, led, not surprisingly, by Holland and the United States, with five each. At one point Baade almost backed out of attending, because his prior commitment to lecture at the Michigan summer school conflicted with the dates first proposed for the Groningen symposium. But Oort moved it up to the end of June, and Baade came. Although

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he had to delay his trip for several weeks, waiting for his reentry permit to the United States, in the end he was able to spend nearly two months in Holland. He, Oort, and Adriaan Blaauw (who returned to the Netherlands briefly that summer) discussed M 31 and our Galaxy, radio and optical results, and new programs almost daily. They also planned the symposium, the order in which the various topics would be discussed, and even began to prepare a draft report.13 During the symposium Baade gave the keynote opening talk summarizing recent extragalactic research. He was at his best in the kind of discussions which went on all week, describing his results and throwing out ideas for future programs. For many of the older Europeans, this symposium was their best chance to learn what the most important fresh new research areas were, and to try to carve out parts of them for their observatories and their countries.14 As soon as the symposium ended, Baade had to fly back to America to teach at Ann Arbor, but he had made a powerful impression on the European astronomers and revitalized their research interests. One of the important future programs Baade emphasized in the discussions, sampling the density of RR Lyrae variables (and thus of population II) along several well-chosen lines (or rays) through the Galaxy at high galactic latitude, was later carried out by Lukas Plaut, the local (Groningen) secretary at the symposium. It required the wide-field 48-inch Schmidt, which was nearly completely tied up with the Palomar Sky Survey until 1956, but Baade was able to get a few plates before that to pick out the best fields. Then Plaut came to America, took hundreds of plates, returned to Groningen with them, found the variables, estimated their magnitudes, and published the data years after Baade’s death. Plaut’s results were somewhat inconclusive because of problems with the corrections for interstellar extinction. To strengthen the coverage near the galactic center, Plaut then rediscussed Baade’s original survey with the 100-inch, now using more recent, accurate magnitude sequences, and Oort obtained further counts in two other fields, made by P. Th. Oosterhoff from plates A. David Thackeray had taken later with the 74-inch Radcliffe reflector in South Africa. With all these data, and another long

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Walter Baade visiting Hamburg-Bergedorf Observatory, 1955. (Courtesy of Hamburg Observatory.)

discussion of the interstellar extinction in the field, in 1975 Oort and Plaut finally derived the distance to the galactic center as 8.7 ± 0.6 kpc, differing very little from Baade’s early determination. Oort and Plaut further derived the density distribution of RR Lyrae variables in the central region and in the halo of the Galaxy. All the concepts in the program went back to Baade’s surveys of the fields around high-latitude globular clusters with the Hamburg 1-meter telescope in the 1920s, and of his “window” near the galactic center with the 100-inch in the mid-1940s.15 In the summer of 1954 Baade took his wife back to Germany to visit their surviving relatives and friends. It was a “strictly family visit” to Minden and Hamburg; his mother had died during the war as Richard Schorr had in 1951, but Baade’s brother and two sisters were still alive. In 1955 he returned to Europe for three meetings. The first was more or less of a lark, a celebration of the fiftieth anniversary of Albert Einstein’s publication of his first paper on relativity theory, organized by Pauli, Baade’s friend since their Hamburg days. The meeting was held in Bern in July; many outstanding physicists, a considerable fraction of them German or German emi-

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gre´ s including Max von Laue, Max Born, Walter Heitler, and Eugene Wigner, were present. Robert Trumpler, the Swiss who had been Leopold Ambronn’s student at Go¨ ttingen just before Baade and had emigrated to America in 1915, gave a paper on the observational evidence for the predicted gravitational light deflection and redshift. Baade himself spoke on “the expansion of the universe” and gave his best current value of the Hubble constant, which corresponded to an age of the universe of about 5.4 < 109 yr, with an uncertainty estimated at “still perhaps 20%” according to the report of the meeting. Baade had agreed to speak on this subject at Pauli’s urging, although privately he wrote his old friend that in his opinion the history of the universe would really come from understanding star formation and evolution, not by “chasing the cosmological problem.” Baade never submitted his manuscript, and Pauli eventually published the symposium proceedings without it, including only a two-sentence statement that the paper had been presented orally.16 The main events for which Baade had come to Europe that summer were the dedication of the new Hamburg Observatory 80-cm (32-inch) Schmidt telescope in August, followed by the IAU General Assembly in Dublin. Otto Heckmann, whom Baade and Schorr had both favored, had finally been named director of the observatory in 1942, after a long bureaucratic struggle between the advocates of “German science,” who preferred a real Nazi, and the bulk of the Hamburg University faculty, who wanted a true scientist. Heckmann, a Catholic and a relativity theorist, made only the minimum concessions to Hitlerism necessary to keep the observatory alive, and came out of the war with a relatively untarnished reputation. After the war, Baade supported him in every way, as he had promised Schorr he would. Baade helped arrange Heckmann’s tour of U.S. observatories in 1950, when most Germans were still not welcome, and made sure that he was invited to every important international astronomical conference, beginning with the IAU Symposium 1 at Groningen.17 West Germany had prospered in the 1950s under Konrad Adenauer, and Heckmann persuaded his government to provide the funds to build the 80-cm Schmidt telescope which Schorr had origi-

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nally gotten into the budget, as a symbol of peaceful science. Baade had supported it strongly, and Bowen had provided the plans for the 48-inch Schmidt, which the German engineers used as a starting point. Baade’s homecoming to Hamburg was a sentimental event. He inspected the Schmidt telescope which he had originally demanded in the 1930s and had lived to see built in the 1950s. At the symposium held in Hamburg as part of the dedication ceremonies, Baade gave an invited talk comparing, as always, the structure of M 31 and our Galaxy. Oort gave a paper on the spiral structure of our Galaxy, summarizing the latest Dutch and Australian 21-cm results, and Arno A. Wachmann, another Hamburg astronomer, reminisced on Bernhard Schmidt’s life and career. Along with numerous German astronomers, several Americans who had used Schmidt telescopes were there and presented oral papers.18 Just a week later Baade was at Dublin, for the IAU General Assembly. He, Oort, and Blaauw had outlined the preliminary program for the symposium on the comparison of the structure of our Galaxy with other galaxies. Most of the world experts on galaxies were there, including Viktor A. Ambarzumian, Grigory A. Shajn, and three other Soviet astronomers. Baade gave the keynote paper for the whole symposium, on the large-scale structure of spiral galaxies (which he still referred to as nebulae). He was the acknowledged leader of the field. Again he did not submit a written manuscript, but Nancy Grace Roman, the editor of the symposium, reconstructed one for him from a tape recording, which he then revised for publication.19 In 1956 Baade stayed in the United States, but took part in a conference on the cosmic distance scale “from trig[onometric] parallaxes to galaxies” at the University of Virginia, sponsored by the National Science Foundation. Although Oort could not come, Heckmann and Wilhelm Dieckvoss were there from Germany, Ben Gascoigne from Australia, and many American astrometrists and galactic-structure researchers. Baade’s paper began with his discovery of the two populations, continued through his revision of the distance scale, and ended with his and Swope’s latest results on the Cepheid variables in his outermost field in M 31. He had picked this field in the hope that extinction would be small or nonexistent

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there, but they were finding it was still present, though not as strong as in the fields closer to the nucleus.20 In 1957, the last year before he retired, Baade gave several lectures in England and then took part in the most important conference of his career, at the Pontifical Academy of Sciences in Vatican City. The Royal Astronomical Society had awarded him its Gold Medal in 1954, but Baade could not go to Britain then. Instead, at an RAS meeting that year, the president presented the medal to a high official of the U.S. Embassy, for transmission to Baade. He waited until May 1957 to go to London and deliver the George Darwin Lecture to the society. It was traditionally given by the Gold Medalist, if not a resident of the United Kingdom, for there was a fund to help pay the lecturer’s traveling expenses. At the meeting Baade was admitted to the RAS as one of its foreign associates. Then he gave his lecture, which was excellent. Although he had a long outline of it, and an even longer manuscript, the published summary of his lecture was based on notes taken by Roy H. Garstang and Peter Sweet, two young editors of Observatory magazine. Perhaps Baade had lost or misplaced his notes and manuscript. After the Darwin Lecture, Baade dined with members of the RAS Club, a small, self-perpetuating insiders’ group of senior members of the society, who met for dinner following each meeting. He sat at the head of the table, next to the elderly club president, F.J.M. Stratton, and no doubt regaled his fellow diners with many funny stories about astronomers (including himself), for which he was well known. Baade was also in England to give the Halley Lecture at Oxford, its highest recognition for an astronomer. In this lecture he included, along with his newest stellar population results, a tribute to Hubble, who had been a Rhodes Scholar there for three years before beginning his graduate work in astronomy in America. Probably the published report of that lecture was also based on notes taken by Garstang and Sweet, who had driven from London to Oxford to hear him again. Their report was illustrated with two prints of 200-inch direct photographs of NGC 205, the dwarf companion of M 31, and of the region between it and M 32, both rich with population II giant stars, taken by Baade. Instead of notes, he had

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brought a “pile” of prints with him, and invited Garstang to take as many as he wanted.21 On that same visit to England Baade also gave colloquia at Herstmonceux, the new site of the Royal Greenwich Observatory, and at Jodrell Bank, the big radio observatory near Manchester. By the time he left for Rome, almost every research astronomer in England had had a chance to learn about the two populations, stellar evolution, and the structure of galaxies from him.22 With Daniel J. K. O’Connell, the Jesuit priest who was director of the Vatican Observatory, Oort and Baade had been planning since 1955 for the “study week” on stellar populations, which was to be held under the Pope’s auspices. The idea was that a group of world experts would immerse themselves in discussing the problem in detail for a week at the Vatican, and emerge with a series of conclusions which the Supreme Pontiff would endorse, as he had done following the two previous similar conferences, on cancer in 1949 and on microseisms in 1951. The number of participants was limited to just over twenty top scientists; at Baade’s insistence several of them were theorists, including Hoyle, Schwarzschild, Salpeter, Lyman Spitzer, and Bengt Stro¨ mgren, and also two young American observers who were already world experts, Sandage and George H. Herbig. William A. Fowler, the Caltech nuclear physicist who had become a leader in understanding element formation in stars, was also there.23 The main subject was stellar evolution, the field which had not existed before Baade’s discovery of the two populations. By 1957 it was clear that population II stars were not only old but had relatively low abundances of elements heavier than hydrogen and helium (“metal-poor”). This was clearly an age effect, resulting from nuclear transmutations, and at first Baade had believed that age and heavy-element abundance were strictly correlated; however Morgan (who was at the Vatican conference) and later Arp and others had produced strong observational evidence that they were not, and that there were old stars with nearly normal “metal content,” particularly in the plane of M 31 near its nucleus, and in our Galaxy. Oort and Blaauw had spent considerable effort on preparing a detailed scheme for dividing stars up into six populations

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instead of two, but after prolonged discussion the conferees compromised on five, with an associated evolutionary picture. Baade accepted the evidence for old, high-metal-content stars, but continued to think in terms of just two main populations. The book that came out of this conference, which includes Baade’s three papers, along with all the others presented and edited transcripts of the discussions following them, became an important reference for the next decade.24 After this conference, Baade, Oort, Hoyle, and Blaauw went on to Amalfi, on the Gulf of Salerno south of Naples, both to unwind and to finalize the list of important research problems suggested for further work. Baade, Oort, and Blaauw also discussed their ideas for the symposium on galactic structure, to be held at the IAU General Assembly in Stockholm in 1958. Baade was not to attend it, however, for he would have to retire in June of that year, and had other plans.25 In addition to his many lectures, invited papers, and discussions with European astronomers, Baade played a key role in getting the project which became the European Southern Observatory started. He and Oort probably first conceived and discussed this concept not in Groningen in 1953, as the official history of ESO states, but in Pasadena in 1952.26 Oort taught a graduate course at Caltech during the winter quarter of that year, and spent much of his time at the Mount Wilson and Palomar Observatories office before going on to lecture at Princeton in the spring; years later Horace W. Babcock remembered hearing about Baade and Oort’s discussions at the time.27 Baade had long realized the advantages of an observatory south of the equator. Back in 1927, when he returned to Germany from his year in America on a Rockefeller Fellowship and reported on it to the Hamburg board which governed the observatory, he strongly advocated moving its 1-meter reflector to South Africa or South America. After seeing the big Mount Wilson telescopes in their far superior California climate, he had concluded that it would be impossible to compete with them in Germany. The Americans were already planning a 300-inch (which was later built as the 200-inch Hale reflector at Palomar). Either give up modern observational as-

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Group at lunch in Amalfi, Italy, after the Study Week on Stellar Populations at the Vatican City. Clockwise from the front: William A. Fowler (wearing sunglasses), Barbara Hoyle, Fred Hoyle, Walter Baade, Ardis Fowler (partly hidden), Mieke Oort, Jan H. Oort, Georges Lemaitre. (Courtesy of Adriaan Blaauw, who was part of the group and took the picture.)

trophysics and stick to second-rate projects or move the biggest telescope in Germany to the Southern Hemisphere, was Baade’s youthful, impetuous advice. Hamburg Observatory could seize the initiative in the southern skies, but if the Germans waited too long the Americans would ship their big telescopes south and beat them there too.28

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Hamburg had neither moved its 1-meter reflector south nor given up astronomy, but Baade had departed for Mount Wilson four years later. His friend Erich Schoenberg had headed the German astronomers’ drive to build an observing station in South Africa, but nothing significant resulted.29 A quarter of a century later, Baade could no longer envision a Hamburg Southern telescope or even a German one, but he still considered himself more of a European than an American, as did Oort, who had declined several proffered directorships in the United States, including those at Yerkes and Harvard. Certainly the Groningen symposium in 1953 was where the ESO idea came out in the open. Very probably Oort had insisted on inviting some of the superannuated European directors like Sir Harold Spencer Jones and Paul Couderc, who were not galactic structure experts, to participate in it in the hope of enlisting their countries in the project. Baade was the one person present at the symposium with unrivaled big-telescope experience and a strong sympathy for the idea. Talkative and insightful, he dispensed advice freely to all of the Europeans, then and later. Baade was never an official member of any of the ESO committees, but he frequently advised Oort, who pushed the idea through to reality. Baade always referred to it as the “European” or “Southern” observatory, and more than once as one “we” need[ed]; Oort usually called it the “co-operative” observatory. Probably Baade’s biggest contribution was to emphasize how important it was to make long, careful seeing tests at possible sites everywhere in the Southern Hemisphere, rather than rushing into an existing one in South Africa, as all the European directors, including Oort, had first intended to do.30 In 1957, when an American astronomer publicly proposed that the United States build a large telescope in the Southern Hemisphere, Baade described him to Oort as “a pretty dumb fellow with an outsize ego.” All the real leaders in American astronomy were committed to building the National Radio Astronomy Observatory and “the National Observatory (probably in Arizona)” first, Baade reassured Oort, and “any further projects in the foreseeable future [we]re definitely out.” Baade was “very strong” for ESO, he wrote

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Oort, for without it astronomy in Europe would remain “too stunted,” and he was “far too much the European to give in without a last ditch fight.” Oort incorporated relevant paragraphs from Baade’s letters in his reports to the other ESO committee members, and by the end of 1957 felt that it would become a reality, although the United Kingdom had decided to go its own way.31

Belgium, Wisconsin, Harvard, Princeton, and Australia Baade attended his last international scientific meeting in early June 1958. It was a Solvay conference, the eleventh in a series dating back to 1911, in which a small group of experts, mostly theoretical physicists, discussed a particular topic. This one was on the “structure and evolution of the universe.” The week began with several abstruse “general statements of cosmological theory” by George Lemaitre, Otto Klein, John A. Wheeler, Heckmann, and others. Then Oort, Baade, Sandage, and the Jodrell Bank radio astronomer Bernard Lovell filled them in on “experimental data on the universe.” Pauli and Robert Oppenheimer dominated the questioning, and Baade paid hardly any attention to the cosmologists, but praised Lovell’s results on the strong radio sources which he and his collaborators were beginning to find in clusters of galaxies. As at the relativity conference in Bern three years earlier, Baade did not bother to submit a manuscript for publication, but took a lively part in the discussions on supernovae, nucleogenesis, and stellar populations.32 Then at the end of June, Baade was back in the States, at Madison, Wisconsin, to give the Henry Norris Russell Lecture of the AAS. It is the highest honor of the society, awarded for “a lifetime of eminence in astronomical research.” Fittingly, he gave an inspiring hour-long review of galactic structure and stellar evolution as they were then understood through his concept of the two stellar populations.33 Baade never wrote up his Russell Lecture for publication in the Astrophysical Journal as most (but not all) of the other recipients of that honor did in those days. Probably he did not want to take the time to prepare a long paper. Hynek, the former Ohio State dean, now at HCO, and Charles A. Federer, the Sky and Telescope

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editor who had published Shapley’s 1953 claim that he had doubled the scale of the universe, cooked up a scheme to publish it in that magazine instead. Baade let Federer have the sketchy four pages of notes he had used when he gave the lecture, which the editor supplemented with his own notes and Hynek’s, both taken as they heard the talk. From these Federer put together a clear, straightforward manuscript of what Baade might have said, which followed the fragments of his notes quite closely, filled out with material from the other sources. It seems quite restrained, scientific rather than sensational, and a good moderate-length summary of what Baade probably did say, but he did not approve it and it never was published. No doubt Baade was too busy even to correct the manuscript carefully, and he certainly distrusted both Hynek and Federer.34 But June 30 marked his official retirement, and he was never to observe again with the 200-inch. Under the strict Carnegie Institution of Washington policies then in effect, a staff member had to retire at the end of the fiscal year in which he reached the age of sixty-five. He could keep an office (perhaps a smaller one), continue research with the data he had, and even observe with the 60-inch or occasionally even the 100-inch when no regular staff member wanted them, as Joy and Paul W. Merrill had done, but the 200inch would be out of the question. Baade did not want to do that; he intended to return to Germany and work up all the data he already had there. First, however, he was going to do some teaching. Partly this was to earn money; Baade was a well-paid senior staff member but also a free spender, and the CIW retirement pensions were low. Under the option he chose, he would receive $236 each month for the rest of his life, approximately the same salary a young assistant professor then earned. After his death his wife, Muschi, would receive half that amount.35 Long before Baade’s retirement, Cecilia Payne-Gaposchkin conceived the idea of inviting him to lecture for a semester at Harvard, and he had indicated that he would like to do so in the fall of 1958, just after he retired. Donald H. Menzel, who had finally been appointed director of HCO in 1954, enthusiastically endorsed the idea. Dean McGeorge Bundy allotted $6,000 for Baade’s salary for

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Walter Baade, probably in England, 1957. (From the Olin Eggen Collection, courtesy of NOAO/NSF.)

the one semester, a quite significant addition to his retirement income. Payne-Gaposchkin’s plan from the beginning was to have Baade’s lectures recorded and transcribed, so that he would have a clean, typed copy of what he had said to use as the basis for a book. An experienced author and editor herself, she realized how hard it was for Baade to write for publication, but she was determined to get his results into print. She knew how important they were.36 Payne-Gaposchkin arranged for Baade to stay in a visiting faculty apartment in one of the student houses at Harvard, and although he postponed the start of his lectures to remain in Pasadena to lobby for ESO with two visiting prominent German politicians, once he got to Cambridge and began teaching he found that he en-

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joyed it. Baade thought that Harvard had “a remarkably good bunch of able students, much better than the mediocre fellows, which usually crawl around at the Caltech”!37 For Baade’s first lecture on 29 September, forty listeners turned up, including three professors and eight Ph.D. research associates. Payne-Gaposchkin gave an emotional welcome, saying Baade’s lectures were “the fulfillment of a dream, deeply fraught with all sorts of things,” and Baade began with his historical introduction. The attendance gradually tailed off until it reached about twenty-five halfway through the course, and then remained constant; PayneGaposchkin, her husband Sergei, Hynek, and David Layzer attended nearly every lecture. Shapley never turned up for any of them. Baade flew home to California for the three-week Christmas vacation, returned to Cambridge after the New Year, and finished his last lecture on January 13, 1959. It was an excellent course, covering galactic structure and stellar evolution thoroughly.38 Then Baade went on to spend a month at the Institute for Advanced Study in Princeton, where Oppenheimer was director and Stro¨ mgren, professor of astronomy. Several of the theoretical physicists who dominated the Institute were then interested in stellar evolution, especially supernovae and nucleogenesis. Baade’s name was well known to them, particularly because he had been the first author of a paper with Geoffrey and Margaret Burbidge, Hoyle, Fowler, and Robert Christy, all then at Caltech. They had tried to interpret the characteristic near-exponential decay of the light curves of type I supernovae in terms of californium 254, an artificial radioactive isotope, samples of which had recently been synthesized in the Bikini hydrogen-bomb test. At Princeton Baade gave four lectures, one each week, and his notes show that he was right up to date on the latest details of the abundances of the elements in various samples of populations I and II. The Institute had a handsome endowment, and Baade probably received a generous stipend.39

✶

8

✶

The Finale and After ¨ TTINGEN, AUSTRALIA AND GO 1959–1960

Australia After Princeton, the “itinerant preacher,” as Walter Baade now jokingly called himself, returned to Pasadena for little more than two weeks before departing for Australia. Bart J. Bok had left Harvard in 1957 to become director of Mount Stromlo Observatory, with its 74-inch reflector, a part of the Australian National University (ANU), located only a few miles from its campus in Canberra. Although Bok could not have known it, Baade had actually recommended strongly against his appointment to the directorship. In late 1955, Mark Oliphant, the head of the postgraduate research school of physical sciences at ANU had consulted Baade and no doubt several other astronomers. He had asked them about potential successors to Richard v.d.R. Woolley, then director of Mount Stromlo Observatory, who was slated to leave for Britain to become Astronomer Royal. Oliphant, a native Australian, was a renowned nuclear physicist who had been Ernest Rutherford’s student, prote´ge´, collaborator, and ultimately assistant director at the Cavendish Laboratory at Cambridge. Then from 1937 until 1950 Oliphant was head of the physics department at the University of Birmingham, which he built up into an outstanding research organization specializing in cosmic-ray and high-energy particle physics. During World War II he had been highly placed in the secret British radar and atomic-bomb programs and had visited the United States, where he developed a friendship with C. Donald Shane, then Robert Oppenheimer’s assistant director for scientific personnel at Los Alamos. As soon as the war ended, Shane became direc-

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tor of Lick Observatory, and Oliphant very probably consulted him too about the Mount Stromlo post. Baade, asked for his opinion of Bok and Olin J. Eggen as possible directors, replied that Australia was perfectly placed for research on “the most exciting and most promising objects of further research . . . , the Magellanic Clouds and the center of our galaxy[,] both of which have to be tackled from the southern hemisphere. But up to now, very little progress has been made.” Harlow Shapley’s work “never went beyond trivialities,” the Radcliffe Observatory was “poverty stricken and lack[ed] both manpower and auxiliary equipment,” and Co´ rdoba Observatory, in Argentina, “never got into the smooth waters on account of political conditions.” With the Mount Stromlo 74-inch, then just coming into operation, Australia could seize the leadership in attacking “the most promising problems both in the Magellanic Clouds and the galactic nucleus.” Australia was no longer scientifically isolated; international airlines had changed the world. Hence Oliphant should aim very high in choosing the next director, “and try to get one of the top men among the younger group.” The job was “one of the real big plums in the field of astronomy.” Neither Bok nor Eggen could fill the bill in Baade’s opinion. Bok was “a prolific writer and an inspiring classroom teacher[,] but there the inspiration ends. He has never shown a spark of it in his researches because he is without originality.” Eggen would be even worse, Baade thought. He had developed as a “narrow specialist” (in photoelectric photometry) but was unsound even in it. (This referred to Eggen’s claims to have found several very narrow “sequences” in the color-magnitude diagrams of galactic clusters, an idea which was never widely accepted and has long since disappeared.) Eggen’s theoretical background was weak, and though under “proper supervision he could be a desirable staff member since he is very industrious,” he just did not have the qualifications to be a true scientific leader of a large observatory. Instead of either of them, Baade proposed two other potential directors, both “quite young but . . . the very best the U.S.A. may have to offer for quite a while.” They were Allan Sandage and Arthur D. Code, both with “wide interests and knowledge over the

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whole field of astronomy.” Both had “used every opportunity to get thoroughly acquainted with different fields of astronomy and their techniques (spectroscopy, photoelectric photometry, etc).” Both Sandage and Code were among the “rare individuals who are equally at home in the fields of theory and observation” and had real “originality and sound judgement.” Though they were young, Code only two years older than Sandage, and both had “just started,” they were “already now heads and shoulders above Bok and Eggen.” In spite of this advice, the ANU appointed Bok director of Mount Stromlo Observatory, and when he left to return to the United States as director of the University of Arizona’s Steward Observatory in 1966, Eggen succeeded him in Australia. Probably Baade’s advice was so strikingly different from the recommendations Oliphant received from other directors and from Bok’s colleagues at Harvard (who certainly had their own individual motives for recommending him for a job in Australia) that he ignored it. The peppery German was well known as a friendly, talkative, but “impractical” astronomer. Baade’s point of view had been shaped by his own experience, as happens to all of us. Clearly he was thinking in terms of Hamburg or Mount Wilson Observatory as they had been, well provided with instruments and operating funds, and with top-notch scientific staffs. If Germany or Hamburg had followed his recommendation back in 1927 to build a first-class observatory in South Africa, with a 1-meter or larger reflector, he would have jumped to accept its directorship, rather than turning it down as he did the Jena job, with its small telescope, no spectrograph, and an observatory preserved more for showing students the stars than for frontier research. If Baade had been appointed assistant director of Mount Wilson before or during World War II, and if Edwin Hubble had become director of Palomar, then when Walter S. Adams retired after the war, Baade certainly would have accepted a chance to succeed him in that directorship. Clearly Baade saw Sandage and Code as surrogates for himself as a younger man; they knew all about astronomy and could observe with any telescope at least as well as all their colleagues and far better than most of them. They could

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provide wise scientific leadership which would have gained acceptance because of the respect in which their colleagues held them and their research. But that is not all a director does. Even Adams spent a large fraction of his time preparing budget proposals, defending them before the CIW trustees, reporting and publicizing the scientific discoveries made by the Mount Wilson staff members, hiring night assistants and cooks for a mountain-top observatory, and making sure that there would be a station wagon with a driver ready to deliver observers to the telescopes on the mountain on schedule, and bring them back again. Hubble and Baade were completely unprepared for these mundane activities, and Baade did not even recognize their necessity in his letter to Oliphant. Sandage and Code were both outstanding young research workers, who became even better as they matured scientifically, but neither one of them would have been a good director in Australia in the 1950s. Baade had been quite unrealistic. Bok in fact turned out to be the ideal director for Mount Stromlo at that time. Loud, brash, pushy, from the moment he arrived in Canberra he began promoting astronomy and his observatory to the Australian taxpayers. With his “flashing eyes,” as Baade used to like to say, Bok could give excellent popular lectures, and did so over and over again, to every civic group, veterans’ organization, church circle, or government committee or official who would listen to him. If at first they thought they did not want to hear him, his first order of business was to barge in and convince them that they did. Bok’s biographer portrayed him in the title of his book as “The Man Who Sold the Milky Way,” and that is just what he did, except that in Australia he also sold the Magellanic Clouds and the center of our Galaxy. Bok made everyone in Australia astronomyconscious, with press releases, radio interviews, and daring confrontations with Cabinet ministers (which always turned friendly in the end). The bright new graduate students he recruited in Australia may have winced at some of his verbal excesses, and referred to him as an “astronomical godfather,” but they realized he was working hard and successfully to provide support for them, for their observatory, and for astronomy Down Under. And, as a senior

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astronomer from Harvard who knew all the leading American astronomers, he was in an excellent position to bring in the cream of the cream, like Baade, as visiting professors.1 Neither Sandage nor Code was at all equipped to play this role back in 1956, nor did either of them want to try it. Baade had even been wrong about Eggen. Woolley imported him to England as Chief Assistant, essentially the top staff member and deputy director at the Royal Greenwich Observatory, by then located at Herstmonceux. There he upheld and interpreted observational research and big telescopes in what had been a fairly traditional government bureau concerned mostly with time and nautical almanacs. Eggen worked hard with theorists (like Donald Lynden-Bell) and observers who understood theory well (like Sandage) and produced results, just as Baade had predicted, but in doing so he also broadened his horizons. In 1962 the three of them, Eggen, Lynden-Bell, and Sandage published a paper combining observational data on the motions of stars (from proper-motion catalogues), their color indices (measured in many cases by Eggen), and a theoretical discussion which amassed the evidence that the halo of our Galaxy is made up of old population II stars which formed from a primordial gas cloud before it collapsed to the galactic plane, where the population I stars formed more recently. The paper exerted a profound influence on the direction of subsequent studies of galactic evolution. By the time Eggen became director at Mount Stromlo in 1966, he was an effective scientific leader, if not as successful in selling his program to Australian politicians as Bok had been. But Bok had established Mount Stromlo as an important research observatory; Eggen was quite successful in keeping it scientifically productive.2 In 1958 Bok, who knew nothing of this negative recommendation made two years earlier, arranged for Baade to come to Australia for six months in 1959 to lecture and consult with the Mount Stromlo astronomers and the “radiophysicists” of the Commonwealth Scientific Industrial and Research Organization in Sydney, one of the leading radio-astronomy groups in the world. Baade’s stipend was to be £2,000 (roughly $10,000) plus travel expenses, “just about tops” for Australia, according to Bok. The radio astronomers were

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especially anxious to talk to Baade, and he was keen to form new contacts with them.3 Baade flew from Los Angeles to Sydney, with a stop in Honolulu. He arrived in Australia in early March, and gave a colloquium on the optical identifications of radio sources, then plunged into scientific discussions with the radiophysicists. A week later he went on to Canberra, but he had come down with pleurisy and bronchitis, which kept him out of circulation for nearly a month.4 Bok had organized a symposium on cosmology at the ANU, coinciding with a visit by the English theorist Herman Bondi. To Baade it was a waste of time; he consistently thought and said in his lectures that it was much more important to get better data on galaxy magnitudes, populations, forms, spectra, and colors first, before discussing the universe theoretically. Nevertheless, Bok signed him up for a talk on the cosmic distance scale at the symposium, telling Baade he could “leave cosmology out of it” altogether. Baade was still ill when the symposium began, and William G. Tifft, a visiting American postdoc who had taken his graduate course at Caltech, substituted for him. The next day Baade was well enough to put in an appearance.5 He gave a series of eight lectures at ANU in July and August, and discussed research constantly with all the staff members and students, particularly emphasizing important new Southern Hemisphere observing projects. They found him ebullient, colorful, and inspiring. His lectures were right up to date, now including a lot of the latest results of spectroscopists on the differences in metal abundances between different globular clusters, especially between those in the galactic halo and in the disk near the galactic center. While he was at Mount Stromlo, Baade took the opportunity to observe with its 74-inch reflector himself. It was “the most uncomfortable instrument with which [he] ever observed,” but nevertheless, remaining true to his own advice, Baade obtained several series of plates of NGC 6522, the globular cluster in “his” window near the galactic center, to check on Sergei Gaposchkin’s periods of the RR Lyrae variables in it. By then Baade was relaxed about observing; one night he arrived so late at the observatory, after a

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good dinner, that Bok exploded that if he were a graduate student he would fire him! Very probably Baade had gotten there early enough to start as soon as NGC 6522 was sufficiently high in the sky so that he could get good plates of it, but Bok was a fanatic on using every minute of telescope time. At Mount Stromlo Baade also studied all the available plates of the Magellanic Clouds; they fascinated him because by now he was learning to see the past history of star formation in them.6 Even while he was in distant Australia, Baade was still providing advice to a few young American astronomers. One of them was George H. Herbig, who at Lick Observatory was becoming one of the prominent experts on the observational study of star formation. He described several T Tauri stars, involved in nebulosity and prime candidates for recognition as very recently formed stars, to Baade in detail. Baade had encouraged Herbig to work on dwarf M and Me stars, the faintest objects known on the main sequence, and the young Lick astronomer in turn asked him to encourage his friends among the astrometrists to measure the proper motions of very faint stars in the Hyades cluster, to find some more objects that were cluster members for him to study.7 Soon after Baade’s lectures ended in mid-August, Bok hustled him off to the annual meeting of the Australia–New Zealand Association for the Advancement of Science in Perth, in distant Western Australia. There they took part in a symposium, “Radio and Optical Studies of Our Own and Other Galaxies,” together with two visiting Americans then at Mount Stromlo (Tifft and Hugh M. Johnson) and a number of Australian astronomers and physicists. Immediately following it, Bok took Baade on a search for an observing site in Western Australia, seven days of jolting travel in a crowded automobile, sleeping in backcountry accommodations in a land Baade had written reminded him of California thirty years earlier. Bok was a compulsive talker and doer; his heart was in the right place but he did not realize that he was wearing Baade down. A picture of the German astronomer, taken on a rocky desert somewhere in Western Australia, shows a tired, travel-worn old man.8 From Perth Baade flew straight back to Sydney, where he enjoyed a more relaxed two weeks repeating the gist of his Mount Stromlo

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Walter Baade in Australia, 1959. (Courtesy of William G. Tifft.)

lectures in three shorter talks, and discussing research with the radiophysicists individually. In mid-September he left Australia; during his six months there he had seen and talked with practically every astronomer and radiophysicist in the country, and they all loved him and had been inspired by him. En route he stopped for two days’ visit in Honolulu with his old friend C. E. Kenneth Mees, now retired, who as head of the Eastman Kodak Research Laboratory had provided him with the best, most sensitive plates they could produce.9 Then it was on to Pasadena, where, as on each of his earlier brief stops, Henrietta H. Swope reported to him on her progress in measuring the light curves of Cepheid variables on his plates of M 31 and the Draco dwarf system. Baade also packed a few plates and a huge mass of papers containing all the notes,

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measurements, reductions, and calculations he had accumulated in his twenty-seven years at Mount Wilson. Muschi was already in Germany, looking for a permanent home for them; on October 21, 1959, after a flight across the country and a night’s rest in a hotel on Long Island, Baade flew on to join her there. It was to be his final destination.10

Return to Germany and Death Baade arrived back in Germany little more than two weeks before he was to begin lecturing again. Go¨ ttingen University had appointed him its Gauss professor, a prestigious and well-paid shortterm lectureship named for the outstanding mathematician and Go¨ ttingen Observatory director Carl Friedrich Gauss. Baade was to give his course “Evolution of Stars and Galaxies,” this time in German for the astronomy graduate students at his alma mater. He had gone full circle with a vengeance; in 1921, a recent Go¨ ttingen Ph.D. and a young Hamburg Observatory staff member, he had begun his little notebook on the evolution of the stars, and in the intervening years he had not only made that subject a real, quantitative branch of science, but had added another to it, the evolution of galaxies.11 But now he only got through the first lecture, on November 7, 1959. From the summer of 1958, when he had retired, Baade had begun confiding to his closest friends that he felt old and tired. The strenuous lecturing schedules at Harvard and in Australia and his constant travel had worn him down. Living on his own for long periods of time with students had not helped; he got little sleep and to some of the more straitlaced of them seemed to exist on a steady diet of cigarettes, coffee, and scotch whiskey or brandy, with few healthy green vegetables. His congenital hip defect had worsened with age, and bone spurs had grown inward from two of his vertebrae, pinching the nerves in his back and causing intense pain. By late 1959 Baade could no longer stand or sit; he could only achieve partial comfort by lying prone, and he had to cancel his series of lectures.12 Muschi had found an apartment for them in Bad Salzuflen, a pleasant little spa close to Herford, not far from Minden, where

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Baade’s favorite sister, Ka¨ the Wehmer, still lived with her family (his brother, Martin, had died). It was a two-hour journey by train or automobile from Go¨ ttingen. They had planned to move into it after Baade’s lectures were to end in March 1960, but because of his painful condition they left Go¨ ttingen and went to Bad Salzuflen just before the end of 1959. Baade had spent much of his time since arriving in Germany on his back, but he believed his problem was a “harmless but painful affair” which the doctors would soon “fix.” However, he was never able to write again after his first Go¨ ttingen lecture; he dictated the few letters he sent after that to Muschi, who typed them neatly or wrote the shorter, more personal ones by hand.13 Baade’s physicians tried massage, then radiation (probably Xrays), but neither helped. In January his condition deteriorated seriously, and he suffered from palsy, then “lameness” or paralysis of his legs. His ingrowing vertebrae were pressing on his spinal column and pinching his nerves, and the only possible remedy was a serious operation. Baade underwent it on January 27, 1960, in the neurosurgical clinic at Go¨ ttingen, one of Germany’s best medical schools. The operation was described as successful but in order to heal, Baade was ordered to remain prone, in bed in the hospital, for many months, and his recovery was slow.14 He tried to keep his illness secret, so Ira S. Bowen, Swope (who was reducing and analyzing her measurements of his plates), and his other friends in Pasadena did not know why they had not heard from him, but feared the worst. In February Baade allowed Muschi to send Swope an upbeat report on the operation and his condition. The previous year, while he was in Australia, Baade had asked the new U.S. National Science Foundation if it would provide travel support and research support for him at Pasadena, Leiden, and Hamburg Observatory to finish analyzing his observational data and write up the results for publication. The infant agency was flush with post-Sputnik funds intended to “restore” America’s “lead” over the Soviet Union, and Geoffrey Keller, the Ohio State theoretical astrophysicist who was its temporary program director for astronomy, well aware of the “very high . . . scientific merit” of Baade’s work, had assured him there would be no problem at all in

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meeting his modest request for $3,500 for three years. An American institution would have to administer the funds, so if he were appointed a postretirement research associate at Mount Wilson and Palomar Observatories, even if the CIW, with its well-known reluctance to touch federal grants, would not accept this one for him, Caltech no doubt would take it. Not to be outdone, Caryl P. Haskins, the CIW president, acting on Bowen’s recommendation, not only appointed his star retired German astronomer to the research associateship but offered him $5,000 for four months in Pasadena, if he would do the work there and repeat his lectures as a course at Caltech in the fall quarter of 1960. Bowen, writing to wish Baade a rapid recovery, urged him to accept the appointment; according to Muschi he intended to do so. She had been in the hospital herself for a minor operation, cutting off all communication between her husband and America for nearly a month. When she could write again herself, in mid-April, she at least feigned to believe that he had made “great strides” toward recovery. They made plans for the future which they “hope[d] will most certainly be bright and wonderful again.”15 Donald H. Menzel, at Harvard, also wanted to get Baade to come back there for another year. No doubt Cecilia Payne-Gaposchkin had initiated this idea, planning that with her assistance he would convert the lecture notes of his 1958 course into a book. Although McGeorge Bundy, the Harvard dean, claimed he had no available funds for a special appointment in 1960–61, he suggested that since Baade seemed to be “perennially young,” Menzel could try again another year.16 Muschi had also sent Jan H. Oort a guarded report on her husband’s condition. Baade had agreed to be one of the main lecturers in a summer course on the structure and evolution of our Galaxy, to be held at Nijenrode in the Netherlands in July and August 1960. Oort hoped that Baade would be able to come by then; he also asked him “if you can easily write in bed” to let him know his “exact plans” and how soon he would come to Leiden to work on his data. Oort thought Baade should also begin writing his papers at Hamburg Observatory, and even wanted to get him involved in

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an ESO committee meeting in Heidelberg in July. From Pasadena, in contrast, William A. Fowler sent Baade the latest Caltech nucleogenesis results and only wished that “you were here to keep me on the straight and narrow path.”17 Alas, none of their plans were to come true. Baade was no longer a perennial youth. His doctors finally allowed him to sit up in a wheelchair for a few minutes late in June, nearly five months after his operation. Just a few days later, only the third time he sat up according to one report, he collapsed and died suddenly. Undoubtedly the months of lying prone had thrown his circulatory system into complete disarray, and probably a blood clot or embolism was the immediate cause of death. It was “the saddest event in astronomy for years to come” in the words of Otto Heckmann, the Hamburg Observatory director who was soon to become the first director of ESO. It was a terrible surprise to all of Baade’s friends, only a few of whom knew how ill he had been. His body was buried in Bad Salzuflen, on a gentle slope with a view over a pleasant valley, under a massive boulder bearing only his last name, BAADE. In a grisly aftermath, the pathologist who had prepared a death mask from Baade’s corpse offered it to Mount Wilson and Palomar Observatories for $285, but Bowen politely declined it, writing that the observatory had no way of paying for “this kind of material.” More appropriately in 1999 the municipality of Bad Salzuflen, on the recommendation of the German Astronomical Society and with the support of the IAU and several individual astronomers, designated Baade’s grave as a permanent memorial to him.18 Muschi lived on for many years after her husband, treasuring his memory to the end. She continued to receive regular monthly payments from the CIW, her surviving spouse’s share of her husband’s pension, and U.S. Social Security payments as well. In 1963 she moved to Bad Oeynhausen (also near Herford), then in 1965 to Bremen, in 1967 to Neuminster, and finally in 1976 to Lu¨ beck, where she eventually entered a nursing home. The CIW group hospitalization plan paid thousands of dollars each year for her care. As Muschi grew older, her memories of long ago became less accurate, but “Walter” was always the hero of them. The nurses thought she

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Walter Baade in profile, 1955. (Courtesy of The Observatories of the Carnegie Institution of Washington.)

had told them that he was a famous author, but they did not recognize astronomy as his subject or his scientific papers as his masterpiece. She died in the nursing home in Lu¨ beck on August 31, 1988, one day before her ninety-fifth birthday.19

Always a German but Never a Nazi Baade was born, lived, and died a German. He never wanted to be anything else. He loved his country, but best of all he loved his native region, Westphalia, where he was born, educated through the Gymnasium, returned as often as he could to visit his mother and his brother and sisters, and where he was buried. After one year at Mu¨ nster, Baade spent the rest of his university years at Go¨ ttingen, both of them close to but just outside Westphalia. Hamburg, where he worked twelve years, is somewhat further away, but not too far.

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For Baade, astronomy took precedence over his homeland; he hoped to go to America to observe with the big telescopes as soon as he received his Ph.D. in 1919, but that was out of the question for a “Hun” so soon after the armistice that ended World War I. When Adams offered him a position at Mount Wilson in 1931, Baade accepted it immediately, but he agonized long and hard about returning to Hamburg as director of its observatory in 1937. The attraction of the big telescopes in the land of clear skies won out, so in the end he decided to stay in California. But Baade never wanted to become an American citizen, and always planned to return to Germany as soon as he retired. He and his wife, childless, spoke German at home all their lives. Baade told Americans who asked that he had taken out “first papers” to become a citizen before World War II, but had “lost” them and so for bureaucratic reasons could never follow through. His students who wrote his obituaries dutifully reported this tale, but surely it was only a cover story designed to divert attention from his firm desire to remain German.20 A declaration of intent to become a citizen, which is what those “first papers” really were if they existed, is a legal document, and losing his copy of it, if he actually did so accidentally or on purpose, would not vitiate the step he said he had taken. The high-priced, super-effective, Carnegie Institution of Washington lawyers could certainly have “found” those first papers in the Los Angeles County Court records or could have produced a barrage of affidavits and testimony as a substitute for them if Baade had only asked Milton L. Humason, Adams, or Vannevar Bush to unleash them on his case. As a student with a congenital hip defect at Go¨ ttingen, Baade had served on a German weapons-development program during World War I. His able-bodied younger brother, Martin, had been a soldier at the front who survived and later joined the Stalhelm (or “Steel Helmets”), a right-wing veterans’ organization similar to the American Legion or the Veterans of Foreign Wars, except that the German veterans had been on the losing side. However, Martin Baade was never a Nazi; after World War II he was one of the leading spirits in forming a branch of the Christian Democratic Union, Konrad Adenauer’s party, in Minden. Martin’s son Eberhard, Walter

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Baade’s nephew, served as an officer on a German submarine during World War II. They were patriotic Germans, but were not Nazis.21 Baade doubtless worried about his nephew during the war, as he certainly did about Richard Schorr’s sons, who were all of military age when the war began. Wilhelm Dieckvoss, a young Hamburg Observatory astronomer, was called up in the German army and ended the war in a prisoner-of-war camp in the United States. From it he wrote Baade, who sent him the latest news of his studies of the RR Lyrae variables near the galactic center and also a book on mathematics for which Dieckvoss had asked.22 During the war Baade, an “enemy alien,” could not have participated in the Mount Wilson Observatory’s weapons development program if he had wanted to, but he was not considered a real security risk. He continued his peaceful astronomical research in the midst of this war project very successfully, as we have seen. When F.J.M. Stratton, the elderly British nova spectroscopist, traveled to California in 1942, clearly on a secret war project, he had no hesitation in visiting Baade in Pasadena, nor did Fred Hoyle two years later, on a similar mission.23 In the war years the American government carried out a gigantic home-front program to sell war bonds to everyone in the country. They helped finance the war, sopped up excess cash which otherwise would have fueled inflation, and provided all loyal citizens with a sense of participation in the war effort. Politicians, famous athletes, aviators, war heroes, and movie stars vied in urging every wage earner to sign up to “buy bonds” through regular payroll deductions. Every business, factory, university, and research center tried to achieve 100% participation in this program, and Mount Wilson Observatory was no exception. In the spring of 1942 Baade, no doubt under considerable moral pressure to prove his “loyalty,” authorized taking $37.50 from his salary every month, but in March 1945, with Germany obviously about to be finally and totally defeated, he began cashing in his bonds one a month, just as he had bought them. By then his need to conform was over. Muschi, speaking with American friends after the war, sometimes would use “we” to refer to the German people.24

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After the war Baade always supported Heckmann, whom he had recommended so strongly for the Hamburg directorship. Baade’s support meant a lot in America, and he helped greatly in organizing Heckmann’s visit to California and colloquium tour through the United States in 1950. He also put the Hamburg director in close touch with Bowen and the Caltech engineers who had designed and built the 48-inch Schmidt telescope. Eventually they gave Heckmann hundreds of blueprints and drawings to use in planning the Hamburg Schmidt telescope. Later Baade made sure that Heckmann was invited to speak at important scientific meetings, such as the study week on stellar populations at the Pontifical Academy of Sciences in 1957, where they were the only two German scientists present. Before long Heckmann was playing a leading role in planning ESO and its big telescope in the Southern Hemisphere, in no small measure because of his close association with Baade.25 Baade apparently wrote only one published popular article on astronomy in his life, and it was in German. It represents the text of a talk he gave as one of a series called “Of Earth and Universe” on the South German radio network, probably in the summer of 1955 when he was in Germany. Twelve noted German scientists, including Baade, Heckmann, Albrecht Unso¨ ld, Walter Fricke, Hans Kienle, Johannes Larink (who had worked with him at Hamburg Observatory in the 1920s), and Heinrich Siedentopf, all astronomers, the geophysicist Julius Bartels, and a few others gave one talk each. Baade’s was entitled “The Milky Way System and the Size of the Universe”; very probably he and the others gave impromptu talks in advance to a science writer, who then converted them into scripts for the broadcasts. In Baade’s papers in the Huntington Library there is an undated two-page draft in his handwriting, in German, entitled “New Measuring Rods for the Universe.” It is written in popular style and may have been the starting point of his talk. It is considerably more detailed and technical, dwelling on Cepheid variables and the two stellar populations, than the published text, which deals more in broad concepts, but includes much of Baade’s version of the history of research on our Galaxy, from Thomas Wright to Palomar. This article shows that Baade could cooperate

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with a writer who respected him and followed his line of reasoning, rather than bringing in Hubble or Shapley, as all the American writers had done. The result was an excellent popular account of galactic structure.26 Although he had dropped out of the German Astronomical Society soon after becoming a member of the Mount Wilson staff, Baade rejoined it after the end of World War II. He was now proclaiming that he planned to return to his homeland to stay, after he retired. Baade’s office and home were destinations for every German astronomer who visited Pasadena, and there were many of them in the 1950s. He and his wife welcomed them all; they both loved to speak their native tongue and catch up on the news from “home.” The visitors loved it too, for Baade was a great raconteur, Muschi was a vivacious hostess as well as a skilled cook, and neither of them stinted the cocktails or wine. The visiting Germans enjoyed every minute with them.27 On the other hand Baade disliked Russians, no doubt in part because he knew from many relatives and friends how the Soviet armies had shot and pillaged their way through Germany on their way to Berlin. As all of us are prone to do, he ignored the horrible excesses his own country’s soldiers had committed against their enemies’ people. After the war the Russians, as victors, played a much larger role in the IAU than previously, while the Germans were barred from membership in it for a number of years. In the immediate postwar period, Soviet scientists were under orders to publish their papers in Russian, rather than in the languages of the “decadent, capitalist powers,” and to use their own tongue in speaking and writing. It was not one of the languages which Baade had learned in the Gymnasium in Herford. He was infuriated to receive letters in Russian from high-level Soviet astronomers, which he had to have translated by Alexander Pogo, the Mount Wilson Observatory librarian who was an emigre´ scholar. In one case Baade was doubly incensed when he read the translation and found that it was a demand that more Soviet astronomers be appointed to the IAU commission on star clusters, which he chaired. He exploded in a bitter letter of complaint, threatening to return future letters in Rus-

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sian to their senders unopened, but very probably the Russian astronomers were named to his commission.28 Although Baade was a patriotic German, he never was a Nazi, contrary to the statement which Fritz Zwicky is said to have made to his face. As we have seen, Baade worked long and hard to create a haven in Pasadena for his friend Rudolph Minkowski, officially a Jew in Hitler Germany. Minkowski had to flee with his family from his native land, and Baade’s careful, diplomatic pressure on Adams made it possible for him to come to America, get a research position eventually, and in the long run become another outstanding research astrophysicist. Baade and Minkowski remained close friends and scientific collaborators until the older man’s death. Baade was also close personally and scientifically to Martin Schwarzschild, a Jew under Hitler’s infamous racial “laws,” who had fled Germany to Norway and then to the United States. Likewise Baade liked and respected Leo Goldberg and Jesse L. Greenstein, both outstanding American astrophysicists. Zwicky, who disliked Baade, Greenstein, and many other American astronomers intensely by the time he insulted Baade, had let his hatred overcome the reality he could see and surely knew.29

Honors and Honorary Societies Baade received many honors during his lifetime. In America he was named the Henry Norris Russell Lecturer of the American Astronomical Society and the Catherine Wolfe Bruce Lecturer of the Astronomical Society of the Pacific, the highest awards of those two societies. He also received the Gold Medal of the Royal Astronomical Society. In Germany he was elected to membership in the Scientific Academies of Go¨ ttingen, Bavaria, and Mainz, and in England was made a foreign associate of the Royal Astronomical Society, in Canada an honorary member of its Royal Astronomical Society, and in Holland an honorary member of the Royal Academy of the Netherlands. In 1953 Baade was elected to membership in the American Philosophical Society, founded by Benjamin Franklin in Philadelphia

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over two centuries earlier. It is America’s oldest and perhaps most prestigious honorary society for academics, many of them scientists. Adams, Frederick H. Seares, Paul W. Merrill, Joel Stebbins, and Shapley were all members then, as George Ellery Hale had been until his death, and as Lyman Spitzer, Martin Schwarzschild, and Allan Sandage would later become. Baade was also elected to the American Academy of Arts and Sciences, in Cambridge, the largest of the “big three” honorary societies, but never to the National Academy of Sciences, which is highly prestigious and to which all those named above, as well as John A. Anderson, Alfred H. Joy, and Ralph E. Wilson of the Mount Wilson group, had been elected. Membership in the National Academy was and is open only to American citizens, so Baade was not eligible for it. He certainly would have been elected had he been naturalized. But it is perhaps strange that the academy never chose Baade as one of its foreign associates, to which citizens of all other countries may be elected. In the 1950s only a few foreign associates were elected to the National Academy, on the average three each year. Almost all of them were resident abroad, the great men of science in their countries, often with ties to the United States through their students or through their own education. But it was very unusual, almost unheard of, for a foreign national, living in the United States, to be elected a foreign associate. In 1953, of the forty-nine foreign associates of the NAS, only two lived in the United States, one of them from China, the other from England. There was a strong feeling then (which still exists to some extent today) that if a scientist from abroad took a regular faculty position in this country and became a permanent resident, he “should” become a citizen. In fact this feeling is the reason that Baade was not elected a foreign associate. The nominations came from the various sections, and in 1955 the astronomy section considered Baade, then at the height of his fame. In that year Subrahmanyan Chandrasekhar (who had become an American citizen just two years earlier) was elected to the academy, and Bertil Lindblad, of Sweden, was elected a foreign associate. There were only two other astronomers who were foreign associates then, Oort and Sir Harold Spencer

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Jones, the retired Astronomer Royal. Otto Struve, then chairman of the section of astronomy, told the other members who were present at the meeting that April that he would probably be asked to recommend another astronomer as a potential candidate. He asked for their suggestions and got a total of seven names, two of whom he thought were already too old to be considered. The other five were Baade and Heckmann, Viktor Ambarzumian of the Soviet Union, and Pol Swings and Georges Lemaitre, both of Belgium. They were all extremely good scientists, but in any ranking by American astronomers at that time Baade would certainly have come out far above the other four. Gerard P. Kuiper, who, like Struve, had been born abroad and had become a naturalized American citizen as an adult, had suggested Baade’s name. Struve more or less vetoed him as a candidate, although he greatly admired his research and was his good friend as well. Struve, by then a professor at Berkeley, frequently dined with the Baades when he visited Pasadena. “I should be inclined to think that we would not be justified in proposing the name of a person, no matter how distinguished, who has the opportunity of becoming an American citizen, but has not availed himself of it. I should, therefore, personally welcome it if Mr. Baade were to receive one of the medals of the Academy for his distinguished work but would not be elected as a [f]oreign [a]ssociate,” Struve wrote. He invited all the section of astronomy members to send him their views, but evidently few, if any, contested what he had written, for Baade was not nominated or ever elected. Ambarzumian was elected a few years later, in 1959, and Swings in 1966. Probably there would have been a little anti-German feeling against Baade that soon after World War II, not from the astronomers who all knew him, but from the scientists in other fields who did not. Werner Heisenberg was elected in 1961, and Ludwig Biermann in 1976, but Albrecht Unso¨ ld and Carl F. von Weizsa¨ cker never were. Very probably if Baade had been well when he retired and returned to Germany, and had then published one or more papers there based on the research he had done at Palomar, he would have been elected as a nonresident foreign associate fairly quickly, perhaps before or soon after Heisenberg.30

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Baade’s Legacy Baade had expected to live many years in Germany, working up all the observational data he had accumulated and publishing a long series of postretirement papers. Oort, not realizing the seriousness of his friend’s condition, had begun pressing him to get to work on his research even while he was still in the hospital. Very soon after Baade’s death the Leiden director urged his widow to turn over her husband’s notes and work in progress to him, his Dutch colleagues, and Heckmann’s group at Hamburg, who would convert them into papers and publish them. In Pasadena, Swope had been tirelessly measuring magnitudes, colors, and Cepheid-variable light curves on Baade’s 200-inch plates of M 31 since his departure for Germany. She had slowly come to recognize how ill he must be from his failure to reply to her letters reporting progress and asking for further guidance. Swope was a close friend of Muschi, and now, almost simultaneously with Oort, she wrote the grieving widow, asking her to send Baade’s observing records, notes, papers, and the few plates he might have back to Pasadena. She, Minkowski, Sandage, and Arp would use these materials, together with the measurements she already had and others she would continue making on Baade’s plates, to prepare and publish papers in his name, giving his results.31 Swope, although undoubtedly sincerely moved by Baade’s death, and doing her best to console her friend, had clearly written parts of her letter with Bowen’s advice. As director of Mount Wilson and Palomar Observatories, he wanted his best-known staff member’s posthumous papers published from it, and he had written so to Muschi, in a letter of sympathy the day after Baade’s death. Oort, in contrast, thought that the California astronomers had more than enough data of their own; he believed that the Europeans would do a better job of working up Baade’s material and would publish it more quickly. Bowen and Oort jousted politely and gingerly over who would do what, but the Palomar director held all the cards; Baade’s plates, which belonged to the observatory where they had been taken, were still in Pasadena. Baade’s notes were in a “big wooden box” in their garage in Bad Salzuflen,

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Henrietta H. Swope, c. 1965. (Author’s collection.)

and though Oort had them moved to Leiden, inspection revealed that it would be impossible to publish anything without going back to the plates.32 Both Bowen and Oort regarded Baade’s ideas as his intellectual property, and took it for granted that his widow, his sole heir, therefore at least in principle owned them and should ultimately control the publication of his unfinished work. But they both wanted to be sure that she made the “right” decision as to just how that should be done. No doubt Muschi, depressed and alone, felt caught in the middle between these two quiet, polite, determined directors. She came up with a plan under which Lo Woltjer, whom Baade had regarded as “the best among the younger astronomers,” should be given “all of Walter’s unfinished work” unconditionally. He was to do what he thought best with all this material, using whatever he wished from it for his own research and giving other parts of it to

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those he thought best qualified to make good use of it in theirs. Oort was surprised; clearly he felt that he himself was the obvious choice for this role, not his young former student who was “primarily a theoretician and no expert on photometri[c] problems,” but he was “a first rate man” and the Leiden director “would fully trust him even for a task like this.” Oort and Heckmann “would continually discuss matters with Woltjer and keep a general eye on” him. Bowen was skeptical, but Woltjer was then working at Yerkes Observatory, and Minkowski, who did not know him well and feared he would be “as stubborn as a Dutchman,” met him there and discussed the matter with him thoroughly. Their conference must have been a success, for Minkowski concluded that no real problems would arise, and on his advice Bowen did not object to this plan but made it clear that any of Baade’s data which Mount Wilson and Palomar astronomers wanted would remain in Pasadena.33 In the end, publication of Baade’s remaining research results went smoothly. Swope completed her measurements of his plates and published them in three long papers (listing him as the first author and herself as second), summarizing his conclusions on the Draco dwarf system, one of the lowest-luminosity population II galaxies then known, and on three of his variable-star fields in the Andromeda galaxy. Arp published the coordinates of the H II regions in M 31 he had measured as a graduate-student research assistant from Baade’s direct photographs, outlining its spiral arms, in another joint paper. Arp showed how well the spiral arms they defined matched Baade’s sketches, and included a map of them. He emphasized that these arms did not fit a logarithmic spiral (the theoreticians’ dream of previous decades), and showed that the distribution of the H II regions could best be understood in terms of a warp (or bend) in the disk of M 31.34 Stephen van Agt, Oort’s student, published the one paper that came from Leiden, on the Ursa Major dwarf galaxy. One other direct fruit of Baade’s observational data was Virginia Trimble’s Caltech thesis on the expansion of the Crab nebula. Nicholas U. Mayall, Baade, and Oort had all used John C. Duncan’s 1939 measurements of the proper motions of the filaments, from blue-sensitive and orthochromatic plates he had taken with the 100-inch tele-

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scope. Baade had realized that red-sensitive plates, obtained with his filter that isolated Hα and the [N II] lines, suppressed the continuum radiation of the “amorphous mass” (the relativistic electrons) and made possible much better measurements of the filaments. He had taken first-epoch plates this way with the 100-inch, beginning in 1939, and with the 200-inch, in 1950; in the mid-1960s Guido Mu¨ nch, later Trimble’s thesis adviser, repeated these exposures and she measured the proper motions from them. Her paper, published eight years after Baade’s death, immediately became the definitive reference on the subject.35 Baade’s magnum opus appeared in print years before these last papers based on his observational data. After his death, the 1960 summer course on the Galaxy had gone on in Nijenrode without him. No single lecturer could take Baade’s place, but several of them, especially Oort and Plaut, expanded their previously planned series of talks to include subjects that would have been his.36 Quite a few of the participants, mostly advanced graduate students, postdocs, and young faculty members, were from the United States; one of them had brought along a copy of the mimeographed notes of Baade’s lectures at Harvard in 1958–59. They provided a much better statement of his ideas and results on stellar populations, stellar evolution, and galactic structure than anything then in print. This copy of the notes was kept in the reading room at Nijenrode, and Blaauw suggested that they might be copied and distributed to the participants. He and Oort had a secretary begin retyping them on mimeograph stencils, while the student, John Gaustad, wrote to Harvard to request permission to distribute them. The reply which came back was pleasant but firm; because of “certain references, especially to other astronomers, which might be misunderstood unless the context was known,” Payne-Gaposchkin and Baade had decided long ago not to distribute the notes except to those who had attended the Harvard course. Furthermore, Frances W. Wright, who replied to Gaustad, made it clear that Payne-Gaposchkin had been responsible for bringing Baade to Harvard to give the course, and had seen that his lectures had been recorded and transcribed, so she certainly deserved to be the one to edit a book based on them.

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In fact, Payne-Gaposchkin had already begun converting the notes into a book-length manuscript, working sporadically in her “spare time.” Now this exchange galvanized her into action. She was in Pasadena on sabbatical leave that fall, and switched to fulltime work on the volume. She planned to publish it as a book by Baade, edited by herself; everyone recognized that she was the right person to do it. Harvard University Press wanted to publish the book, and Oort suggested that the royalties be split equally between Baade’s widow and Payne-Gaposchkin. This was just what Muschi had thought appropriate, but evidently Payne-Gaposchkin insisted that her own share be reduced to 20%, for that is what the final contract specified, with the rest to the widow. Payne-Gaposchkin finished the manuscript by March 1961, cutting out all Baade’s criticisms of other astronomers’ work (which she considered appropriate in a lecture but not in a book), somewhat rearranging the text and rewriting it completely. She corrected a few actual errors that had crept into the notes, but remained true to Baade’s ideas and concepts, not trying to recast his thoughts to fit her own.37 Harvard University Press did not publish Evolution of Stars and Galaxies until 1963, but then it became an immediate success among astronomers. It was the first clear, book-length exposition of stellar evolution, galactic structure, high- and low-velocity stars, the two stellar populations, the types of galaxies, and how they perhaps had formed and would evolve that had been published. All astronomical libraries ordered it, and many astronomers bought their own personal copies. For years teachers of galactic-structure courses assigned it for supplementary reading. The first printing of 2,500 copies sold out completely, and a second printing in 1968 sold nearly 800 more copies. Then in 1975 the MIT Press reissued it as a paperback. It had a great influence on research astronomers in the United States and abroad for almost two decades.38 Besides his papers and his book, Baade’s legacy included not only his two Ph.D. thesis students, Sandage and Arp, but a host of European and American astronomers he had taught, trained, inspired, and guided. They were the primary leaders in the next two decades of research on stellar and galactic evolution. Although his friend and collaborator Oort was the moving spirit in creating ESO,

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Baade’s support, realistic advice, and will to make it succeed were crucial, and made it part of his legacy too. ESO’s first three directors, Heckmann, Blaauw, and Woltjer, were all very close to Baade. At Mount Wilson and Palomar Observatories, Horace W. Babcock, the stellar spectroscopist who had discovered magnetic fields in stars, succeeded Bowen as director, but after him Maarten Schmidt, another Oort and Baade prote´ ge´ , became the last director of the joint observatories. Baade was an outstanding scientist. He combined an excellent mind with excellent training and experience in Germany and the United States. He had tremendous drive, and lived for astronomical research. Dedicated to doing science himself, he avoided all administrative posts and hardly ever agreed to serve on committees, no matter how “important” or prestigious, except those directly concerned with operating his own observatory. Yet he spent hours, days, and weeks in scientific discussions with individuals and small groups of research workers and in lecturing to advanced students and working scientists. Baade had tremendous personal magnetism, and by the end of his life he had had a direct, stimulating effect on just about every astronomer in America, Germany, and Australia, as well as on many in Holland and England. Practically every one of them loved him, or at least admired him at the same time they were drawn to him, and he used that personal attraction to communicate his scientific ideas effectively to other scientists. Stellar evolution and population research dominated astronomy for two decades or more; today galaxy evolution, based on those fields, including the ages of globular clusters and other systems, is one of the most important research topics. The Magellanic Clouds are prime targets of the Hubble Space Telescope for astronomers everywhere for exactly the same reasons Baade wanted to observe them from South Africa, Argentina, Chile, Australia, or wherever he could, to see star formation and stellar evolution close up and in detail. Other hot subjects in astronomy and astrophysics at the turn of the millennium were the distance scale to galaxies and the closely related age of the universe, based on Cepheid variables and supernovae; star formation in galaxies; galactic haloes; and the nature of dwarf galaxies and their role in galactic evolution. All these

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Walter Baade 6.5-meter telescope, nearly completed, in its enclosure (octagonal dome), Las Campanas, Chile, 1998. (Photo by Frank Perez, The Observatories, Carnegie Institution of Washington.)

were were part of Baade’s intellectual legacy to the current generation, along with his insistence that accurate measurements must come before speculation. Although Baade’s name is not particularly well known to astronomers today, his ideas still have a most important place in the research they are doing. Baade’s name will live on in the 6.5-meter (260-inch) telescope, the first telescope of the Magellan Project, a joint venture of the Carnegie Institution of Washington, the University of Arizona, Harvard University, the Massachusetts Institute of Technology, and the

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University of Michigan at the CIW site at Las Campanas, Chile. The Baade Telescope, which went into full scientific operation in early 2001, has a very advanced, alt-azimuth design, with a lightweight f/1.25 primary mirror. The mirror was cast in 1994 at the University of Arizona’s Mirror Laboratory in Tucson, and in 1999 CIW president Maxine F. Singer announced the decision to name the telescope, then close to completion on Las Campanas, the Walter Baade Telescope, in honor of the great former CIW research astronomer.39

✶

A B B R E V I AT I O N S

✶

NAMES WSA RGA HA HB WB EBB JWB BJB ISB MGB VB AE CAF EBF GG CPG SG WMG LG GWG GEH EH HVH MLH JAH AHJ GEK GPK MM

Walter S. Adams Robert G. Aitken Halton Arp Johanna (“Hanni” or “Muschi”) Baade Walter Baade E. B. Biesecker John W. Boise Bart J. Bok Ira S. Bowen McGeorge Bundy Vannevar Bush Albert Einstein Charles A. Federer Edwin B. Frost George Gamow Cecilia Payne-Gaposchkin Sergei Gaposchkin Walter M. Gilbert Leo Goldberg George W. Gray George Ellery Hale Edwin Hubble Hendrik van de Hulst Milton L. Humason J. Allen Hynek Alfred H. Joy Gerald E. Kron Gerard P. Kuiper Max Mason

JCM NUM DHM RM JHM ERM JJN JHO DEO CPG JLP FER HNR AS LRS RS MS FHS CDS HS LS JS OS HHS MAT AEW LW WHW FZ

PUBLICATIONS A&A AJ AN

Astronomy and Astrophysics Astronomical Journal Astronomische Nachrichten

John C. Merriam Nicholas U. Mayall Donald H. Menzel Rudolph Minkowski Joseph H. Moore Edward R. Murrow Jason J. Nassau Jan H. Oort Donald E. Osterbrock Cecilia Payne-Gaposchkin Joseph L. Pawsey Frank E. Ross Henry Norris Russell Allan Sandage Lawrence R. Schmieder Richard Schorr Martin Schwarzschild Frederick H. Seares C. Donald Shane Harlow Shapley Lyman Spitzer, Jr. Joel Stebbins Otto Struve Henrietta H. Swope Merle A. Tuve Albert E. Whitford Lo Woltjer William H. Wright Fritz Zwicky

230 ApJ BAN BMNAS CIW IAU JHA JRASC MAG MHSB MNRAS PA PAAS PAPS PASP PNAS QJRAS S&T VJS ZfA

AIP

ANU ATNF BL

CIT CIW CWR

GO HCO

A B B R E V I AT I O N S

Astrophysical Journal Bulletin of the Astronomical Institutes of the Netherlands Biographical Memoirs of the National Academy of Sciences Carnegie Institution of Washington Year Book International Astronomical Union Journal for the History of Astronomy Journal of the Royal Astronomical Society of Canada Mitteilungen der Astronomischer Gesellschaft Mitteilungen Hamburger Sternwarte Bergedorf Monthly Notices of the Royal Astronomical Society Popular Astronomy Publications of the American Astronomical Society Proceedings of the American Philosophical Society Publications of the Astronomical Society of the Pacific Proceedings of the National Academy of Sciences Quarterly Journal of the Royal Astronomical Society Sky and Telescope Vierteljahrsschrift der Astronomischer Gesellschaft Zeitschrift fu¨r Astrophysik

Archives American Institute of Physics Microfilm Sources for the History of Modern Astrophysics Correspondence—Jan Hendrik Oort Australian National University, Canberra, A.C.T. Australia Telescope National Facility, Epping, N.S.W. Bancroft Library, University of California, Berkeley Otto Struve Papers University of California Archives California Institute of Technology Archives, Pasadena Ira S. Bowen Papers Carnegie Institution of Washington, Washington, D.C. Institution Files Special Collections Department, Case Western Reserve University, Cleveland, Ohio Jason John Nassau Papers Go¨ttingen University Observatory Records Harvard College Observatory Records, Harvard University Archives, Pusey Library, Cambridge, Massachusetts S. I. Bailey Director’s Papers Bart J. Bok Administrative Files

A B B R E V I AT I O N S

HHL

HO HPM LC

NAS OGC RAC SLO

UAL

ULL UWA

YOA

231

Donald H. Menzel Director’s Papers Harlow Shapley Director’s Papers Mount Wilson Observatory Collection, Henry E. Huntington Library, San Marino, California Walter S. Adams Papers Walter Baade Papers Ira S. Bowen Papers Edwin Hubble Papers Alfred H. Joy Papers Frederick H. Seares Papers Henrietta H. Swope Papers Hamburg Observatory, Hamburg, Germany George Ellery Hale Papers, Microfilm Edition, Carnegie Institution of Washington and California Institute of Technology, Pasadena Library of Congress, Washington, D.C. George Gamow Papers Merle A. Tuve Papers National Academy of Sciences Archives, Washington, D.C. Owen Gingerich Collection, Cambridge, Massachusetts Rockefeller Archive Center, North Tarrytown, New York International Education Board Records Mary Lea Shane Archives of the Lick Observatory, McHenry Library, University of California, Santa Cruz Directors’ Papers Gerald E. Kron Papers Nicholas U. Mayall Papers C. Donald Shane Papers Special Collections Department, University of Arizona Library, Tucson Gerard P. Kuiper Papers Bart J. Bok Papers University of Leiden Library, The Netherlands Letters and Papers of Jan H. Oort University of Wisconsin Archives, Madison Department of Astronomy Papers Joel Stebbins Papers Yerkes Observatory Archives, Williams Bay, Wisconsin Director’s Papers McDonald Observatory Correspondence

NOTE: AIP and ULL strongly overlap in their coverage, but I have listed the source in which I actually read each letter myself.

✶

NOTES

✶

PREFACE 1. DEO, “Walter Baade, observational astrophysicist (1): The preparation, 1893–1931, JHA, xxvi (1995), 1–32; (2): Mount Wilson, 1931–1947, JHA, xxvii (1996), 301–48; (3) Palomar and Go¨ttingen, 1948–1960 (Part A), JHA, xxviii (1997), 283–316; (Part B), JHA, xxix (1998), 345–77.

1. THE PREPARATION 1. [WB], “Entwicklung der Fixsterne” [a notebook], HHL; [WB], [untitled text for invited paper at AAS meeting, symposium at Perkins Observatory, 30 Dec. 1947], 15 pages, large handwritten sheets, HHL. 2. WB, “Tagebuch Mechanik, Wintersemester 1913/1914, Prof. Hilbert” [a bound, lined class notebook, with notes written in ink], HHL. 3. Heckmann (1976) contains many facts of Baade’s life and career, and is used as a source throughout this book. Heckmann’s obituary article (note 6 below) is reprinted in it, but without the list of Baade’s publications. Two other good obituary articles, emphasizing Baade’s training and early life in Germany, are A. A. Wachmann, “Walter Baade,” Die Sterne, xxxvi (1960), 204–7, and E. Schoenberg, “Walther Baade,” Bayerische Akademie der Wissenschaften Jahrbuch (1960), 177–81. Th. Schmidt-Kaler, “Walter Baades wissenschaftliche Genealogie,” Die Sterne, lxx (1994), 90–100, gives a complete account of Baade’s education and of his teachers’ research careers. 4. J. Hartmann, “Go¨ttingen” [annual reports of observatory for years 1916 through 1919], VJS, lii (1917), 198–205, ibid.; liii (1918), 186–88, ibid., liv (1919), 228–30; ibid., lv (1920), 87–89; WB, “Bahnbestimmung des spektoskopischen Doppelsterns β Lyrae nach Spekrogrammen von Prof. Hartmann” [Ph.D. thesis summary], Jahrbuch der Philosophischen Fakulta¨t in Go¨ttingen, Teil 2 (1922), 81– 88; ibid. [Ph.D. thesis in full, 40 typewritten pages], 23 July 1919, Niedersa¨chsische Staats- und Universita¨tsbibliothek, Go¨ttingen. 5. M. A. Hoskin, “The ‘great debate’: what really happened,” JHA, vii (1976), 169–82. 6. O. Heckmann, “Walter Baade,” MAG, cxiv (1961), 5–11; WB to [RS], 16 Aug., 21 Aug. 1919, HO. 7. DEO, R. A. Brashear, and J. A. Gwinn, “Young Edwin Hubble,” Mercury, xix (1990), 2–14; L. Ambronn to [RS], [∼ 21 Aug. 1919], J. Hartmann to [RS], 28 Aug. [19]19, RS to WB, 19 Sept. 1919, HO. 8. RS to [L. Ambronn], 19 Aug. 1919, [RS] to [Hamburg] University Authorities, [∼31] Aug. 1919, HO.

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9. RS, “Hamburg-Bergedorf” [combined annual reports of observatory for the three years 1919–1921], VJS, lvii (1922), 103–11. ¨ ber die Periode und den Lichtwechsel von SS Can10. K. Graff and WB, “U cri,” MHSB, v (1922), 22–24. 11. M. Wolf, “Nova oder Variabilis 3.1920 Cancri,” AN, ccx (1920), 374–76; WB, “The absolute photographic magnitude of supernovae,” ApJ, lxxxviii (1938), 285–304. 12. J. Larink, “Die vera¨ nderlichen Sterne in Kugelsternhaufen Messier 3,” Astronomische Abhandlungen der Hamburger Sternwarte in Bergedorf, ii, no. 6 (1924), 1–59. 13. See note 11; J. C. Duncan, “Three variable stars and a suspected ova in the spiral nebula Messier 33 Trianguli,” PASP, xxx (1922), 290–91; EH, “Cepheids in spiral nebulae,” PA, xxxiii (1925), 253–55; EH and AS, “The brightest variable stars in extragalactic nebulae. I, M 33,” ApJ, cxviii (1953), 353–61. 14. WB to [S. I. Bailey], 4 May 1921, WB to [HS], 7 May 1921, S. I. B[ailey] to WB, 2 June 1921, H. S. Leavitt to WB, 15 July 1921, HCO. 15. HS to WB, 5 July, 28 Aug. 1922, 5 Jan. 1923, WB to [H]S, 15 July, 11 Aug. 1922, 17 Jan. 1923, HCO. 16. WB, “7 Vera¨ nderliche in der Umgebung des Kugelhaufens M 53,” MHSB, v (1922), 35–39. 17. RS to WB, 12 Apr. 1920, HO; WB to W. W. Campbell, 7 Apr. 1923, SLO. 18. RS, “Hamburger-Bergedorf” [Annual reports of observatory for 1922 through 1925], VJS, lviii (1923), 107–12; ibid., lix (1924), 99–106; ibid., lx (1925), 94–100; ibid., lxi (1926), 119–29; WB and K. G. Malmquist, “Die Verwendung der Seareschen Methode zur Bestimmung der Farbenindex (exposure-ratio) fu¨ r Durchmusterungszwecke,” MHSB, v (1924), 135–47. 19. RS, Report on the observation of the total solar eclipse of Jan. 24, 1925, at sea, [∼15 Mar. 1925], HO. 20. RS to HS, 15 Feb. 1925, HS to RS, 13 Mar. 1925, HCO; HS to International Education Board, 13 Mar. 1925, HS to A. Trowbridge, 12 Apr. 1925, RAC. 21. WB to International Education Board, 11 Apr. 1925, RS to A. Trowbridge, 12 Apr. 1925, RS to HS, 14 Apr. 1925, HO. 22. A. Trowbridge to W. Rose, 16 Apr. 1925, W. Rose to A. Trowbridge, 27 Apr. 1925, W. W. Brierly to WB, 25 June 1925, W. Lund to W. E. Tisdale, 8 Oct. 1925, RAC. 23. WB to [HS], 13 Apr., 1 June, 13 July 1925, HS to WB, 14 Apr., 2 May, 20 June, 3 Aug. 1925, HCO. 24. W. E. Tisdale to WB, 15 Sept. 1926, W. E. Tisdale to W. Lund, 29 Sept. 1926, WB to [W. E.] Tisdale, 24 Nov. 1926, RAC. 25. [Anon.], “Anlegenheiten der Gesellschaft,” VJS, lx (1925), 54–55. 26. WB, “Photographische Beobachtungen von Kometen und Planeten,” AN, ccxxvii (1926), 119–24. 27. See note 9.

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28. WB, “17 Neue Vera¨ nderliche in Kugelhaufen Messier 53 (NGC 5024),” MHSB, vi (1926), 67. 29. WB, “Der kugelfo¨ rmige Sternhaufen NGC 5466,” MHSB, vi (1926), 61–65. 30. WB, “5 isolierte Haufenvera¨ nderliche in der Umgeburg des Kugelhaufens NGC 5466,” MHSB, vi (1926), 66. 31. RS, “Hamburg-Bergedorf” [Annual report of observatory for 1926], VJS, lxii (1926), 85–94; W. Lund to Commissioner of Immigration, 14 Feb. 1926, RAC. 32. “List of I-E-B Fellowships, Germany,” 10 May 1939, RAC. 33. BJB, “Harlow Shapley, 1885–1972,” BMNAS, xlix (1978), 241–91; O. Gingerich, “Through rugged ways to the galaxies,” JHA, xxi (1990), 77–88. 34. It is impossible to know with certainty or to document Baade’s impressions of American astronomy in 1926–27, for he was far too diplomatic to express them openly. He may have written about them to close personal friends in Germany (such as Hanni Bohlmann, his future wife), but if he did, no letters have survived in archives known to me. However, he was remarkably consistent in most of his views all of his life. My comments in this section are therefore based on my own memories of Baade’s later attitudes, expressed in many conversations I had with him in the years 1953–1958, of lectures I heard him give (in which he was by then much less guarded than in his letters) at the University of Michigan in 1953 (mimeographed notes exist, prepared by the auditors, which give a little of the flavor of his remarks), of a Caltech graduate course which he gave and I attended, and of several colloquia I heard him give in those years. They are also based on the mimeographed notes from a graduate course he gave at Harvard in 1958, prepared by R. B. Rodman from a tape recording, which were eventually published in facsimile as WB, Evolution in Stars and Galaxies (Ann Arbor, 1980). These notes served as the basis for Baade’s 1963 book, Evolution of Stars and Galaxies, edited by Cecilia Payne-Gaposchkin, who had organized and attended the course. She believed that the original notes reproduced too much of Baade’s “very personal and often critical tone” which was “quite appropriate in a lecture but most unfortunate on a printed page” and cut all vestiges of it out of the book. (CPG to JHO, 14 Nov. 1960, HHL.) 35. C. Payne-Gaposchkin (1984). 36. D. E. Osterbrock (1997), 47–76. 37. RS to EBF, 29 Oct., 30 Nov. 1925, EBF to RS, 12 Nov. 1925, WB to [EBF], 21 Dec. 1925, 21 Mar., 4 Apr. 1926, EBF to WB, 23 Mar., 13 Apr. 1926, YOA; EBF, “Yerkes Observatory” [Annual report for 1925–26], PAAS, v (1927), 445–48. 38. J. S. Plaskett to [W]B, 14 June 1926, HHL. 39. RS to RGA, 14 Oct. 1923, 15 Feb. 1924, 29 Oct. 1925, WB to RGA, 21 Dec. 1925, 2 June, 13 July 1926 (telegram), RGA to WB, 7 June 1926, SLO.

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40. RS to WSA, 29 Oct. 1925, WSA to RS, 2 Dec. 1925, HHL. 41. WB to [FH]S, 29 July 1930, FHS to WB, 23 Oct. 1930, HHL. 42. WB to [H]S, 7 Oct. 1926, HCO. 43. WB, “Der Sternhaufen NGC 5053,” AN, ccxxxii (1927), 193–99. ¨ ber eine Mo¨ glichkeit, die Pulsatsionstheorie der δ Cephei Ver44. WB, “U a¨ nderlichen zu pru¨ fen,” AN, ccxxviii (1926), 359–62. 45. A. J. Wesselink, “The observations of brightness, colour, and radial velocity of δ Cephei and the pulsation hypothesis,“ BAN, x (1946), 91–99. ¨ ber den auf die Teilchen in den Kometen46. WB and W. Pauli, Jr., “U schweifen ausgeu¨ bten Strahlungsdruck,” Naturwissenschaften, xv (1927), 49–51. 47. [GEH] to AE, 2 Dec. 1926, 21 Jan. 1927, [GEH] to WB, 6 Dec. 1926, AE to GEH, 25 Dec. 1926, HPM. 48. WB to [W. Lund], 20 Dec. 1926, RAC; HS to WB, 15 Feb. 1927, HCO. 49. WB to [H]S, 5 Mar. 1927, HCO. 2. THE PATH TOWARD THE TWO POPULATIONS 1. RS, “Hamburg-Bergedorf” [Annual report of observatory for 1927], VJS, lxiii (1928), 159–72. 2. Higher Educational Authorities, Hamburg, to [Hamburg] Observatory, 21 Sept. 1927, HHL; ibid., 10 Nov. 1927, HO. 3. RS, “Hamburg-Bergedorf” [Annual report of observatory for 1928], VJS, lxiv (1929), 193–212. 4. WB, “Untersuchung von zwei Milchstrassenfeldern auf Vera¨ nderliche,” AN, ccxxxii (1927), 65–70. 5. See chapter 1, note 43. 6. HS to WB, 16 Jan. 1928, HCO. 7. WB to [H]S, 20 Aug. 1928, HS to WB, 5 Sept. 1928, HCO. 8. See note 1; see also W. J. Luyten, “Total solar eclipse of July 29, 1927 at Jokkmukk,” PA, xxxvi (1928), 75–78. 9. A. A. Wachmann, “From the life of Bernhard Schmidt,” S&T, xv, no. 1 (1955), 17–23; S. Marx and W. Pfau (1992). 10. WB, “Bericht u¨ ber die Hamburgische Sonnenfinsternes-Expedition nach den Philippinen,” [∼ 15 June 1929], [15-page typed report], HO. 11. [W. A. Kinney], “Fame for Eccentric Genius in Historic Sky Survey” [5page press release, based on an interview with WB], 18 June 1949, HHL. 12. B. Schmidt, “Ein lichtstarkes, komafreies Spiegelsystem, MHSB, vii (1931), 15. An English translation appears in S. Marx and W. Pfau (1992). 13. WB to CPG, [~30] Apr. 1946, OGC. 14. [W.] Pauli to [W]B, 2 Mar. 1927, HHL. 15. WB to Higher Educational Authorities, Hamburg, 2 Jan. 1928, HO; WB to Herr Oberregierungsrat, [Jena], 14 Jan. 1928, HHL.

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16. Herr Oberregierungsrat Stier to WB, 5 Apr. 1928, WB to Herr [Stier], 18 Apr. 1928, [WB], “Denkschrift u¨ ber den Ausbau der Jenaer Sternwarte” [typed 7-page plan], 3 May 1928, WB to Prof. [Straubel], 4 May 1928, WB to Herr Oberregierungssrat, 4 May 1928, Herr Stier to WB, 26 June 1928, HHL. 17. O. Knopf to WB, 19 Apr., 27 Sept. 1928, HHL. 18. WB to Higher Educational Authorities, Hamburg, 18 Sept. 1928, RS to ibid., same date, HO. 19. WB to Dean of the Mathematical-Physical Faculty, Hamburg University, 7 Dec. 1928 (endorsed as approved 12 Dec. 1928), HO. 20. Printed announcement of inaugural lecture, Hamburg University, 30 Jan. 1929, HO; WB, “Extragalaktische Nebel als Sternsysteme: Habilitationvorlesung; H[am]b[ur]g” [handwritten 14-page manuscript], [30 Jan. 1929], HHL. 21. J. Schramm (1996), 199–200: RS, “Hamburg Bergedorf” [Annual report of observatory for 1929], VJS, lxv (1930), 122–40. 22. Baade and Muschi told Martin Schwarzschild “one long, dark night” of their rootless feeling, always living in rented quarters “with their bags packed,” MS to DEO, 16 Aug. 1993. ¨ ber die Entfernungen und Dimensionen der extragalaktischen 23. [WB], “U Nebel” [handwritten text for a lecture to the Naturwiss. Verein zu Hamburg], 19 Dec. 1928; WB, “Neuere Untersuchungen u¨ ber extragalaktischen Sternsysteme[,] Nach einem Vortrage auf dem Naturforschertage in Ko¨ nigsberg” [typed 13-page manuscript], 8 Sept. 1930, HHL. 24. WB, “Der Nebel NGC II 1613,” AN, ccxxxiii (1928), 407–8; see also note 1. 25. “Bericht u¨ ber die Versammlung der Astronomische Gesellschaft zu Hei¨ ber einen bemerkdelberg, 1928, Juli 18–21,” VJS, lxiii (1928), 248–71; WB, “U samswerten neuen Nebelhaufen in Ursa Major,” AN, ccxxxiii (1928), 65–70. 26. See note 3. 27. WB, “Der kugelfo¨ rmige Sternhaufen NGC 4147,” AN, ccxxxix (1930), 353–58. 28. WB, “Schwache Haufenvera¨ nderliche in hohen galaktischen Breiten,” AN, ccxliv (1931), 153–58. Baade had not mentioned finding any RR Lyrae variables in the Cygnus and Sagitta fields in his 1927 paper on them. 29. WB to HS, 8 Apr. 1930, HCO. 30. WB to [H]S, 15 Feb. 1930, HS to WB, 27 Feb., 22 Apr. 1930, HCO; see also note 27. 31. See note 3; RS, “Hamburg-Bergedorf” [annual report of observatory for 1929], VJS, lxv (1930), 122–40; WB to [J. G.] Hagen, 15 Nov. 1929, 24 Jan. [1930], Vatican Observatory Archives, Vatican City. ¨ ber den photographischen Lichtwechsel von Eros,” MHSB, vii 32. WB, “U (1930), 39; WB, “Bestimmung der photographischen und photovisuellen Helligkeit von Eros,” AN, vii (1933), 355–84.

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33. WB, “Beobachtungen des Pluto an Spiegelteleskop der Hamburger Sternwarte,” MHSB, vii (1931), 44. 34. WB, F. Goos, P. P. Koch, and R. Minkowski, “Die Intensita¨ tsverteilen in den Spektralinien des Orion-Nebels,” ZfA, vi (1933), 87–88. 35. H. Zanstra, “An Application of the Quantum Theory to the Emission of Light by Diffuse Nebulae” [ms.], July 1925, RAC; RS, “Hamburg-Bergedorf” [annual reports of observatory for 1930 and 1931], VJS, lxvi (1931), 164–83; ibid., lxvii (1932), 199–214. ¨ ber einen zweiten, sehr entfernen Nebelhaufen in Ursa major,” 36. WB, “U AN, ccxliii (1931), 107–10. 37. MLH, “The apparent radial velocities of 100 extra-galactic nebulae,” ApJ, xxc (1936), 10–22. 38. WB, “Photographische Beobachtungen von Nebeln in Spiegelteleskop der Hamberger Sternwarte in Bergedorf,” AN, cccxliii (1931), 303–8. ¨ ber der Periode von RV Canum ven,” AN, ccxliii (1931), 179–80. 39. WB, “U 40. WSA, “Memorandum regarding Dr. Baade of the Hamburg Observatory,” [∼ 15 Sept. 1930], HHL. 41. WSA to FER, 22 Dec. 1930, WB to [WS]A, 20 Jan. 1931, HHL. 42. WSA to WB, 2 Feb., 21 Mar. (telegram), 24 Mar. 1931; JCM to WSA, 25 Feb., 11 Mar. 1931; WSA to JCM, 21 Feb., 21 Mar. 1931; [W]B to [WS]A, 26 Mar. 1931 (telegram), HHL. 43. [JCM] to EH, 27 Apr. 1929, WSA to JCM, 19 Mar. 1934, JCM to WSA, 21 Mar. 1935, CIW; WSA to I[SB], [∼ 5 Dec. 1947], CIT. 44. RS to the University Authorities, Hamburg, 25 Jan. 1930, HO. 45. WB to the University Authorities, Hamburg, 20 Apr. 1931, RS ibid., 20 Apr. 1931, Extract from proceedings of the Hamburg Senate, 22 June 1931, HO. 46. WB to [WS]A, 11 Apr. 1931; WSA to JCM, 30 Apr. 1931; WSA to WB, 18 May 1931, HHL. 47. JCM to WB, 23 May 1931; WB to [WS]A, 10 June, 22 July, 1931; WSA to WB, 30 June 1931; [WSA] “To whom it may concern,” 30 June 1931, HHL. 48. WB to [WS]A, 22 July 1931, HHL; “Prof. Dr. Baade Departs for America,” Bergedorfer Zeitung, 1 Sept. 1931. 3. BEFORE THE WAR 1. FER to WSA, 5 Feb. 1931, HHL. 2. WSA, “Annual report of the director,” CIW, xxxi (1932), 135–72. 3. WB to [H]S, 15 Mar. 1937, HCO. 4. Some of Baade’s unexpurgated statements about Hubble’s hopeful treatment of magnitude scales are preserved in WB, Evolution in Stars and Galaxies (Ann Arbor, 1980), a facsimile reprint of the mimeographed notes from a graduate course he gave at Harvard in 1958, prepared at the time by R. B. Rodman from a tape recording.

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5. Baade’s progress is briefly described in successive annual reports, similar to the one for 1931–32 of note 2, WSA, “Annual report of the director,” CIW, xxxii (1933), 127–65; xxxiii (1934), 125–57; xxxiv (1935), 157–90; xxxv (1936), 157–94. These annual reports, drawn up by Adams on the basis of draft paragraphs and sections prepared by individual staff members like Baade, are full of information and are used throughout this chapter without being referenced each time. 6. WSA, ibid., xxxvi (1937), 3–39; xxxvii (1938), 2–32; xxxviii (1939), 3–31. 7. HS to WB, 3 Mar. 1937, HCO. 8. JS to WB, 23 Nov. 1935, 19 Nov. 1942, UWA. 9. WB to [NU]M, 19 Nov. 1945, SLO; WB to [IS]B, 12 Sept. 1946, HHL. 10. WSA, “Annual report of the director,” CIW, xxxix (1940), 3–26. 11. WB to [NU]M, 10 May 1936, SLO. 12. WB, “The distance of the globular cluster NGC 5694,” PASP, xlvi (1934), 52–53. 13. WB, “The globular cluster NGC 2419,” ApJ, lxxxii (1935), 396–412; HS to WB, 4 Feb. 1935, HCO. 14. WB, “A new, very distant cluster-type variable,” PASP, xlviii (1936), 274–76. 15. WB to [RS], 21 Oct. 1935, [RS] to WB, 18 Nov. 1935, HO. 16. WB, “The distance of the Cygnus cloud,” ApJ, lxxix (1934), 475–82. 17. See chapter 2, note 20. 18. H. D. Curtis, “The nebulae,” Handbuch der Astrophysik (Berlin), v/ii (1933), 774–936. 19. K. Lundmark, “The pre-Tychonic novae,” Lund Observatory Circular, viii (1932), 216–18; D. Hoffleit, “Observations of supernova,” PAPS, lxxxi (1939), 265–76. 20. WB and FZ, “On super-novae,” PNAS, xx (1934), 254–59. 21. WB and FZ, “Cosmic rays from super-novae,” PNAS, xx (1934), 259–63. 22. WB and FZ, “Remarks on super-novae and cosmic rays,” Physical Review, xlvi (1934), 76–77. 23. EH and G. Moore, “A super-nova in the Virgo cluster,” PASP, xlviii (1936), 108–10. 24. J. A. Anderson to GEH, 5 Oct. 1931, HPM. 25. R. Mu¨ ller (1986). This is by far the best source for an outline of Zwicky’s life and career, but any statement in it based on retrospective writings by the subject himself must be taken with a large grain of salt, if it is not corroborated by contemporary written evidence. Mu¨ ller reveals this indirectly many times in the book, not in the text itself but in the references in the appendix. Several more discrepancies have turned up in the research for the present book between Zwicky’s later statements and written letters from the 1930s in archives cited or in published papers. The same is true of FZ, “Basic results of the international search for supernovae,” in M. Dufay (1965), 166–80.

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26. WB to [RS], 7 June 1935, [RS] to [W]B, 19 Aug. 1935, HO. 27. WB to [RS], 28 July 1936, HO; C.F.J. Overhage and FZ, “Some magnitudes of nova 618.1936 Aquilae (I),” PASP, xlviii (1936), 321–23; WB and FZ, “Photographic light-curves of the two supernovae in IC 4182 and NGC 1003,” ApJ, lxxxviii (1938), 411–21. 28. FHS to FZ, 31 Aug. 1937, HHL. 29. HS to W[B], 19 Aug., 4 Sept. 1936, HCO; FZ to CPG, 8 Aug. 1936; R. Mu¨ ller (1986). I have not had access to this letter, which Shapley and Mu¨ ller describe; Mu¨ ller quotes directly the one sentence from it which I have quoted in the text, translated back into the English in which Zwicky presumably wrote it. 30. CPG, “On the physical condition of the supernovae,” PNAS, xxii (1936), 332–36; FZ, “Characteristic temperatures in super-novae,” PNAS, xxii (1936), 557–61. 31. FZ, “On the frequency of supernovae,” ApJ, lxxxviii (1938), 529–41. 32. WB, “Further notes on the super-nova in NGC 4273,” PASP, xlviii (1936), 226–29; WB, “The absolute photographic magnitude of supernovae,” ApJ, lxxxviii (1938), 285–304. 33. DEO, “Nicholas Ulrich Mayall,” BMNAS, lxix (1996), 3–26. 34. WB to [NU]M, 15 May, 30 June 1936, 13 Jan., 13 Mar. 1937, SLO, are just a few of the early examples of such letters. 35. WB to [NU]M, 16 Jan. 1937, SLO. 36. NUM, “The spectrum of the Crab nebula in Taurus,” PASP, xlix (1937), 101–5. 37. WB to [NU]M, 3 Nov., 10 Nov. 1938, SLO; NUM, “The Crab nebula, probable supernova,” ASP Leaflets, iii (1939), 145–54. 38. DEO, “Rudolph Leo Minkowski,” BMNAS, liv (1983), 271–98; Th. Schmidt-Kaler, “Rudolph Minkowski,” Sterne und Weltraum, xxxiv (1995), 436–40. 39. WB to [WS]A, 15 June 1933, HHL. 40. W. E. Tisdale memorandum, 24 July 1933, WSA to MM, 31 July 1933, RAC. 41. RM to [WS]A, 20 July 1933, MM to WSA, 13 Oct. 1933, R. Ladenburg to WSA, 1 May 1934, WSA to MM, 24 May 1934, ERM to WSA, 21 July, 19 Sept. 1934, WSA to ERM, 27 Sept. 1934, HHL. 42. ERM to WSA, 19 Oct. 1934, WSA to W. Weaver, 26 Oct. 1934, Weaver to WSA, 1 Nov., 16 Nov. 1934, RM to WSA, 25 Feb. 1935, HHL. 43. ERM to WSA, 17 May 1934, FHS to JCM, 27 May, 3 June (telegram and letter) 1935, JCM to FHS, 29 May, 31 May (both telegrams), 31 May (letter) 1935, R. F. Hanson to JCM, 24 June 1935, JCM to Hanson, 13 July 1935, WSA to JCM, 9 Aug. 1935, HHL. 44. WB and RM, “The spectrum of comet Peltier (1935a),” PASP, xlviii (1936), 277–78.

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45. WB and RM, “The Trapezium cluster of the Orion nebula,” ApJ, lxxxvi (1937), 119–22; WB and RM, “Spectrophotometric investigations of some Oand B-type stars connected with the Orion nebula,” ApJ, lxxxvi (1937), 123–35. 46. WSA to Director, Emergency Committee, 25 Mar. 1936, J. Whyte to WSA, 10 Apr., 14 May 1936, WSA to Whyte, 30 Apr. 1936, WSA to JCM, 18 May 1936, HHL. 47. WSA to JCM, 14 Jan., 3 May 1937, JCM to WSA, 8 Mar. 1937, HHL. 48. WSA to JCM, 11 Apr., 25 Apr., 17 May 1938, JCM to WSA, 14 Apr., 5 May 1938, HHL. 49. RM, “The spectra of the supernovae in IC 4182 and NGC 1003,” ApJ, lxxxix (1939), 156–217. 50. RM, “Spectra of the supernova in NGC 4275,” PASP, lii (1940), 206–7; RM, “Spectra of supernovae,” PASP, liii (1941), 224–25. 51. WMG to WSA, 19 Dec., 26 Dec. 1942, 5 June 194[2], WSA to WMG, 22 Dec. 1941, HHL. 52. WB to [RS], 22 Oct. 1934, 1 Apr. 1935, HO. 53. RS to [W]B, 13 May, 14 May 1935, WB to [RS], 7 June 1935 (second letter of this date), HO. 54. WB to [RS], 21 Oct. 1935, 21 July 1936, HO. 55. RS to Prof. Backer, 23 Jan. 1936, WB to [RS], 24 Mar. 1936, RS to [R.] Mentzel, 6 Apr. 1936, Lecture announcement for 14 Feb. 1936, HO. 56. RS to [W]B, 14 June, 30 Sept., 15 Nov. 1936, WB to [RS], 7 Dec. 1936, HO. 57. WSA to JCM, 29 Sept., 7 Oct. 1936, JCM to WSA, 1 Oct. 1936, HHL. 58. WB to [RS], 7 Oct. 1936, HO. 59. WB to [RS], 24 Jan. (cablegram), 2 Mar. (cablegram and letter), 22 Mar., [15] Apr., 30 May 1937, [RS] to [W]B, 25 Jan., 15 Apr., 24 Apr., 29 Apr., 24 June 1937, HO. 60. WSA to MM, 6 July 1937 (telegram), 10 July 1937, MM to WSA, 7 July (telegram) 1937, WSA to JCM, 7 July, 19 July, 28 July, 16 Aug. 1937, JCM to WSA, 8 July, 9 July, 21 July, 1 Aug. 1937, G. L. Streeter to WSA, 20 Aug. 1937, HHL. 61. [W]B to [RS], 19 July [1937] (cablegram), RS to [W]B, 20 July 1937, HO. 62. WSA to RS, 19 July 1937, HHL. 63. WB to Councilor Niemann, 29 July 1937, WB to [RS], 4 Aug. 1937, 13 Mar. 1938, WB and HB to [RS], 10 Aug. 1937, HO; GPK to “Dear Colleague,” 14 June 1946, YOA. Kuiper was a member of the ALSOS mission, which followed close behind the Allied armies, debriefing and interrogating many Dutch and German astronomers and physicists during and immediately after the war. His letter of 14 June 1946 summarizes his belief at the time that there were no anti-Nazi astronomers left, except K.-O. Kiepenheuer. The letter from Baade to Councilor Niemann is a typed copy, sent to Schorr from the Hamburg Education Board, so it is not certain that Baade actually wrote “Heil Hitler!” in his original letter, but it is quite likely.

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64. [NUM] to WB, 19 Oct. 1937, 8 Mar. 1938, WB to [NU]M, 24 Oct., 24 Nov. 1937, 14 Jan., 11 Mar., 18 May, 26 May 1938, SLO. 65. WB, “Stellar photography in the red region of the spectrum,” PAAS, ix (1939), 31–32. 66. WB to [RS], 12 July 1938, [R]S to [W]B, 14 July 1938, HO; WB, [Untitled report on the advantages of the new red-sensitive plates], Transactions IAU, vi (1939), 452–53. 67. WB to [FH]S, 31 Aug. 1938, WB to [WS]A, 11 Sept. 1938, HHL; WB to RS, 26 Sept. 1938, HO. 68. HS to WB, 2 Nov. 1938, HCO. 4. WAR AND A GREAT DISCOVERY 1. WB to [O]S, 2 Sept. 1932, YOA. 2. WB to [O]S, 28 Nov. 1938, YOA; WB to RS, 8 May 1939, HO; D. S. Evans and J. D. Mulholland (1986). 3. WB, “Paper at dedication of the McDonald Observatory [handwritten text in parts and notes in other parts], [∼ 6 May 1939], HHL. 4. WB to [NU]M, 20 May 1939, SLO; JHO to AHJ, 23 Dec. 1939, HHL; WB to [JH]O, 17 Oct. 1940, 6 Oct. 1941, [JHO] to WB, 23 June, 22 Nov. 1941, AIP. 5. WB to RS, 13 July 1939, HO; WB to E. P. Varela, 5 June 1939, CIW; WB to [H]S, 1 Sept. 1939, HCO; “Programme du Colloque de la Fondation SingerPolignac, Paris, Juillet 1939,” HHL. 6. WB, “Supernovae,” in A. J. Shaler (1941), 177–96. 7. WB to WSA, 22 July 1939, HHL; WB to RS, 31 July, 4 Aug., 15 Aug., 8 Nov. 1939, RS to [W]B, 24 Aug., 10 Nov. 1939, HO. Baade was not listed as a member in the directories of the German Astronomical Society published in VJS, lxviii (1933), 432; ibid., lxx (1935), 381; ibid., lxxii (1937); ibid., lxxiv (1939), 281–304, although many American and English astronomers were still members of it in 1939, including CPG, HNR, HS, JS, OS, A. S. Eddington, F.J.M. Stratton, and H. Spencer-Jones, then Astronomer Royal and director of the Royal Greenwich Observatory. However, no Mount Wilson astronomers belonged to it at that time. 8. WB to [O]S, 24 July 1940, OS to WB, 6 Aug. 1940, YOA; WB to [WH]W, 27 July 1940, JHM to WB, 30 July 1940, WHW to WB, 6 Aug. 1940, SLO. 9. WB to [GP]K, 17 Sept. 1940, 28 Nov. 1941, 10 Apr., 17 June 1942, [GPK] to WB, 27 Dec. 1940, 31 Mar. 1942, UAL; WB to [O]S, 9 Oct. 1940, OS to WB, 14 Oct., 19 Oct. 1940, YOA. 10. WB, “Nova Herculis 1934—the nebula and central star,” PASP, lii (1940), 386–88; WB, “The expanding shell around the nova Herculis,” PASP, liv (1942), 244–49. 11. JHO to WHW, 13 May, 20 June, 24 July 1939, WHW to JHO, 22 June, 1 July, 26 Aug. 1939, WB to [NU]M, 19 Oct. 1940, 27 Feb. 1941, SLO.

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12. JHO to NUM, 22 July, 26 July, 6 Sept. 1940, 16 Jan., 6 May, 26 June, 28 Sept. 1941, 30 Aug. 1945, NUM to JHO, 2 Oct. 1940, 27 May, 3 Dec. 1941, SLO; J.J.L. Duyvendak, “Further data bearing on the identification of the Crab nebula with the supernova of 1054 A.D. Part I. The Chinese records,” PASP, liv (1942), 91–94; NUM and JHO, “Part II. The astronomical aspects” PASP, liv (1942), 95–104. 13. WB, “Eine Rotaufnahme des Krebsnebels,” Die Himmelswelt, l (1940), 20–21. 14. WB, “The Crab nebula,” ApJ, xlvi (1942), 188–98; RM, “The Crab nebula,” ApJ, xlvi (1942), 199–213. 15. WB to [NU]M, 13 Nov., 27 Nov., 6 Dec. 1941, 14 Jan., 29 Jan. 1942, SLO. 16. WB, “Brightness and lightcurve of Nova Ophiuchus (1604)” [his undated, handwritten, careful notes, all in German, on the “primary observations,” containing long passages copied out in Latin and Italian with various phrases underlined for emphasis], [c. 1937?], HHL; WB, “Nova Ophiuchi of 1604 as a supernova,” ApJ, xcvii (1943), 119–27; WB, “B Cassiopeiae as a supernova of type I,” ApJ, cii (1945), 309–17. ¨ hman, 29 Feb. 1944, 14 Oct. 1946, O ¨ hman to WB, 9 June 1944, 17. WB to Y. O 9 Oct. 1946, Center for History of Science, Royal Swedish Academy of Sciences, ¨ hman to P. Swings, 8 Jan. 1945, UWA. Stockholm; O 18. EH, “Problems of nebular research,” Scientific Monthly, li (1940), 391–408; EH, “Zwicky’s systems in Sextans and in Leo,” Scientific Monthly, lii (1941), 486; F. Zwicky (1976). The phrase quoted in the text is a sample “description” from this book, and several of those named are also criticized in it. See also R. Mu¨ ller (1986), however, which downplays Zwicky’s hostilities and outbursts. 19. HS, “A stellar system of a new type,” HCO Bulletin, cmviii (1938), 1–11; HS, “Two stellar systems of a new kind,” Nature, cxlii (1938), 715–16. 20. WB to [H]S, 2 Feb. 1939, HCO. 21. WB and EH, “Two new stellar systems in Sculptor and Fornax”, PASP, li (1939), 40–44. EH to V. M. Slipher, 11 June 1941, HHL, states very clearly Hubble’s position on observers producing data and “theoretical men” speculating. 22. E. Schoenberg to WB, 13 July 1939, K. Wurm to WB, 18 July 1939, HHL. 23. WSA to VB, 6 Apr. 1942, VB to WSA, 8 Apr. 1942, WSA “To Whom It May Concern,” 8 Apr. 1942, FHS to ibid., 8 Apr. 1942, WSA to Selective Service Draft Board, Altadena, 9 Apr. 1942, WSA to Col. Severin, 30 Apr., 14 May 1942, 100-inch Telescope Logbooks 1 and 2, HHL; GPK to OS, 17 May 1942, YOA. 24. WB, “The resolution of Messier 32, NGC 205, and the central region of the Andromeda nebula,” ApJ, c (1944), 137–46; WB, “NGC 147 and NGC 185, two new members of the local group of galaxies,” ApJ, c (1944), 147–50. 25. WB to [H]S, 14 Aug., 28 Aug. 1942, 20 Apr. 1943, HS to WB, 25 Aug., 2 Sept. 1942, 9 Apr., 28 July, 18 Oct. 1943, MLH to HS, 28 Feb. 1944, HCO.

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26. JHM to NUM, 19 July, 14 Oct. 1943, 11 Jan., 11 Mar. 1944, NUM to [JH]M, 7 Oct. 1943, 5 Jan., 17 Jan. 1944, WB to [JH]M, 19 Oct. 1943, SLO. 27. WB to [O]S, 26 June, 8 July, 31 Aug. 1944, OS to WB, 1 July, 12 July, 6 Sept. 1944, YOA. 28. F. Hoyle (1994), 221–30. 29. WB, “The globular clusters NGC 5634 and NGC 6229,” ApJ, cii (1945), 17–25. 30. WB to [W.] Dieckvoss, 26 Sept. 1945, HHL. 31. WB, “A search for the nucleus of our Galaxy,” PASP, lviii (1946), 249–52. 32. R. L. Sanford, “Reno meeting of the Astronomical Society of the Pacific,” PASP, lviii (1946), 236–38; anon., “New members [of the ASP],” PASP, xlv (1933), 78. 33. ISB to WB, 4 Jan. 1946, HHL. 34. WB to [NU]M, 3 Oct., 10 Oct., 18 Oct., 4 Nov. 1946, 9 Jan., 12 July, 31 July, 25 Aug., 23 Sept., 6 Oct., 23 Oct. 1947, SLO. 35. WB, “Population II; 1947 Group meeting (January); Journal Club; Structure of Our Galaxy (comparison with M 31)” [folder of notes for four different talks, one from January 1947, one dated “Journal Club 1947 April 30,” another clearly from 1946 from an internal reference, fourth probably also for 1947], HHL. 36. WB, [Survey of the problem of the two stellar populations], [1947], 15 handwritten pages of text of his invited talk, HHL. 37. C. H. Gingrich, “The seventy-fifth meeting of the American Astronomical Society,” PA, lvi (1948), 59–63; J. L. Gossner and S. D. Gossner, “Columbus meeting,” S&T, vii (1948), 91–93. The abstracts of many of the papers, including CPG’s and RM’s, are printed in AJ, liii (1948), 193–207, but not WB’s. 38. WB to [CP]G, 3 Feb. 1947, OGC; WB to F. L. Whipple, 2 July 1947, HHL. 5. YOUNG STARS AND OLD 1. See chapter 4, note 12. 2. A. Blaauw and M. Schmidt, “Jan Hendrik Oort (1900–1992),” PASP, cv (1993), 681–85; H. C. van de Hulst, “Jan Hendrik Oort (1900–1992),” QJRAS, xxxv (1994), 237–42; S. van den Bergh, “An astronomical life: J. H. Oort (1900– 1992),” JRASC, lxxxvii (1993), 73–75; J. K. Katgert-Merkelyn, “A short biography of Jan Hendrik Oort,” in J. K. Katgert-Merkelyn (1997), xv–xxx. 3. [JHO] to WB, [∼ 28 Aug. 1945], WB to [JH]O, 29 Oct. 1945, AIP. 4. WB to [JH]O, 7 Feb., 22 Apr., 18 July, 23 Sept. 1946; [JHO] to WB, 4 Apr., 15 Apr., 17 June, 18 July, 2 Sept. 1946, AIP; JHO, “Some phenomena connected with interstellar matter,” MNRAS, cvi (1946) 159–79. 5. JHO to WB, 7 Sept. 1946, WB to [JH]O, 23 Sept. 1946, AIP. 6. [JHO] to WB, 11 Jan., 24 Oct. 1947, WB to [JH]O, 31 Oct. 1947, 19 Mar. 1948, AIP; WB to HVH, 7 Sept. 1950, HHL.

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7. WB to C.E.K. Mees, 4 June 1945, WB to W. F. Swann, 23 July, 10 Aug. 1948, 6 Jan. 1949, WB to D. McIntosh, 19 Oct. 1949, WB, “U.S. Achievement Award to Mount Wilson and Palomar Observatories” [his acceptance remarks on behalf of the observatory, probably written by ISB], 8 Nov. 1949, HHL. 8. WB to [NU]M, 1 Sept. 1948, SLO; WB to [JH]O, 7 Mar. 1949, AIP. 9. WB to D. ter Haar, 22 Oct. 1948, HHL. 10. WB to [NU]M, 3 July, 23 Aug., 18 Sept., 18 Nov. 1946, 29 Aug., 4 Dec. 1947, 22 Sept. 1948, SLO. 11. WB to [NU]M, 10 Aug. 1948, 6 June, 27 July 1949, 20 Aug. 1951, LRS to N[UM], 11 May 1950, SLO. 12. GG to Director, Hamburg Observatory, 10 Apr. 1966, G. Traving to GG, 9 May 1966, HB to GG, 11 May 1966, LC. 13. WB to NUM, 7 Nov., 14 Nov. 1945, SLO. 14. JS to WB, 27 July 1947, WB to R. H. Stoy, 7 Dec. 1950, HHL; WB to [AE]W, 12 July 1947, 15 Jan., 22 Mar. 1949; AEW to WB, 1 Dec. 1948, 20 Mar. 1950, UWA; GEK to WB, 19 Aug. 1948, 24 Oct. 1950, W[B] to N[UM], 16 Feb. 1950, SLO. 15. CDS to MLH, 13 Dec. 1946, WB to CDS, 1 Feb. 1947, WB to [NU]M, 20 Jan., 28 Apr. 1948, 20 Jan. 1949, SLO. 16. ISB, “Annual report of the director,” CIW, xl (1948), 3–26; WB to [NU]M, 28 Nov. 1947, SLO. 17. WSA to I[SB], [∼ 5 Dec. 1947], CIT; WB to [NU]M, 20 Jan. 1948, MLH to NUM, 19 Apr. 1948, SLO. 18. WB, “A program of extragalactic research for the 200-inch Hale telescope,” PASP, lx (1948), 230–34; O. J. Lee, “The seventy-ninth meeting of the American Astronomical Society,” PA, lvi (1948), 341–48; G. H. Herbig, “Astronomical meeting—Pasadena, June 28–July 1, 1948,” PASP, lx (1948), 219–24. 19. DEO, “The appointment of a physicist as director of the astronomical center of the world,” JHA, xxiii (1992), 155–65. 20. EH, [“Scientists and the Ballistic Research Laboratory, Aberdeen Proving Ground, Md. during World War II”], “Address delivered to the Sunset Club, 1946” [undated handwritten ms. by EH, and typed reading copy of same], HHL. 21. ISB, “Annual report of the director,” CIW, xlviii (1949), 3–27; xlix (1950), 3–50. These and the other similar annual reports up through lx (1961), 59–99, are used as sources for facts later in this chapter without being cited again. 22. EH, “First photographs with the 200-inch Hale telescope,” PASP, lx (1949), 121–24. 23. W[B] to N[UM], 21 July, 22 Oct. 1949; M[LH] to NUM, 27 Feb. 1951, 21 Oct. 1953, SLO; N[UM] to [Grace] Hubble, 28 Sept. 1953, HHL; G. E. Christianson (1995). 24. EH, “Explorations into space: the cosmological program for the Palomar telescope,” PASP, xlv (1951), 461–70.

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25. WB, “Provisional determination of the limiting magnitude of the 200inch” [3-page typed report], 5 Feb. 1949, HHL; AEW to WB, 7 Mar., 18 May 1949, UWA. 26. WB to [JH]O, 29 May 1950, 25 Apr. 1951, AIP. 27. WB to Sister M. Therese, 27 Feb. 1951; WB to HHS, 7 Jan. 1952, HHL. 28. S. E. Gates to WSA, 17, 20, 23 Jan. 1931, HHL; HS to HHS, 15 Sept. 1939, 21 Aug. 1950, HHS to HS, 10 May 1942, 20 Apr. 1943, 1 Oct. 1945, 9 Jan. 194[6], 8 May 1947, 13 Jan., 19 Mar. 1948, 27 Sept. 1951, HCO. 29. HS to HHS, 23 Nov. 1951, 27 June 1952, HCO. 30. See note 21; WB to [JH]O, 22 Sept. 1948. 31. WB to [RS], 10 Nov. 1937, 4 Aug. 1939, HO. 32. W. A. Kinney to WB, 10 June 1949; [W. A. Kinney], “Fame for Eccentric Genius in Historic Sky Survey” [5-page press release], 18 June 1949, HHL. 33. WB, “Galaxies—present-day problems,” Pub. Michigan Obs., x (1951), 7–17. 34. R. S. Richardson, “A new asteroid with the smallest mean distance,” PASP, lxi (1949), 162–65. 35. HS to ISB, 13 July 1949, ISB to HS, 22 July 1949, HCO. 36. RM, “The diffuse nebula in Monoceros,” PASP, lxi (1949), 151–53. 37. “Symposium—Current problems of Schmidt telescopes,” AJ, lv (1950), 65–84; C. H. Gingrich, “The eighty-second meeting of the American Astronomical Society,” PA, lviii (1950), 1–7; CAF, “Tucson trail,” S&T, ix (1950), 109–11. Baade’s paper is listed by title only, “The forty-eight inch Palomar Schmidttelescope” on p. 84 of the first reference, following the abstracts of the other papers at the symposium and the meeting. An article by Mount Wilson and Palomar Observatory “computer” (assistant) A. Beach, “A new celestial camera surveys the universe,” ASP Leaflet, ccxliv (1949), 1–16, is probably close to Baade’s thinking at that time. 38. [JHO] to WB, 1 Mar., 4 Apr., 29 Apr., 27 June, 21 July 1949; WB to [JH]O, 18 Apr. 1949, AIP; W[B] to N[UM], 9 Aug. 1949, 27 Oct. 1950, SLO. 39. WB and NUM, “Distribution and motions of gaseous masses in spirals,” in J. M. Burgers and H. C. van de Hulst (1951), 165–84. 40. HVH, “The shell of Nova Aquilae,” in J. M. Burgers and H. C. van de Hulst (1951), 116–17; HVH to DEO, 6 May 1998. 41. HNR, “On the probable order of stellar evolution,” Observatory, xxxvii (1914), 165–75, “On the sources of stellar energy,” PASP, xxxi (1919), 205–11, “On the problem of stellar evolution,” PA, xxxiv (1926), 244–45; H. N. Russell, R. S. Dugan, and J. Q. Stewart (1927); H. Bethe, “Energy production in stars,” Phys. Rev., lv (1939), 434–56. See also K. Hufbauer, “Astronomers take up the energy-generation problem, 1917–1920,” Historical Studies of the Physical Sciences, xi (1981), 277–303; K. Hufbauer (1991); O. Gingerich, “Report on the progress in stellar evolution to 1950,” in P. C. van de Kruit and G. Gilmore (1994), 3–20; and D. H. DeVorkin (2000).

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42. L. Spitzer, Jr., and J. P. Ostriker (1997), 3–20; LS to DEO, 19, 31 Aug. 1993; MS to DEO, 16 Aug. 1993. 43. HNR, “Present state of the theory of stellar evolution,” Scientific Monthly, lv (1942), 233–38. 44. CPG, “Problems of stellar evolution,” S&T, ii [July issue] (1942), 5–7, 19; F. L. Whipple, “Concentrations of the interstellar medium,” ApJ, civ (1946), 1– 11. The latter paper was presented at the 1942 Tonanzintla dedication, but not published until after World War II had ended. 45. WB to [HN]R, 9 June, 24 June 1944; HNR to WB, 14 June 1944, HNR Papers, Special Collections Department, Princeton University Library. 46. GG to [WS]A, 8 Mar. 1944, WSA to GG, 6 Apr. 1944, HHL. 47. WB to [CP]G, 3 Feb. 1947, OGC; WB to [BJ]B, 25 Mar., 21 Apr. 1947, BJB to WB, 7 Apr. 1947, UAL; WB to F. L. Whipple, 2 July 1947, HHL. 48. HNR, “On the distribution of absolute magnitude in populations I and II,” PASP, lx (1948), 202–4. 49. HNR, “Notes on the constitution of the stars,” MNRAS, xc (1931), 951–66. 50. WB to LS, 27 Jan. 1949, HHL; LS, “Princeton University Observatory” [annual report for 1949–50], AJ, lv (1950), 198–201; WB to [JH]O, 2 June 1952, AIP. 51. GPK to WB, 10 Nov., 9 Dec. 1949, WB to GPK, 5 Dec. 1949, UAL; H. Weaver to [W]B, 3 Jan. 1950, HHL; C. F. von Weizsa¨ cker, “The evolution of stars and galaxies,” ApJ, cliv (1951), 165–86. 52. LS and WB, “Stellar populations and collisions of galaxies,” ApJ, cxiii (1951), 413–18. 53. HA, W. A. Baum, and AS, “The H-R diagrams for the globular clusters M 92 and M 3,” AJ, lvii (1952), 5–6. 54. MS, “Inherited and acquired characteristics of stars,” AJ, lvii (1952), 5–6; AS, “The search for the curvature of space,” Physica Scripta, xlviii (1992), 7–21. 55. LS, “Princeton University Observatory” [annual report for 1951–52], AJ, lvii (1952), 187–88; AS and MS, “Inhomogeneous stellar models II. Models with exhausted cores in gravitational contraction,” ApJ, cxvi (1952), 463–76. 56. WB to [A]S, 9 Feb., 17 May, 4 June 1953, DEO Collection. 57. W[B] to N[UM], 16 Mar. 1955, SLO; MLH, NUM, and AS, “Redshifts and magnitudes of extragalactic nebulae,” AJ, lxi (1956), 97–162. 58. AEW to WB, 26 Sept., 12 Dec. 1950, 22 Jan. 1951, 8 Mar., 7 June 1952, WB to [AE]W, 27 Nov. 1950, 9 Apr., 4 June 1952, UWA. 59. WB to [CP]G, 14 May 1951, OGC; W[B] to N[UM], 9 Nov. 1952, WB to G[EK], 10 Feb. 1953, 1 Apr. 1954, [GEK] to [W]B, 22 Dec. 1953, 21 Apr. 1954, 1 Feb. 1956, SLO; H. Weaver to [W]B, 14 Apr., 20 June 1953, HHL. 60. WB to [P. J] van Rhijn, 13 Dec. 1950, AIP. 61. WB to [CP]G, 30 Jan. 1948, OGC.

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62. WB to C[PG], 27 Mar., 17 July 1947, 11 Mar., 2 Nov. 1952, OGC; Cecilia Payne-Gaposchkin (1954). 63. SG to W[B], 3 Feb., [∼ 28 Apr.], 2 June 1950, 28 Mar. 1951, 3 Mar., 9 Mar., 4 Dec. 1953, HHL. SG, “285 stars toward the galactic nucleus,” Variable Stars, x (1955), 337–81; J. B. Alexander, “RR Lyrae variables in the direction of the galactic centre,” Observatory, lxxx (1960), 110–12; F. D. A. Hartwick, J. E. Hesser, and G. Hill, “The RR Lyrae variables in Baade’s field near NGC 6522,” ApJ, clxxiv (1972), 573–82. 64. D. A. MacRae, “Explorer of the Milky Way,” S&T, xxx (1965), 7–8; V. M. Blanco, “Dr. Jason John Nassau,” QJRAS, vii (1966), 79–81. These two obituary articles are the sources for most of the facts of Nassau’s life given in this section. 65. JJN to RGA, 6 Oct. 1924, SLO; JJN to WSA, 11 Mar. 1935, HHL. 66. W. Seely to OS, 4 Feb. 1939, Anon., “Guests traveling to Texas in special cars” [memo], 11 Apr. 1939, “Program of the Dedication . . . May 5–8, 1939” [which lists both Baade and Nassau among the guests at the Prude Ranch], YOA; WB to [JJ]N, 17 Jan. 1942, CWR. 67. JJN to WB, 18 May 1939, WB to [JJ]N, 17 Jan., 3 Mar. 1942, CWR; JJN, “The addition to the Warner and Swasey Observatory,” PA, xl (1941), 257–59; JJN, “The Burrell Telescope of the Warner and Swasey Observatory,” ApJ, ci (1945), 275–79.

6. RADIO ASTRONOMY AND THE SIZE OF THE UNIVERSE 1. W. T. Sullivan (1982, 1984), B. K. Malphrus (1996), and some of the references listed in them are the primary sources for this section and parts of the next. 2. See chapter 5, note 2. 3. NUM to JHO, 4 Feb. 1947, SLO; see also chapter 5, note 6. 4. [JHO] to WB, 4 Sept. 1950, 23 May 1951, AIP. 5. JHO, “Problems of galactic structure,” ApJ, cxvi (1952), 233–50. 6. W. W. Morgan, S. Sharpless, and DEO, “Some features of galactic structure in the neighborhood of the sun,” AJ, lvii (1952), 3; O. Gingerich, “The discovery of the spiral arms of the Milky Way,” in H. van Woerden, R. J. Allen, and W. B. Burton (1985), 59–70; DEO, “William Wilson Morgan, 1906–1994,” BMNAS, lxxii (1997), 289–313; WB to C[PG], 21 Mar. 1952, HCO. 7. [JHO] to WB, 14 July 1952, WB to [JH]O, 7 Aug. 1952, AIP; W[B] to N[UM], 15 Sept. 1952, SLO. 8. HVH, C. A. Muller, and JHO, “The spiral structure of the outer part of the galactic system determined from the hydrogen emission at 21-cm wavelength,” BAN, xii (1954), 117–49; M. Schmidt, “A model of the distribution of

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mass in the galactic system,” BAN, xiii (1956), 15–41; [JHO] to WB, 15 May 1950, ULL. 9. [JHO] to WB, 22 Feb., 25 Oct. 1954, WB to [JH]O, 12 Oct. 1954, M. Schmidt to WB, 8 Feb. 1955, AIP. 10. HVH, E. Raimond, and H. van Woerden, “Rotation and density distribution of the Andromeda nebula derived from observations of the 21-cm line,” BAN, xiv (1957), 1–16; M. Schmidt, “The distribution of mass in M 31,” BAN, xiv (1957), 17–19; ISB, “Annual report of the director, Mount Wilson and Palomar Observatories,” CIW, lvii (1958), 51–88. 11. J. G. Bolton, “Radio astronomy at Dover Heights,” Proc. Astron. Soc. Australia, iv (1982), 349–58; DEO, “Rudolph Leo Minkowski, 1895–1976,” BMNAS, liv (1983), 271–298; see also note 1. 12. F. G. Smith and B. Lovell, “On the discovery of extragalactic radio sources,” JHA, xiv (1983), 155–65. This paper includes long quotations from several letters between Baade, Smith, and Lovell. Most of the original incoming letters to WB are in his papers in the HHL, but no copies of his own outgoing letters (which he only rarely kept). 13. WB to [JH]O, 20 May, 2 June 1952, [JHO] to [W]B, 24 May 1952, AIP. 14. HS to WB, 27 Sept. 1951, WB to [H]S, 28 Sept. 1951, DHM to WB, 24 Jan. 1952, WB to D[HM], 29 Jan. 1952, HCO. 15. WB and RM, “Identification of the radio sources in Cassiopeia, Cygnus A and Puppis A,” ApJ, cxix (1953), 206–14; WB and RM, “On the identification of radio sources,”ApJ, cxix (1953), 215–31; RM, “Cygnus loop and some related nebulosities,” Rev. Modern Physics, xxx (1958), 1048–57. 16. W. T. Sullivan (1984); B. K. Malphrus (1996); see also A. A. Needell, “Lloyd Berkner, Merle Tuve, and the federal role in radio astronomy,” Osiris, lix (1987), 261–88. 17. MAT to JHO, 17 Mar. 1953 (cablegram), JHO to MAT, 23 Mar. (cablegram), 24 Mar. 1953, MAT to M. Ryle, 30 Mar. 1953 (cablegram), LC; MAT to HVH, 1 Apr. 1953, MAT to M. Ryle, 1 Apr. 1953, MAT to JHO, 1 Apr. 1953, MAT to D. Bronk, 2 Apr. 1953, “National Academy of Sciences Annual Meeting, Sessions for the Presentation of Scientific Papers” (printed program), [27 Apr. 1953], NAS. 18. “Tentative Program–Joint Meeting USA National Committee, URSI Professional Group on Antennas and Propagation, April 27, 28, 29, 30, NAS Washington,” LC. 19. JJN to WB, 29 Apr. 1954, W[B] to J[JN], 7 May 1954, CWR; [Anon.], “Pulkovo Observatory rebuilt,” S&T, xiii (1954), 298; I. Shklovsky (1968), 272–81; I. Shklovsky (1991), 14–15, 109–12. 20. [JHO] to WB, 21 Jan., 24. Feb. with postscript added 12 Apr. 1955, AIP.

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21. [JHO] to WB, 26 Apr. 1955, WB to [JH]O, 30 Apr. 1955, AIP, WB to [JH]O, 29 Apr. 1955, HHL. The last is an earlier, longer, unsent draft of the letter of 30 Apr., containing calculations Baade made as he wrote it. 22. WB to [JH]O, 2 May, 5 June 1955, [JHO] to WB, 26 May, 31 May, 27 June, 1955, AIP. 23. H. L. Giclas to WB, 12 Sept. 1955, WB to [JH]O, 29 Sept., 12 Oct., 24 Oct., 21 Nov., 8 Dec. 1955, [JHO] to WB, 11 Oct., 14 Nov. 1955, 20 Jan., 2 Feb. 1956, AIP; JHO to ISB, 4 Oct. 1955, HHL. 24. WB to [JH]O, 10 Feb., 19 Feb., 19 Mar. 1956, [JHO] to WB, 21 Jan., 10 Feb., 21 Feb. (cablegram), 30 Mar., 3 Apr. 1956, AIP. 25. WB to [JH]O, 3 Apr. 1956, [JHO] to WB, 11 Apr. 1956, AIP; FZ, “Composite analytical photography of polarized objects,” PASP, lxviii (1956), 121–24; WB, “Polarization in the jet of Messier 87,” ApJ, cxiii (1956), 350–51. 26. JHO and Th. Walraven, “Polarization and continuum of the Crab nebula,” BAN, xii (1956), 285–308; WB, “The polarization of the Crab nebula on plates taken with the 200-inch telescope,” BAN, xii (1956), 312; [JHO] to [W]B, 19 May, 6 June 1956, WB to [JH]O, 23 May 1956, AIP. 27. [JHO] to WB, 30 July, 21 Aug., 25 Sept. 1956, AIP; LW to WB, 28 Feb. 1957, HHL; Th. Walraven, “Photo-electric observations of the polarization and surface brightness of the Crab nebula made at the Observatoire de Haute Provence,” BAN, xiii (1957), 293–301; LW, “The polarization and intensity distribution in the Crab nebula derived from plates taken with the 200-inch telescope by Dr. W. Baade,” BAN, xiii (1957), 301–11; LW, “The Crab nebula,” BAN, xiv (1958), 39–80; LW, “Emission nuclei in galaxies,” ApJ, cxxx (1959), 38–44; GPK, “Report of Yerkes Observatory and McDonald Observatory [for 1957– 1959],” AJ, lxiv (1959), 473–82. 28. WB to CPG, 3 Feb., 27 Mar. 1947, OGC. 29. WB to [H]S, 19 Feb. 1948, HHL. 30. E. Gaviola to WSA, 9 June, 10 Oct. 1942, WSA to Gaviola, 22 June, 21 Oct. 1942, HHL; WSA, “Algunas posibles investigaciones para un observatorio austral,” Revista Astronomica, xiv (1942), 243–46. 31. E. Gaviola to ISB, 26 Nov. 1946, EH to Gaviola, 3 Jan. 1947, [WB] to Gaviola, [∼ 15 Feb. 1947], ISB to Gaviola, 11 Jan., 5 Apr. 1947, Gaviola to ISB, 25 Mar. 1947, HHL. 32. HS to WB, 17 Mar. 1948, 10 Feb. 1949, 21 Nov. 1950, WB to [H]S, 9 Feb. 1949, HCO; WB to [JH]O, 19 Mar. 1948, 18 Apr. 1949, AIP. 33. HS to AHJ, 28 Oct., 25 Nov. 1949, AHJ to HS, 18 Nov. 1949, HS to WB, 11 July 1950, HCO. 34. WB to [JH]O, 2 June 1952, AIP. 35. A. D. Thackeray and A. J. Wesselink, “Distances of the Magellanic clouds,” Nature, clxxi (1955), 693; A. D. Thackeray, “RR Lyrae Variables in the Magellanic Clouds,” Monthly Notices of the Astronomical Society of Southern Africa, xxxiii (1974), 66–70; M. Feast, “Stellar populations and the distance scale:

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The Baade-Thackeray correspondence,” JHA, xxxi (2000), 29–36. The last paper contains long quotations from some of the letters between Baade and Thackeray, and lists and summarizes all thirty-one of them, now housed in the Archives of the University of Cape Town, South Africa. 36. F. Hoyle, “Report of meeting” [of IAU Commission 28], Trans. IAU, viii (1954), 397–99; WB, “Basic facts of stellar evolution,” ibid., 682–89. 37. S. Chandrasekhar to NUM, 10 Nov. 1952, NUM to Chandrasekhar, 12 Nov. 1952, SLO; W. Dieckvoss to AS, 21 Jan. 1968 (which includes a photograph of Baade at Hamburg Observatory dated 28 Sept. 1952), DEO Collection. 38. Science Service, “Yardstick of the universe needs to be shortened,” Future release, 16 Dec. 1952, HCO; GWG to ISB, 5 Jan. 1953, HHL; CAF to OS, 17 Jan. 1953 (two letters of same date), BL. See also F. Hoyle (1994), 263, which characterizes Shapley’s action as plagiarism without naming him. Hoyle was the acting secretary who took minutes of what Baade, Shapley, and other speakers said at the IAU Commission 28 meeting. 39. WB to S[G], 13 Jan. 1953, HCO (there are three earlier, more extreme drafts of this letter, two dated 9 Jan. and one 10 Jan., HHL); WB to DHM, 29 Jan. 1953, GO; WB to [GP]K, 30 Jan. 1953, GPK to WB, 11 Feb. 1953, UAL; WB to [GW]G, 2 May 1953, HHL; WB to [GE]K, 14 June 1954, SLO. 40. OS to WB, 13 Jan. 1953, WB to [O]S, 18 Jan. 1953, OS to BJB, 16, 22 Jan 1953, BJB to OS, 20 Jan. 1953, OS to CAF, 20 Jan. 1953, UCA; CPG to W[B], 16 Jan. 1953, OGC; B[J]B to D[HM], 20 Jan. 1953, HCO; ISB to DHM, 26 Jan. 1953, DHM to ISB, 26 Jan. 1953, DHM to WB, 2 Feb. 1953, HS to WB, 26 Jan. 1953, HHL; HS, “The distance of the Magellanic Clouds,” AJ, lviii (1953), 47; DHM to WB, 10 Feb. 1953, HHL. 41. “Harvard Observatory Council” [minutes], 15, 26, 30 Jan. 1953, DHM, “Interim report on Observatory problems” [11-page typed report], 13 Feb. 1953, HCO; WB to C[PG], 9 Feb. 1953, OGC; ISB to DHM, 17 Feb., 11 May 1953, DHM to ISB, 8 May 1953, HHL; OS to P. Th. Oosterhoff, 3 Mar. 1953, UCA; HS, “Magellanic Clouds. VI. Revised distances and luminosities,” PNAS, xxxix (1953), 349–57; OS, “The distance scale of the universe,” S&T, xii (1953), 203– 5, 238–40; HS, “Note on the extragalactic distance scale,” AJ, lviii (1953), 227– 28. 42. ISB to GWG, 9, 27 Jan., 10 Feb., 16 Mar., 18 May, 1 June 1953, GWG to ISB, 5 Feb., 6 Mar., 21 Apr., 11 May 1953, WB to [GW]G, 2 May 1953, GWG to WB, 8 May 1953, HHL. 43. A. Behr, “Zur Entfernungskala der extragalaktischen Nebel,” AN, cclxxix (1951), 97–104; GG to [W]B, 12 May [1951], GO. 44. M. Dartayet and J. Landi Dessi, “Studies of variables in the Magellanic Clouds. I. Twenty new variables in a region in the Small Cloud,” ApJ, cxv (1952), 279–83; see also note 35. 45. WB, “The period-luminosity relation of the Cepheids,” PASP, lxviii (1956), 5–15.

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7. TELLING THE GOOD NEWS 1. JAH to WB, 16 Feb., 6 Mar. 1951, ISB to JAH, 14 Mar. 1951, [WB], “Swarthmore 1953” [folder containing notes for seven lectures in course and one Sigma Xi lecture], [March 1953], HHL; WB to JJN, 1 Mar. 1953, JJN to WB, 30 Mar. 1953, CWR; P. van de Kamp, “Sproul Observatory, Swarthmore College” [annual report for 1952–53], AJ, lviii (1953), 265. 2. WB to [H]S, 29 May 1940, WB to [L]G, 23 Apr. 1943, HCO; WB to [JH]O, 5 Dec. 1952, AIP. 3. WB to [JH]O, 7 Jan. 1954, AIP; LG, ed., Symposium on Astrophysics (Ann Arbor, 1953) [mimeographed notes of the lectures at the Michigan symposium, prepared by the participants]; LG, “The Observatory, Ann Arbor” [annual report for 1953–54], AJ, lix (1954), 346–50; D. H. DeVorkin, “The Harvard summer school in astronomy,” Physics Today, xxxvii (1984), July issue, 48–55: O. Gingerich, “The summer of 1953: A watershed for astrophysics,” Physics Today, xlvii (1994), Dec. issue, 34–40. The last of these is an excellent article by a participant in the Michigan symposium, which gives the full flavor of the experience. I was also a participant and was greatly inspired by Baade’s lectures, as I had been earlier by his invited paper at the dedication of the Michigan Schmidt telescope in 1950 (see chapter 5, note 33). 4. WB to [O]S, 7 Feb., 1 Apr., 26 Apr., 1 June, 1954, OS to WB, 26 Feb. 1954, BL; [WB], “Hitchcock Lectures” [spiral notebook of his handwritten notes for the lectures], 4–24 May 1954, WB to ISB, 9 May 1954, HHL. 5. [WB], “The structure and composition of galaxies, with special consideration of our own galaxy[,] Lecture course given at the Caltech, 1956 Jan. 23–” [Baade’s handwritten lecture notes for the course], HHL; W. G. Tifft, “Baade— Extragalactic nebulae—Ay 212” [61 pages of handwritten notes, rewritten soon after the lectures from rough notes taken in class], 23 Jan.–7 Mar. 1956. I am grateful to Dr. Tifft for lending me a copy of these notes, which are in his possession. I sat in this course for all the lectures I could, and the remarks on Baade’s teaching skills are based on my own observation. They were corroborated by the former students in the course with whom I was able to discuss them in 1996–97, Drs. Tifft, Walter K. Bonsack, and Ray L. Newburn. 6. WB to [JH]O, 24 Jan. 1956, AIP. 7. HA, “Novae in the Andromeda galaxy,” AJ, lxi (1956), 15–34; HA, “Threecolor photometry of cepheids, W Virginis, M 5 nos. 42 and 84 and M 10 nos. 2 and 43,” AJ, lxii (1957), 129–36; HA, “Southern hemisphere photometry: Photoelectric measures of bright stars,” AJ, lxiii (1958), 118–27; HA, “Wilhelm Heinrich Walter Baade,” JRASC, lv (1961), 113–16; AS, “Wilhelm Heinrich Walter Baade,” QJRAS, ii (1961), 118–21; AS to DEO, 19 Mar. 1992, 5 Dec. 1996, HA to DEO, 31 Dec. 1996. See also Th. Schmidt-Kaler, “Walter Baades wissenschaftliche Genealogie,” Die Sterne, lxx (1994), 90–100.

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8. D. M. Popper, “Spectral types of stars in the globular clusters Messier 3 and Messier 13,” ApJ, cv (1947), 204–18. 9. WB to [A]S, 12 May 1952, DEO Collection; W. A. Baum, “Globular clusters. I. Photoelectric and spectroscopic observations in M 3 and M 92,” AJ, lvii (1952), 222–27; AS, “The color-magnitude diagram for the globular cluster M 3,” AJ, lviii (1953), 61–75. 10. F. Hoyle to WB, 15 Apr. 1954, DEO Collection; F. Hoyle and MS, “On the evolution of type II stars,” ApJ Suppl., ii (1955), 1–40. 11. AS and H. L. Johnson, “The galactic cluster M 67 and its significance for stellar evolution,” ApJ, cxxi (1955), 616–27; AS, “Observational approach to cosmology. II. A computed luminosity function for K0–K2 stars from Mν = +5 to Mν = −4.5,” ApJ, cxxv (1957), 435–44; AS, “The ages of M 67, NGC 188, M 3, M 5, and M 13 according to Hoyle’s 1959 models,” ApJ, cxxxv (1962), 349–65, are three selected examples. For a more detailed discussion with many more references see AS, “The first fifty years at Palomar: 1949–1999: The early years of stellar evolution, cosmology, and high-energy astrophysics,” ARAA, xxxvii (1999), 445–86. 12. WB, “Planetary nebulae in M 31,” AJ, lx (1955), 151; WB and HHS, “Note on planetary nebulae in M 31,” AJ, lxvi (1963), 470. 13. [JHO] to WB, 23 June, 17 Oct., 29 Nov., 24 Dec. (2 letters) 1952, 20 Jan. 1953, WB to [JH]O, 5 Jan., 1 Apr., 20 Apr. 1953, AIP. 14. A. Blaauw (1955). 15. [JHO] to WB, 24 July 1953, 10 Apr., 6 Sept. 1954, 30 Aug. 1956, WB to [JH]O, 17 Apr., 18 Sept. 1954, 16 Aug. 1956, AIP; P. J. van Rhijn to ISB, 23 Feb. 1955, Van Rhijn to WB, [10] Mar. 1955, L. Plaut to ISB, 4 Apr. 1955, HHL; L. Plaut, “Variable stars in a field centered at l = 0°, b = +29°, BAN Supplement, ii (1966), 105–76; JHO and L. Plaut, “The distance to the galactic center derived from the RR Lyrae variables, the distribution of these variables in the Galaxy’s inner region and halo, and a rediscussion of galactic rotation constants,” A& A, xli (1975), 71–86. 16. WB to [W.] Pauli, 6 Feb. 1954, 1 Nov. 1955, W. Pauli Papers, CERN Archive, Geneva, Switzerland; W. H. McCrea, “Jubilee of relativity theory conference at Berne,” Nature, clxxvi (1955), 330–31; A. Mercier and M. Kovaire (1956). 17. J. Schramm (1996); O. Heckmann (1976) [Heckmann dedicated this autobiography to Baade]. 18. O. Heckmann, “Der neue Schmidt-Spiegel der Hamburger Sternwarte,” MAG, vi (1955), 57–60; “Vortra¨ ge der astronomischen Tagung in HamburgBergedorf,” MAG, vii (1956), 11–88, esp. WB, “Wie weit sind Milchstrasse und Andromeda-Nebel im Aufbau vergleichbar?” 51–59. 19. N. G. Roman (1958). See esp. WB, “Large-scale structure of spiral nebulae,” 1–4. 20. WB to [JH]O, 18 Sept. 1954, AIP; WB, “Problems in the determination of distances of galaxies,” AJ, lxiii (1958), 207–10.

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21. “Meeting of the RAS, Friday 1954 May 14,” Observatory, lxxiv (1954), 106–7; “Meeting of the RAS, Friday 1957 May 10,” Observatory, lxxvii (1957), 117–21; WB, “Galaxies and their stellar population,” Observatory, lxxvii (1957), 165–71; F.J.M. Stratton to [W]B, 1 Feb. [19]57, WB, “George Darwin Lecture delivered by Dr. Walter Baade on 1957 May 10” (his written 27-page manuscript), WB, “Fahrplan” [outline for the talk, 12 pages handwritten]; R. H. Garstang to DEO, [22] July 1998. 22. [WB] to H. H. Plaskett, 30 Oct. 1956, H. Bondi to WB, 18 Dec. 1956, A.C.B. Lovell to WB, 15 Nov. 1956, R.v.d.R. Woolley to WB, 26 Mar., 13 Apr. 1957, HHL. 23. D. O’Connell to WB, 15 Dec. 1955, 31 May, 11 Oct., 12 Nov. 1956, 4 Mar. 1957, WB to O’Connell, 4 May, 5 June, 2 July, 21 Nov., 10 Dec. 1956, 18 Jan. 1957, HHL. 24. D.J.K. O’Connell (1958). 25. WB to [JH]O, 25 Nov. 1956, 18 Jan., 22 Apr., 17 Sept. 1957, [JHO] to WB, 14 Dec. 1956, 16 Apr. 1957, ULL; Hoyle (1994). 301–2. 26. A. Blaauw (1991), 4–6. This excellent book is the source for much of the section on the European Southern Observatory; however, Dr. Blaauw was naturally not aware of Oort and Baade’s earlier discussion of this concept. 27. H. W. Babcock to J. M. Beckers, 22 Jan. 1993. I am greatly indebted to Dr. Babcock for telling me of his recollection, and sending me a copy of this letter. He thought their conversation was “in 1953 (or possibly in the preceding year)”; since Oort was not in Pasadena in 1953 but was for nearly three months in early 1952, it must have been then. At least partial corroboration is in WB to [JH]O, 14 Oct. 1957, ULL, a retrospective letter Baade wrote after learning from Oort that the ESO project would go ahead. In it Baade said he was as enthusiastic as he was when they discussed it on the automobile trip from Amsterdam (when Baade arrived by air in May 1953) through the tulip fields to Leiden. Its seems unlikely that this topic would have come up spontaneously under those circumstances, but quite natural if Oort were reminding Baade of their early discussion, and telling him that now was the time to go ahead with it. 28. WB to the Higher Educational Authorities in Hamburg, [∼ 1 July 1927], HHL. 29. E. Schoenberg, “Astronomische Arbeiten in Su¨ dwestafrika,” AN, cclvii ¨ ber Extinktion und Himmelshelligkeit in (1935), 201–10; H.-U. Sandig, “U Windhuk, Su¨ dwestafrika,” AN, cclxii (1937), 385–98; E. Schoenberg to [W]B, 13 July 1939, HHL. 30. [JHO] to WB, 16 Nov. 1954, 4 Oct. 1956, WB to [JH]O, 25 Jan. 1955, 1 Oct. 1956, AIP; B. Lindblad to [W]B, 15 Nov. 1954, HHL. 31. WB to [JH]O, 24 Mar., 17 Nov., 8 Dec. 1957, [JHO] to WB, 4 Nov. 1957, ULL.

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32. La structure et l’evolution de l’universe: onzie´me conseil de physique tenu a l’Universite´ de Bruxelles du 9 au 13 juin 1958 (Brussels, 1957); J. Mehra, (1975); [WB], “Vortrag in Brussel: The basic observational data” [handwritten notes, in English, for his report at the Solvay conference], [∼1 June] 1958, HHL. 33. “Program of the One Hundredth Meeting of the AAS, . . . , Madison, Wisconsin, June 29–July 2, 1958”; [Madison] Wisconsin State Journal, 30 June, 1 July 1958. I was at the AAS meeting in Madison and remember Baade’s Russell Lecture well. At midnight June 30 he, Albert E. Whitford, who at that same instant was officially transferring from the University of Wisconsin faculty to the University of California (as director of Lick Observatory), and I, moving from Caltech to Wisconsin (as an assistant professor) were talking over a few quiet glasses of beer at Baade’s motel near the campus. 34. JAH to AAS Executive Committee, 29 July 1958; [WB], “Russell Lecture” [Baade’s 4 pages of handwritten notes, very fragmentary], [∼ 28 June 1958]; [CAF], [8 pages of handwritten notes, very neat], [1 July 1958]; [JAH], [5 pages of handwritten notes, scrawled], [1 July 1958]; [WB], “Galaxies and Stellar Evolution” [6-page typed manuscript, written by Federer for S&T), [∼ 15 Aug. 1958]; HHL. 35. JWB to WB, 15 Apr., 31 July 1958, WB to CIW, 5 June 1958, CIW. 36. WB to C[PG], 29 Mar. 1957, DHM to WB, 4 Apr., 27 Sept. 1957, MGB to CPG, 10 Apr. 1957, DHM to MGB, 25 Feb., 15 July 1958, MGB to DHM, 10 Mar. 1958, HCO. 37. C[PG] to W[B], 20 July 1958, HHL; W[B] to N[UM], 9 Oct. 1958, SLO. 38. WB to [HH]S, 6 Dec. 1958, WB, “Harvard Lectures (Fall Term 1958–59)” [a spiral notebook containing his detailed notes for the first 19 lectures, plus outlines on sheets of paper for the rest], HHL; WB, Evolution in Stars and Galaxies: Lectures delivered at the Harvard College Observatory, Transcribed from Tape Recordings by Richard S. Rodman (Cambridge, Mass., 1959). (Typed, mimeographed notes, 432 pages, later published as a facsimile, Ann Arbor, 1980.) 39. WB, G. R. Burbidge, F. Hoyle, E. M. Burbidge, R. F. Christy, and W. A. Fowler, “Supernovae and Californium 254,” PASP, lxviii (1956), 296–300; WB to [HH]S, 1 Dec. 1958, WB, “Lectures at the Institute for Advanced Study in Princeton[:] The observational basis for our present ideas about the evolution of stars and galaxies” [his handwritten notes for four lectures 23 Jan., 30 Jan., 6 Feb., 13 Feb. 1959], HHL; WB to [EB]B, 10 Jan. 1959, CIW. 8. THE FINALE AND AFTER 1. WB to M. L. Oliphant, 25 Nov. 1955, DEO Collection; J. B. Whiteoak, “Student memories of Bart Bok, an astronomical godfather,” Proceedings of the Astronomical Society of Australia, v (1984), 608–10; D. H. Levy (1993). Drs. Sandage and Code told me in separate interviews in 1998 and 1999 respectively that they knew nothing of Baade’s recommendation at the time. They did remem-

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ber being asked if they would be interested in being considered for the directorship, and both of them were surprised but answered in the negative. 2. O. J. Eggen, D. Lynden-Bell, and AS, “Evidence from the motions of old stars that the Galaxy collapsed,” ApJ, ciiivi (1962), 748–66. 3. BJB to WB, 28 May 1958, HHL; WB to Registrar, ANU, 4 June 1958, ANU; JLP to WB, 11 Nov. 1958, WB to JLP, 13 Dec. 1958, ATNF. 4. BJB to WB, 19 Feb. 1959, BJB to D. Westley, 3 Mar. 1959, ANU; JLP to WB, 18 Mar. 1959, BJB to I[S]B, 14 Apr. 1959, HHL; WB to D[HM], 28 Apr. 1959, HCO. 5. BJB to JLP, 2 Jan. 1959, JLP to BJB, 9 Jan. 1959, BJB to WB, 30 Jan. 1959, ATNF; Programme, “Symposium on Cosmology,” 2–3 Apr. 1959, ANU; BJB, “Mount Stromlo Observatory annual report for 1959,” 18 Jan. 1960, W. G. Tifft Collection. 6. WB to [JH]O, 29 June 1959, ULL; WB to H[HS], 17 July 1959, HHL; WB, “7 Lectures at the Australian National University July 2–Aug. 13, 1959” [his notes], HHL; R. A. Bell, “Evolution of galaxies” [handwritten notes Dr. Bell, then a student, took in this course], Bell Collection. The attitudes of the young astronomers and graduate students are based on my interviews of Roger Bell and John Whiteoak (students) in 1994 and on a memo from Alex Rodgers (research associate), written in May 1995. 7. G. H. Herbig to WB, 25 Apr. 1949, 3 Mar. 1959, HHL. 8. “Visit to Western Australia by Mount Stromlo Astronomers and Visitors from the United States” (press release), 24 July 1959, W. G. Tifft Collection; WB to [EB]B, 26 May 1959, CIW. 9. JLP to WB, 28 May, 25 June, 26 Aug. 1959, WB to JLP, 2 June 1959, JLP to [B. Y.] Mills, [C. M.] Wade et al., 14 Sept. 1959, ATNF; WB, “First Sydney lecture” [actually notes for several lectures he gave there], [∼ 1 Sept. 1959], [C.E.]K. Mees to BJB, 20 Apr. 1959, WB to H[HS], 29 July 1959, HHL. 10. [HHS] to [W]B, 4 Dec. 1958, 6 Jan. 1959, WB to H[HS], 21 Oct. 1959, HHL. 11. See chapter 1, note 1. 12. JJN to WB, 16 Sept. 1958, GO; “Astronomie und Astrophysik, Vorlesungsankundingung fu¨ r das Winter-Semester 1959/60” [mimeographed onepage course announcement including WB, “Entwicklung von Sternen und Sternsystem”], “Go¨ ttingen, Klassifikation der Sternsystem nach Typus” [Baade’s notes in German for only one lecture, closely following his notes for his second lecture at Harvard in the fall of 1958], HHL. 13. WB to [EB]B, 26 May 1959, HB to [EB]B, 10 June, 31 July 1959, WB to [JW]B, 14 Jan. 1960, CIW; WB to D[H]M, 3 Jan. 1960, HCO. 14. [H]B to J[HO], 5 Feb. 1960, ULL; HB to [EB]B, 1 Mar. [1960], HHL; O. Heckmann to LRS, 18 July 1960, P. ten Bruggencate to [LRS], 28 July 1960, SLO. The last two letters were originally written in German to Baade’s old friend the Marine chaplain, who knew the language very well but was not familiar with medical terminology. Schmieder sent his own translations into

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English to Mayall (and to Bowen), and these translations are all that I have seen. They and Muschi Baade’s letters are the main sources for most of the details of Baade’s illness and death. O. Heckmann (1976) also states Baade was optimistic, and eagerly discussed science while confined to his hospital bed, up to his death. 15. G. Keller to WB, 25 Aug. 1959, C. P. Haskins to ISB, 3 Feb. 1960, ISB to WB, 19 Feb. 1960, [HHS] to W[B], 19 Feb. 1960, [HB] to H[HS], 18 Apr. 1960. H[HS[ to [IS]B, 25 Apr. 1960, HB to [IS]B, 7 July 1960, HHL. 16. DHM to MGB, 24 Nov. 1959, MGB to DHM, 1 Dec. 1959, DHM to WB, 7 Dec. 1959, HCO. 17. JHO to WB, 15 Feb., 14 June 1960, ULL; W. A. Fowler to WB, 6 May 1960, GO. 18. F. K. Jungklaas to [ISB], 24 Sept. 1962, ISB to Jungklaas, 16 Oct. 1962, HHL; Th. Schmidt-Kaler to W. Pfau, 14 Oct. 1997, DEO to W. Pfau, 23 Oct. 1947, R. P. Kraft to W. Pfau, 19 Feb. 1998, W. Pfau to DEO, 14 June 1998, DEO Collection. I have seen the original photograph of the death mask; it has also been published by Th. Schmidt-Kaler (chapter 1, note 3). It does not look in the slightest like Baade, even in the few moments when he was in repose. 19. [H]B to [JW]B, 2 Dec. 1963, 25 July 1965, 1 Oct. 1967, 12 Sept. 1976, U. Albrecht to CIW, 18 Sept. [19]82, 17 Sept. 1988, K. R. Henard to [H]B, 23 Sept. 1982, A. J. Feiss, [Memo] re [H]B, 6 June [19]88, CIW. 20. HA, “Wilhelm Heinrich Walter Baade,” JRASC, lv (1961), 113–16; AS, “Wilhelm Heinrich Walter Baade,” QJRAS, ii (1961). 21. Th. Schmidt-Kaler to DEO, 9 Sept. 1995, reports this information on Baade’s brother and nephew. Prof. Schmidt-Kaler tracked it down in interviews and records in Germany, and I am most grateful to him for letting me use it here. 22. WB to [RS], 8 Nov. 1939, RS to [W]B, 10 Nov. 1939, HO; [WB] to [W.] Dieckvoss, 26 Sept. 1935, HHL. 23. F.[J.M.] Stratton to [W]B, 24 July, 17 Aug. [19]42, HHL. 24. WB, “Application for Payroll Deduction for U.S. Series E Bonds,” [∼ 1 May, 1942], CIW; [WB], Diary pages 8 Jan.–19 Nov. 1945, HHL. These diary pages contain mostly jottings of Baade’s research ideas and thoughts, along with these records of war-bond redemptions. Regarding Muschi, A. E. Whitford told me on two or three occasions that at a cocktail party at the Baades’ house in the late 1940s or early 1950s he had heard her say, without bitterness but matter-of-factly, something along the lines of, “Well, we lost the war, and now we have to suffer,” speaking of conditions in Germany then. 25. O. Heckmann (1976), 40–43, 54–56, 214–19, 260–79. 26. [WB], “Neue Ma[s]ssta¨ be fu¨ r das Universum” (2-page handwritten manuscript) [∼ 1954?]; WB, “Das Milchstrassensystem und die Gro¨ sse der Welt,” in J. Schlemmer (1956), 129–38

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27. WB is included in published membership lists of the German Astronomical Society as of June 1950 in MAG, i ([1949]), 23, and as of 15 May 1954 in ibid., v (1955), 41, as well as in a short list of changes of members’ addresses in MAG, xi (1959), 61; G. Miczaika to [W]B, 4 Oct. 1953, HHL. 28. B. V. Kukarkin to WB, 19 Feb. 1952 (English translation), WB to [B.] Stro¨ mgren, 10 Mar. 1952, HHL. 29. G. Christianson (1995), 292, reports that Zwicky called Baade a Nazi to his face. MS to DEO, 16 Aug. 1993, cited several of the examples given in the text as disproof of Zwicky’s charge. See also O. Gingerich, “Leo Goldberg,” PAPS, cxxxvi (1992), 297–301. 30. OS to WB, 7 Oct. 1953, WB to [O]S, 9 Oct. 1953, BL; OS to “Members of the Academy Section NAS,” 6 May 1955, HHL; E. B. Wilson, “Vital statistics of our foreign associates,” PNAS, xxxix (1953), 1295–98. 31. [JHO] to HB, 12 July 1960, ULL; HHS to W[B], 6 Mar., 2 Apr., 2 June, 1960, H[HS] to [HB], 5 May, 6 July 1960, HHL. 32. ISB to [H]B, 27 June 1960, JHO to ISB, 19 Aug., 15 Sept. 1960, ISB to JHO, 26 Aug., 25 Oct. 1960, HHL. 33. HB to [JH]O, 15 July, 21 Aug., 1 Sept., [∼ 16 Sept.] 1960, 26 Feb. [19]61, JHO to HB, 27 Aug., 14 Sept. 1960, 31 Jan. 1961, ULL; ISB to JHO, 28 Sept., 1 Dec., 20 Dec. 1960, R[M] to H[HS], 7 Oct. 1960, RM to ISB, 21 Oct. 1960, HHL; LW to DEO, 7 Aug. 1997. 34. [HHS] to [IS]B, 12 Oct. 1960, HHL; WB and HHS, “The Draco system, a dwarf galaxy,” AJ, lxvi (1961), 300–47; WB and HHS, “Variable star field 96¢ south preceding the nucleus of the Andromeda galaxy,” AJ, lxviii (1963), 435–69; WB and HHS, “Variables in the Andromeda galaxy—fields I and III,” AJ, lxx (1965), 212–68; WB and HA, “Positions of emission nebulae in M 31,” ApJ, cxxxix (1964), 1027–44; HA, “Spiral structure in M 31,” ApJ, cxxxix (1964), 1045–57. 35. S. L. Th. J. van Agt, “A discussion of the Ursa Major dwarf galaxy based on plates obtained by Walter Baade,” BAN, xix (1968), 275–302; V. Trimble, “Motions and structure of the filamentary envelope of the Crab nebula,” AJ, lxxiii (1968), 535–50. 36. L. Plaut to H[HS], 12 Aug. 1960, HHL; V. C. Rubin, “Structure and evolution of the galactic system,” Physics Today, xiii (1960), Dec. issue, 32–35. 37. F. W. W[right] to J. [Gaustad], 8 Aug. 1960, JHO to CPG, 12 Oct., 22 Nov. 1960, CPG to JHO, 14 Nov. 1960, 16 Mar. 1961, JHO to HB, 22 Nov. 1960, HB to [JH]O, 24 Nov. 1960, 12 Apr. [19]61, JHO to T. Wilson, 6 Apr. [19]61,ULL; C[PG] to D[HM], 24 Oct. 1960, DHM to CPG, 1 Nov., 16 Nov. 1960, H. S. Federer to DHM, 2 Nov. 1960, HCO. 38. W. Baade (1963); M. G. Fisher to DEO, 14 Aug. 1997. 39. [R. Bowers], “About Project Magellan,” Spectra: The Newsletter of the CIW, March (1994), 1–16; [T. McDowell], “Magellan I Telescope to be named in honor of former Carnegie astronomer Walter Baade,” ibid. (April 1999), 6.

✶

BIBLIOGRAPHY

✶

Baade, Walter. 1963. Evolution of Stars and Galaxies. Edited by Cecilia PayneGaposchkin. Cambridge, Mass. Blaauw, Adriaan, ed. 1955. Co-ordination of Galactic Research: IAU Symposium No. 1. Cambridge. .1991. ESO’s Early History: The European Southern Observatory from Concept to Reality. Garching ber Mu¨nchen. Burgers, J. M., and H. C. van de Hulst, eds. 1951. Problems of Cosmical Aerodynamics. Dayton, Ohio. Christianson, Gale E. 1995. Edwin Hubble: Mariner of the Universe. New York. DeVorkin, David H. 2000. Henry Norris Russell: Dean of American Theoretical Astrophysicists. Princeton. Dufay, M., ed. 1965. Novae, Novoa˚des, et Supernovae: Colloque Internationaux de Centre National de la Recherche Scientifique No. 121. Paris. Evans, David S., and J. Derral Mulholland. 1986. Big and Bright: A History of McDonald Observatory. Austin. Heckmann, Otto. 1976. Sterne, Kosmos, Weltmodelle: Erlebte Astronomie. Munich. Hoyle, Fred. 1994. Home Is Where the Wind Blows: Pages from a Cosmologist’s Life. Mill Valley, Calif. Hufbauer, Karl 1991. Exploring the Sun: Solar Science since Galileo. Baltimore. Katgert-Merkelyn, J. K., ed. 1997. The Letters and Papers of Jan Hendrik Oort. Dordrecht. Levy, David H. 1993. The Man Who Sold the Milky Way: A Biography of Bart Bok. Tucson. Malphrus, Benjamin K. 1996. The History of Radio Astronomy and the National Radio Astronomy Observatory: Evolution toward Big Science. Miami. Marx, S., and W. Pfau. 1992. Astrophotography with the Schmidt Telescope. Cambridge. Mehra, J. 1975. The Solvay Conferences in Physics: Aspects of the Development of Physics since 1911. Dordrecht. Mercier, A., and M. Kovaire. 1956. Fu¨nfzig Jahre Relativita¨tstheorie: Verhandlungen. Basel. Mu¨ller, Roland. 1986. Fritz Zwicky: Leben und Werk des grossen Schweizer Astrophysikers, Raketenforschers und Morphologen. Glarus, Switzerland. O’Connell, D.J.K., ed. 1958. Stellar Populations: Proceedings of the Conference Sponsored by the Pontifical Academy of Science and the Vatican Observatory, May 20–28, 1957. Amsterdam and New York. Osterbrock, Donald E. 1997. Yerkes Observatory, 1892–1950: The Birth, Near Death, and Resurrection of a Scientific Research Institution. Chicago.

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Payne-Gaposchkin, Cecilia. 1954. Variable Stars and Galactic Structure. London. . 1984. An Autobiography and Other Recollections. Edited by K. Haramundanis. Cambridge, Mass. Roman, Nancy G., ed. 1958. Comparison of the Large-scale Structure of the Galactic System with That of Other Stellar Systems: IAU Symposium No. 5. Cambridge. Russell, Henry Norris, Raymond S. Dugan, and John Q. Stewart. 1927. Astronomy. II. Astrophysics and Stellar Astronomy. Boston. Schlemmer, J., ed. 1956. Von Erde und Weltall: Eine Vortragsreihe. Stuttgart. Schramm, Jochen. 1996. Sterne u¨ ber Hamburg: Die Geschichte der Astronomie in Hamburg. Hamburg. Shaler, A. J., ed. 1941. Novae and White Dwarfs. Paris. Shklovsky, Iosif S. 1968. Supernovae. London. . 1991. Five Billion Vodka Bottles to the Moon: Tales of a Soviet Scientist. New York and London. Spitzer, Lyman, Jr., and J. P. Ostriker, eds. 1997. Dreams, Stars, and Electrons: Selected Writings of Lyman Spitzer, Jr. Princeton. Sullivan, W. T., III, ed. 1982. Classics in Radio Astronomy. Dordrecht. , ed. 1984. The Early Years of Radio Astronomy: Reflections Fifty Years after Jansky’s Discovery. Cambridge. van de Kruit, P. C., and Gary Gilmore. 1994. Stellar Populations: IAU Symposium No. 164. Dordrecht. van Woerden, Hugo, Ronald J. Allen, and W. Butler Burton, ed. 1985. The Milky Way Galaxy. Dordrecht. Zwicky, Fritz. 1976. Catalogue of Selected Compact Galaxies and of Posteruptive Galaxies. Guemlingen, Switzerland.

✶

INDEX

✶

(Prepared by Irene H. Osterbrock)

References to illustrations are printed in italics Abbot, Charles G., 44 abundances of elements: differences in, among globular clusters, 205; differences in, between the two populations, 137, 183–85, 192, 205 Adams, Walter S., 6, 21, 47–48, 99, 108, 110, 218; anti-German sentiments of, 44, 82–83; as director of Mount Wilson, 43, 45–46, 49, 54, 60, 76, 82–83, 136, 164, 202–3; efforts to hire Minkowski, 66–69, 217; efforts to retain Baade on staff, 74– 76; helps secure wartime movement privileges for Baade, 99–100; recommendation of Baade, 44–45 Agfa photographic company, 78 Aitken, Robert G., 20–21 Alfe´n, Hannes, 157 Allegheny Observatory, 4, 18 Aller, Lawrence H., 178, 179 Ambarzumian, Victor, 190, 219 Ambronn, Leopold, 3, 5, 6, 189 American Academy of Arts and Sciences, 218 American Astronomical Society, 145; Amherst meeting (1952), 171; Boulder meeting (1953), 173; Cleveland meeting (1951), 140, 149; Columbus meeting (1947), 2, 109–11, 115–16, 143; Pasadena meeting (1948), 121–23; Princeton meeting (1955), 185; Tucson meeting (1949), 131; Williams College meeting (1937), 79 American Philosophical Society, 217–18 Ames, Adelaide, 35 Anderson, John A., 99, 218; and 200-inch telescope, 60, 122 Andromeda galaxy (M 31), 2, 12, 25, 152; Cepheids in, 163–64, 166, 190–91, 207; distance to, 76, 185–86; globular clusters in, 163–64, 167; H II regions in, 108–9, 116, 222; interstellar gas and dust in, 132, 136, 148; novae in, 32, 117, 181; nucleus of, 22, 101; planetary nebulae in, 185; spiral arms of, 109, 117, 132,

148, 222; supernovae in, 32, 57–58, 70; W Vir variables in, 164, 166 S Andromedae, 57, 70–71, 93 η Aql, 167 Arp, Halton C., 141, 170, 220; as Baade’s student, 139–40, 181, 224; and discovery of H II regions in Andromeda galaxy, 222; and nova search in Andromeda galaxy, 181; and galactic research, 181–82 asteroids, 7, 8, 16, 31, 129. See also individual objects astrograph, Lick Observatory, 120–21 Astronomical Journal, 173 Astronomical Society of the Pacific, 12, 70, 107; Bruce medal of, 175, 217; 1947 meeting of, 121–22 Astrophysical Journal, 104–5, 161 Australian National University, 200, 202, 205 Baade, Charlotte (mother), 2, 73, 79, 85, 87, 188 Baade, Eberhard (nephew), 213–14 Baade, Elizabeth (sister), 2, 188 Baade, Johanna Bohlman (Mrs. Walter), 16, 49, 77, 119, 214, 216; during Baade’s illness, 209–10; following Baade’s death, 211–12, 220–22, 224; death of, 212; and disposition of Baade’s unpublished research material, 220–22; marriage of, 33; move to Germany of, 208; visits to Germany of, 79–81, 188 Baade, Katherine (sister), 2, 188, 209 Baade, Konrad (father), 2 Baade, Martin (brother), 2, 73, 79, 188, 209, 213–14 Baade, Walter, 14, 19, 26, 29, 36, 53, 74, 80, 86, 99, 119, 138, 151, 160, 168, 179, 188, 194, 198, 207, 212; Andromeda nebula (galaxy) research, 22, 101–2, 116, 185; anti-Soviet sentiments of, 216–17; asteroid discoveries and observations of, 16, 31, 40, 129; Australian visit of, 200–207;

262

INDEX

Baade, Walter (cont.) birth and family history of, 2; California Institute of Technology lectures, 180–81; Cepheid (also cluster-type) variable studies of, 10, 16–17, 22–23, 27, 37, 84, 114, 176, 207; clusters of galaxies discovery of, 56–57; collaboration and friendship with Mayall, 63–64, 118–19; collaboration with Minkowski, 40–41, 65, 67–68, 92–94, 117, 153–56, 178; collaboration with Oort, 112–15; comet discoveries and research of, 12, 16, 23; consulted about director for Mount Stromlo, 201–3; controversy with Shapley regarding revised distance scale publication, 171–74; Crab nebula research of, 63–64, 87–88, 90–92; death of, 211; discovery of variable stars in M 33, 9; disposition of unpublished research after death of, 221–22; and distance scale of universe, 162–63, 174–76; eclipse expeditions of, 13, 28, 29–30; education of, 2–3; and ESO planning, 194–96, 198; European trip of (1939), 84–88; evaluations of, 15, 31, 44, 74; Evolution of Stars and Galaxies (edited by Payne-Gaposchkin), 224; expertise in choosing best photographic emulsions for his observations, 39, 78, 101–2, 116– 17; extragalactic nebulae research of, 9, 35–37, 42, 83, 94, 103; friendship with Nassau, 144–49; galactic research of, 15, 21, 106–07, 152; as Gauss professor in Go¨ ttingen, 208; German sentiments of, 75–76, 88, 186, 212–13, 216; gives Halley Lecture, 191; gives Hitchcock lectures, Berkeley, 180; globular cluster research of, see globular clusters; at Hamburg Observatory, 1, 6, 25–26, 32, 44, 60; Hamburg Observatory directorship offer to, 72–73, 76–77, 213; Harvard College Observatory lectures, 197–99; Harvard visit on Rockefeller grant of, 18–19; health problems, 208– 11; honors and awards, 191, 217–19; at IAU symposium on galactic research, 187–89; and identification of radio sources, 153–56; at Institute for Advanced Study, 199; Jena University offer to, 30–32; joins Mount Wilson staff, 43–49; legacy of, 224–26; β Lyrae research of, 3–5; RR Lyrae research of,

see RR Lyrae stars; magnitude determinations by, 51–53, 83; at McDonald Observatory dedication, 83; “method” for determining radius and absolute magnitudes of Cepheids, 22–23; and National Academy of Sciences, 218–19; nebular research of, 41, 68; novae research of, 8, 32–33, 88–90, 117; observing skills of, 7–8, 11, 30, 56, 78, 101–2; at Paris conference, 84–85; and periodluminosity relation, 84, 163–64, 166–70; personality of, 1, 6, 18, 20, 44, 57, 89– 90, 153; at Pontifical Academy conference, 192–93; popular writing of, 215– 16; presents paper on stellar populations at AAS meeting, 1947, 109–11; Princeton University lectures, 137–39; promotion to Observator at Hamburg Observatory, 25–31; radio source identifications of, 153–56; relationship and collaboration with Zwicky, 58–61; retirement from Mount Wilson, 197; Rockefeller grant of, 12–13, 17–20; Russell Lecture, 196; and Schmidt telescope concept, 29–30, 32; at Solvay conference, 196; stellar populations concept of, see stellar populations; supernovae research of, 8, 57–59, 85–88, 93, 117; Swarthmore College lectures, 177; as teacher, 1, 180–81, 205; telescope named for, 226; thesis of, 3–5; and 200-inch tests, 125–26; at the University of Michigan symposium, 178–80; variable star observations of, 7–11, 15–16, 21, 27, see also Cepheid variables, RR Lyrae variables, W Virginis variables; views on cosmology of, 189, 196, 205; visits to Germany of, 1936, 73; 1938, 79–81, 188; during WW I, 3, 213; during WW II, 98–101, 214; W Virginis variables research of, 115, 163–64, 166–67 Baade Telescope, 226–27, 226 Baade’s window, 12, 79, 106, 142, 205 Baade-Wesselink method, 22–23, 167 Babcock, Horace W., 160, 193, 225 Bad Salzuflen (burial site of Baade), 208, 211 Bailey, Solon I., 9 Bartels, Julius, 215 Batchelor, George K., 178 Baum, William A., 139–40, 160, 170, 183 Bavaria, Scientific Academy of, 217

INDEX

Behr, Alfred, 175 Bergedorf, 7, 10, 42 Bethe, Hans, 133 Biermann, Ludwig, 219 Blaauw, Adriaan, 116, 187, 190, 192, 223, 225 Bok, Bart J., 136, 172, 177, 186; as director of Mount Stromlo, 200–204; as host of Baade in Australia, 204–6 Bolton, John C., 153 Bondi, Herman, 205 Born, Max, 189 Bowen, Ira S., 41, 46. 117, 160, 190, 209; and Baade’s papers, 220–22; and controversy between Shapley and Baade, 172–74; as director of Mount Wilson and Palomar, 108, 120, 129–30, 150, 211; and 200-inch completion, 121, 124 Boyden Station, 95–96 Brouwer, Dirk, 158 Bundy, McGeorge, 197, 210 Burbidge, Geoffrey, 179, 199 Burbidge, Margaret, 178, 179, 199 Burrell, A. P., 144 Bush, Vannevar, 99–100 California Institute of Technology, 58, 60, 139 Cambridge, England, radio astronomy at, 154, 156, 200 Campbell’s hydrogen envelope star, 89 SS Cancri, 7 Carnegie Institution of Washington, 43, 47, 49, 53, 60, 67, 75, 83, 99, 156, 210, 226; post-doctoral fellowships of, 150, 181 Case Institute of Technology, 144, 177; AAS meeting at, 140, 149 Cassiopeia A, 154–55 Centaurus A (NGC 5128), 153 Cepheid variables, 22, 23, 25, 27, 55, 114, 176, 207; distances to, 10; period-luminosity relation for, 10, 84, 163–67. See also RR Lyrae variables, W Virginis variables Chandrasekhar, Subrahmanyan, 85, 86, 92, 122, 140, 162, 218 Chre´ tien, Henri, 6 Christy, Robert, 199 cluster-type variables, 10–11, 27, 37; at high-galactic latitude, 11–12, 16–17

263

Code, Arthur D., 142, 150; as possible director of Mount Stromlo, 201–4 color-magnitude diagrams, of globular clusters, 139–41, 167, 170, 180, 182–84; comparison with stellar models, 184–85 Comet Baade, 16, 31 Comet Peltier, 67 Comets, 7–8, 16, 23. See also individual comets Commonwealth Scientific, Industrial and Research Organization (CSIRO), 153, 204 Conant, James B., 173 Copenhagen Observatory, 36 Co´ rdoba Observatory, Bosque Alegre station of, 164–65, 175, 201 cosmic rays, 59 cosmology, 49; Baade’s views on, 189, 196, 205 Couderc, Paul, 195 Crab nebula (NGC 1952), 37, 87, 153; expansion of, 62–64, 92, 222–23; polarization in, 158–59, 161–62; as supernova remnant, 88, 90–92 Curtis, Heber D.: and “Great Debate,” 5– 6; nebular research of, 35, 63; nova discoveries of, 32, 57, 93 Curtiss, Ralph H., 4, 5 RS CVn, 43 Y Cygni, 135 Cygnus A, 153–56 Cygnus field, 21, 38, 55 Dartayet, Martin, 165, 175 Darwin Lecture, Baade’s, 191; Oort’s, 115 Department of Terrestrial Magnetism, 156 De Sitter, Willem, 34, 113 Dieckvoss, Wilhelm, 190, 214 distance scale of the universe, revised, 162, 166, 168, 171, 173, 174–76; controversy with Shapley over, 171–74 Dombrovsky, Viktor A., 157 Dominion Astrophysical Observatory, 15, 20, 51 Draco dwarf system, 207, 222 Duncan, John C., 9, 79, 222 Duyvendak, J.J.L., 91 Dwingeloo Radio Telescope, 152 Eastman Kodak Company, 78, 116 eclipse expeditions, Hamburg Observatory, 13, 28–30

264

INDEX

eclipsing binaries, 27 Eddington, Arthur S., 34–35, 85, 86, 133, 144 Edle´ n, Bengt, 85, 86 Eggen, Olin J., 120, 142; as director of Mount Stromlo Observatory, 201–4 Einstein, Albert, 23–24, 34, 45, 188 Emergency Committee in Aid of Displaced German Scholars, 67–68, 73 Eros, 40 European Southern Observatory (ESO), 224–45; planning of, 193–96, 198, 215 Evolution of Stars and Galaxies (Baade), editor: Cecilia Payne-Gaposchkin, 224 Ewen, H. I., 148 Fabry-Perot interferometer, 41 Federer, Charles A., 171, 173, 196–97 Fermi, Enrico, 178 Fleming, Williamina, 57 Fornax system, 96–97, 169 Fowler, William A., 192, 194, 211 Fricke, Walter, 215 Frost, Edwin B., 20, 48, 50 Galactic center, distance to, 107, 187–88 galaxies, clusters of, 34; Andromeda, 56– 57; Cetus, 56–57; Cygnus, 56–57; Ursa Major, 42; Virgo, 59, 62, 96 galaxies, collisions of, 139, 154–55 galaxies, distances of, 164 galaxies, dwarf, 83, 94–95, 97 galaxies, dwarf elliptical, 37 galaxies, dwarf irregular, 35, 94–95 galaxies, elliptical, 97; population II stars in, 102–3, 134 galaxies, “nebulae” recognized as, 22, 25, 49 Galaxy: distribution of mass in, 150; evolution of, 204; extinction in, 79, 142; H II regions in, 149; halo of, 204; nucleus of, 129; planetary nebulae in, 185; RR Lyrae variables in, 35, 106–07, 115, 187–88; similarities to M 31, 106, 152, 185, 190; spiral arms of, 149–50; star formation in, 152. See also Milky Way Gamow, George, 119–20, 178, 179; stellar evolution theories of, 136, 140, 175 Ganymede, 16, 31 Gaposchkin, Sergei, 171, 199; work for Baade, 143–44

Garstang, Roy H., 191–92 Gascoigne, Ben, 190 Gauss, Carl Friedrich, 3, 52, 208 Gaustad, John, 223 Gaviola, Enrique, 164–65 Ginzburg, Vitaly L., 15 globular clusters, 2; absolute magnitudes of, 10, 97, 139, 164, 167; abundances in, 183–84; color-magnitude diagrams for, 170, 180, 182–85; distances to, 37, 54–55, 97, 168; as key to understanding the two-population concept, 14, 17, 118; RR Lyr variables in, see RR Lyrae variables; spectra of stars in, 182–83; variable stars in, 10–11, 16–17, 27; W Vir variables in, 163–64, 166 Goldberg, Leo, 151, 178, 179, 217 Goss, Fritz, 41 Go¨ ttingen Observatory, 3, 21, 208 Go¨ ttingen University, 213 Graff, Kasimir, 7, 26 “Great Debate,” 5 Gray, George W., 174 Greenstein, Jesse L., 157, 160, 183, 217; as chairman of Caltech astronomy dept., 139, 152 H II regions: in our Galaxy, 149; in Andromeda nebula, 108–9, 116, 222 Hagen, John P., 157 Hale, George Ellery, 6, 23, 60, 121, 218 Hale telescope, 60, 76–77, 117, 125–26, 159; dedication of, 121–23 Halley Lecture, 191 Hamburg Observatory, 1, 36, 72, 193, 214; Baade’s tenure there, 6–17, 25–27, 35– 37, 40–41, 44, 60; dedication of Schmidt telescope, 189–90; and eclipse expeditions, 30–33; offer of directorship to Baade, 71–77 Hamburg University, 23, 33, 40, 42, 46, 65 Hartmann, Johannes, 3–6 Harvard College Observatory, 9, 13, 178; Baade’s fellowship there, 17–19; Baade’s lectureship there, 197–99, 223; Boyden station of, 95–96; Peruvian station of, 18; Swope’s career there, 126–28 Harvard University, 173, 226 Haskins, Caryl F., 210

INDEX

Heckmann, Otto, 72, 190, 196, 211, 222; as director of ESO, 215, 225; as director of Hamburg Observatory, 77, 87, 189, 215, 220 Heisenberg, Werner, 139, 219 Heitler, Walter, 189 Hellerich, Johannes, 26, 72, 74, 88 Hendrix, Don O., 121, 123–24, 128 Herbig, George H., 192, 206 Herlofson, Nicolai, 157 Hertzsprung, Einar, 22 Hertzsprung-Russell diagram, 133, 136 Hidalgo, 16, 31, 129 Higgs, A. J., 156 Hilbert, David, 3 Hitler, Adolf, 42, 66, 75, 80, 106, 217 Hoyle, Fred, 177, 192–93, 194; meeting with Baade, 105; stellar evolution work, 184–85, 199 Hubble, Edwin P., 6, 22, 27, 45–46, 52, 53, 63, 75, 82, 121–23, 99, 130, 163, 169, 191, 202; and 200-inch telescope, 124–25; Cepheid research, 25, 34; confirms spiral nebulae are star systems, 9; death of, 124–25; nebular (galaxy) research, 9, 22, 49, 94–97; novae and supernovae searches of, 57, 59, 62; and redshift-distance relation, 35, 56–57, 162; WW II work, 101 Hubble constant, 141, 162 Hubble Space Telescope, 225 Humason, Milton L., 70, 71, 99, 130, 154, 160; as Baade’s friend, 75, 100, 141; and redshift determinations, 35, 42, 56, 71, 118, 141 Hund, Friedrich, 17–18 Huffer, C. Morse, 149 Hynek, J. Allen, 177, 196–97, 199 IC 443, 43 IC 1613, 35, 83–84, 94, 163 IC 4182, supernova in, 69 Icarus, 149 Institute for Advanced Study, 199 Institute for Experimental Aerodynamics, 3 International Astronomical Union: Dublin meeting (1955), 189–90; Paris meeting (1935), 71–72; Rome meetings (1922) 10, (1952), 167–70; Stockholm meeting

265

(1938), 79; symposium on galactic research, Groningen (1953), 186–87 interstellar extinction, 107; dust, 27, 39, 68, 78–79, 116, 134–37, 142, 148; gas, 134–37, 147–48, 184 interstellar radiation. See 21-cm radiation Jansky, Karl, 147 Jena University, 30–32 Jews, discrimination and persecution of, 65–67, 73–74, 113, 138 Johnson, Harold L., 120, 142 Johnson, Hugh M., 206 Johnson, Josef, 61, 70 Jones, Sir Harold Spencer, 218 Joy, Alfred H., 4, 166, 99, 115, 173, 182–83, 197, 218 Kapteyn, Jacobus C., 12, 112 Keenan, Philip C., 103, 115, 183 Keller, Geoffrey, 209 Kepler, Johannes, 93 Kienle, Hans, 215 Kiepenheuer, Karl-Otto, 157 Klein, Felix, 3–4 Klein, Otto, 196 Knopf, Otto, 30–31 Koch, Peter P., 41 Kron, Gerald E., 120, 142, 167 Kuiper, Gerard P., 85, 86, 100, 172, 178; aids Oort in WW II, 114; and Nova Herculis 1934, 89 Ladenburg, Rudolph, 67 Lampland, Carl O., 159 Landi Dessi, Jorge, 175 Laporte, Otto, 17 Larink, Johannes, 215 Layzer, David, 199 Leavitt, Henrietta S., 10, 163 Leiden Observatory, 113–14 Lemaitre, Georges, 34, 194, 196 Lick Observatory, 20, 27, 50, 63, 103, 118, 120, 167, 180, 206 Lindblad, Bertil, 84, 86, 114, 151, 182, 218 Local Group, 25 Lovell, Bernard, 196 Lowell Observatory, 40, 51, 63, 159 Lundin, Robert, 145 Lundmark, Knut, 57–58, 85, 93 Lund University, 12

266

INDEX

Luyten, Willem, 28 Lynden-Bell, Donald, 204 RR Lyrae variables, 205; absolute magnitudes of, 139–40; as distance indicators, 54–55; in our Galaxy, 106–7, 110, 115, 143–44, 187–88, 205; high-latitude, 37– 38, 55, 188; as markers of population II distribution, 55, 102; in NGC 121, 170; search for, in Magellanic Clouds, 164– 66, 175. See also cluster-type variables β Lyrae, 3–5 M stars, magnitudes of, 145 M 3, 27, 141, 170, 182 M 13, 182 M 31. See Andromeda nebula M 32, 102, 191 M 33, 9, 25, 163 M 53, 2, 9–12, 16–17 M 87 (Virgo A), 153–54, 156; polarization of jet in, 159, 161 M 92, 10, 170, 182–83 Magellanic Clouds, 96, 117, 164–65, 206, 225; Large, 94, 163, 166, 169, 175; Small, 168, 175 magnitude scales, 51–54, 83–84; for determining globular cluster distances, 54–55, 84; in selected areas, 51–54, 84 Mainz, Scientific Academy of, 217 Malmquist, K. Gustav, 12–13 Mary Therese, Sister, 126 Mason, Max, 66, 76 Massachusetts Institute of Technology, 226 Mayall, Nicholas, 103, 109, 119, 120, 124– 25, 132, 151; co-researcher with Baade, 63–64; Crab nebula research, 63–64, 90– 93, 162, 222; friendship with Baade, 118, 141 McDonald Observatory, 100; dedication of, 82–84 McGuire, Dorothy, 126 Mees, C. E. Kenneth, 78, 116–17, 207 Menzel, Donald H., 155, 172, 197, 210 Merriam, John C., 67, 69; and hiring of Baade, 43–47; and keeping Baade on staff, 74–76 Merrill, Paul W., 49, 99, 197, 218 Milky Way, 21, 180; radio signals from, 147; star density in, 12; variable stars in, 15, 21, 38, 55, 114. See also Galaxy

Millikan, Robert A., 58–59 Milne, Edward A., 83 Minden, Germany, 73, 79, 87, 188, 208 Mineur, Henri, 85 Minkowski, Hermann, 41, 65 Minkowski, Rudolph, 65, 95, 99, 118, 130, 130, 151, 157, 160; and Baade’s unpublished research, 280, 222; begins Mount Wilson career, 67–69, 217; discrimination problems, 66–67, 217; during WW II, 98; family of, 40, 65; at Hamburg University, 40–41; leaves Germany, 66–67; Orion nebula research, 40–41, 65, 67–68; planetary nebula research, 92–94, 110; and radio source identifications, 153–56, 178; supernova research, 70–71, 92–94, 117 Moore, Joseph H., 21, 103 Morgan, William W., 103, 151, 183, 192; and discovery of galactic spiral arms, 149–50 Mount Stromlo Observatory: Baade’s visiting lectureship there, 12, 204–7; directorship, 200–204 Mount Wilson Observatory, 1–2, 6, 21–22, 43, 47, 50, 126; Bowen becomes director of, 108; during WW II, 98–101; 1930s research program of, 45; staff of, 49, 69 Mount Wilson and Palomar Observatories, 120–21, 152, 225 Muller, C. A., 148, 150 Mu¨ nch, Guido, 160, 223 Murrow, Edward R., 67 966 Muschi, 16 Nassau, Jason J., 151; AAS service of, 145, Baade’s friendship with, 144–46, 177– 78; as director of Warner and Swasey Observatory, 144–45; early life and education of, 144 National Academy of Sciences, 5, 156, 178, 218–19 National Bureau of Standards, 157 National Geographic Society, 131 National Science Foundation, 178, 181, 190, 209 Naval Research Laboratory, 157 Nazi party, 42, 60–67, 72–74, 77, 88, 100, 113 New York Times, 171, 174 NGC 121, 168, 170 NGC 147, 37, 39, 42

INDEX

NGC 185, 102, 170 NGC 205, 102, 170, 191 NGC 1003, supernova in, 69 NGC 1049, 97 NGC 1275, 155 NGC 1866, 169 NGC 1952. See Crab nebula NGC 2244, 130 NGC 2419, 55 NGC 2608, supernova in, 8, 32, 62 NGC 4147, 37, 39, 55 NGC 4273, supernova in, 62 NGC 4725, supernova in, 70–71 NGC 5053, 27 NGC 5128 (Centaurus A), 153 NGC 5466, 16–17, 55 NGC 5634, 55 NGC 5694, 54 NGC 6366, 27 NGC 6522, 107, 205 NGC 6553, 79 NGC 6822, 25, 83–84, 163 Nicholson, Seth B., 49, 99, 122, 160 North Polar Sequence, 51 Nova Aquilae 1918, 132 novae, 8; in our Galaxy, 117; in M 31, 32, 62, 117. See also individual novae Nova Herculis 1934, 89–90 O and B stars, as indicators of spiral arms, 109, 149 Observatory magazine, 191 O’Connell, Daniel J. K., 192 Ohio State University, 177; AAS meeting (1947), 2, 109–11, 115–16, 143 Oliphant, Mark, 200–202 Oort, Jan H., 80, 80, 102, 113, 186–87, 190, 192, 194, 210, 218, 223; collaboration with Baade, 84, 112–16; Darwin lecture, 115, 131; and disposition of Baade’s unpublished research, 220–22; during WW II, 113–14; and ESO plans, 193–96, galactic research of, 83–84; nova and supernova research of, 90–91, 113, 131; radio astronomy and, 147–51, 154, 158– 59, 161 Oosterhoff, P. Th., 187 Oppenheimer, J. Robert, 196, 199, 200 Orion nebula, 9, 10, 41; interstellar dust in, 39, 67–68 θ1 Orionis, 68 Osterbrock, Donald, 160, 178, 179

267

Paris conference on novae, supernovae, and white dwarfs, 84–86 Parker, Eugene N., 178 Pauli, Wolfgang, 30, 178, 188–89, 196; collaboration with Baade, 23–24 Payne-Gaposchkin, Cecilia, 19, 61, 85, 86, 110, 172–73; arranges Baade’s Harvard lectureship, 197–99; Evolution of Stars and Galaxies (editor), 19, 223–24; research of, 142–43; Stellar Atmospheres, 19; stellar evolution theories of, 135–36, 143; Variable Stars and Galactic Structure, 143 Pease, Francis G., 69 period-luminosity relation, 10, 21–22, 84, 163–64; differences among types of Cepheids, 163, 166–67; dispute with Shapley regarding, 171–73 Perkins Observatory, 2, 110 photographic emulsions, importance of, in astronomical research, 13, 39, 67–68, 78, 88–89, 101–2, 116–17, 185, 222–23 photometry, photoelectric, 53, 83, 118, 120, 142 photometry, photographic, 11, 21, 43–44, 51–54, 142 Pickering, Edward C., 9 planetary nebulae, 89, 110; in Andromeda nebula, 185 Plaskett, John S., 20 Plaut, Lukas, 187–88, 223 Pluto, discovery of, 40 Pogo, Alexander, 158, 216 polarization, in Crab nebula, 157, 158–59, 161–62 Pontifical Academy of Sciences conference (1957), 191–93, 215 Popper, Daniel M., 182–83 Populations I and II. See stellar populations Prandtl, Ludwig, 3 Princeton University: Baade’s lectures at, 137–39; Sandage’s work at, 140–41 Pulkovo Observatory, 158 Puppis A, 155 Purcell, Edward M., 148, 156 Radcliffe Observatory, 168–70 radio astronomy, 64, 153, 157, 204; in Australia, 153, 204–5; Baade and Minkowski’s work with, 153–54;

268

INDEX

radio astronomy (cont.) beginnings of, 147–48; in the Netherlands, 147–49; 21-cm observations, 148–50 radio sources, 94, 153–56; emission trigger of, 157 Rauschelback, Heinrich, 6–7 Reber, Grete, 147, 156 redshifts: of clusters of galaxies, 42, 56; of galaxies, 34–35, 42, 71 Ritchey, George Willis, 32 Roberts, Morton S., 180 Rockefeller Foundation: fellowships, 2, 17–18, 58, 73; International Education Board, 13; support for Minkowski, 66– 68; support for 200-inch, 60, 173–74 Roman, Nancy Grace, 179, 190 Rosenberg, Hans, 73 Ross, Frank E., 20, 40, 45, 54 Ross lens, 39–40 Royal Astronomical Society: Darwin Lecture and medal of, 115, 191; Gold medal of, 191, 217 Royal Astronomical Society of Canada, 217 Royal Astronomical Society, Netherlands, 217 Royal Greenwich Observatory, 204 Rubin, Vera C., 178, 179 Rule, Bruce, 122–23, 128 Russell, Henry Norris, 85, 86, 95; and stellar evolution, 133, 135–37 Russell Lecture, 110, 196, 217 Russian astronomers, 216–17, 157–58, 190 Ryle, Martin, 156 Salpeter, Edwin E., 178, 179, 192 Sandage, Allan, 95, 160, 178, 179, 192, 196, 218, 220, 224; as Baade’s thesis student, 181–82, 224; and color-magnitude diagrams for globular clusters, 139–41, 167, 170, 180, 183; as possible Mount Stromlo director, 201–4; at Princeton, 140–41; stellar evolution research of, 140–41, 184–85 Schlesinger, Frank, 113 Schmidt, Bernhard, 28, 29, 30, 61, 128–29, 190 Schmidt, Martin, 95, 150, 152, 225 Schmidt telescope, 29–30, 59, 68, 76, 145; 18-inch Palomar, 60–62; 24-inch Warner and Swasey, (Burrell-Schmidt), 145; 24-

inch Michigan; 32-inch Hamburg, 189; 48-inch Palomar, 78, 128–31, 155–56 Schmieder, Lawrence R., 118–19, 119 Schoenbert, Erich, 195 Scho¨ nberg, Mario, 140 Schorr, Richard, 14, 16, 20, 26, 77, 79, 87– 88, 128, 189, 214; death of, 170; as director of Hamburg Observatory, 6–8, 10, 12, 13, 30–31, 60–61; on eclipse expeditions, 13, 28; efforts to hire Baade as director of Hamburg Observatory, 71– 74, 76; recommendations of Baade, 15, 25, 46 Schro¨ ttinghausen (birthplace of Baade), 2 Schwarzschild, Martin, 134–38, 138, 192, 217, 218; stellar evolution research of, 137, 140, 180, 184–85 Scientific American, 174 Scklovsky, Iosif S., 157–58 Sculptor system, 95–97, 169 Seares, Frederick H., 21, 43–44, 48, 49, 99, 100, 218; magnitude determinations of, 51–52; retirement of, 54, 75 Selected Area 57, 125 Selected Area 68, 52, 83–84 Selected Area 82, 37 selected areas, magnitudes of stars in, 21, 51–52 Seyfert galaxies, 155, 162 Shajn, Grigory A., 190 Shane, C. Donald, 120–21, 141, 200 Shapley, Harlow, 35, 81, 103, 121, 151, 155, 165–66, 169, 178, 201, 218; as director of Harvard College Observatory, 18, 50, 127–28, 129–30; dispute with Baade regarding revised distance scale, 171– 74; and distance scale, 27, 39, 170; efforts to bring Baade to U.S. on fellowship, 13–15; globular cluster research, 9–11, 17, 23, 37; and “Great Debate,” 5– 6; Sculptor and Fornax studies, 95–96 Siedentopf, Heinrich, 215 Singer, Maxine F., 227 Sky Survey plates, 131 Sky and Telescope, 171, 173, 196–97 Slee, O. B., 153 Slipher, Vesto M., 63 Smith, Francis Graham, 154 Smith, Sinclair, 76 Solvay conference, 196 Sommerfeld, Arnold, 178

INDEX

spiral arms: in Andromeda galaxy, 117, 132, 148, 222; in our Galaxy, 114, 149–50 spiral nebulae (galaxies), 84; Sb, 106; SBc, 62; Sc, 62 Spitzer, Lyman, Jr., 137, 139, 218; stellar evolution research of, 133–35, 192 Stanley, Gordon J., 153 star formation, 152, 206; interstellar matter and, 134–35 Stebbins, Joel, 53, 83, 107, 110, 120, 149, 151, 167, 218 stellar evolution, 111, 132–40; H-R diagram and, 133–36, 140; models for, 140–41, 184–85; populations I and II as representing, 136, 137, 139, 141, 192–93 stellar populations: Baade’s presentation of, at AAS meeting, 109–11; clues to concept of, 1–2, 10, 12, 22, 27, 29, 38, 42–43, 47, 55–56, 62; globular clusters as key to identifying, 14, 47, 118; Hubble’s understanding of, 125; identification of Population II stars, 102–4, 106, 110; as indicating stellar evolution, 137–39, 143, 170, 192–93, 204; members of Population I: H II regions, 108–9, supernovae type II, 117, classical Cepheids, 27, 110, 114; members of Population II: W Vir variables, 22, 114, 163; RR Lyr variables, 106, 110; supernovae type I, 117; publication of two-population concept, 104–05, 109 Stern, Otto, 66–67 Steward Observatory, 202 Stratton, F.J.M., 85, 86, 214 Stro¨ mgren, Bengt, 36, 85, 86, 199 Stro¨ mgren, Elis, 36 Struve, Otto, 20, 82, 89, 104, 121, 158, 172–73, 219 sun, 23–24. See also eclipses supergiants: associated with interstellar matter, 117, 134, 136; lifetimes of, 136 supernovae, 8, 57; absolute magnitudes of, 58, 62, 85; in Andromeda nebula, 32; classification of, 117; energy released by, 58, 87; frequency of, 58, 85–86; in galaxies, 32–33, 62–63; historic, 62–63; in our Galaxy, 58, 62; search for, 59–62, 86; spectra of, 69–71, 86 supernova remnants, 37, 43, 62, 63–64, 87, 88; Crab nebula as, 88, 90–93; identification as radio sources, 94, 154, 156 Swann, W. F., 116

269

Swarthmore College, 13, 177 Sweet, Peter, 191 Swings, Pol, 85, 86, 219 Swope, Henrietta, 221; becomes assistant to Baade, 128; Cepheid measurements of, 167, 170, 185–86, 190, 207, 209, 220; education and early career, 126–28; and illness and death of Baade, 220; planetary nebula measurements, 186; publishes papers following death of Baade, 222 T Tauri stars, 206 Taurus A., 153–54 telescopes: Dominion Astrophysical Observatory 72-inch reflector, 20; Hamburg 24-in refractor, 7–8; Hamburg 40inch reflector, 7–8, 11, 36; Lick 36-inch refractor, 20, 50; Lick 36-inch reflector (Crossley), 20–21, 32, 50, 63–64, 118; McDonald 82-inch reflector, 82–84; Radcliffe 74-inch reflector, 168, 17, 187; Yerkes 40-inch refractor, 50. See also Schmidt telescopes telescopes, Mount Wilson and Palomar: 10-inch Cooke, 59, 62; 18-inch Schmidt, 60–62; 48-inch Schmidt, 17, 128–31, 155–56, 187; 60-inch reflector, 6, 49, 62; 100-in reflector, 6, 49, 62; 200-inch reflector, see Hale telescope; Walter Baade telescope, see Baade Telescope Thackeray, A. David, 187; confirmation of revised distance scale by, 168–70, 175 Tifft, William, 205, 206 Tombaugh, Clyde, 40 Trimble, Virginia, 222 Trumpler, Robert J., 21, 27, 189 Tuve, Merle A., 156 21-cm radiation, 148–50, 152 Tycho Brahe’s nova, 57, 62, 93, 155 U. S. Naval Observatory, 13, 51 Universe, age of, 162, size of, 22, 162– 63, 170–71, 174. See also distance scale, revised University of Arizona, 202, 226–27 University of Birmingham, 200 University of California, 180 University of Chicago, 6. See also Yerkes Observatory

270

INDEX

University of Michigan, 226; symposium on astrophysics at, 178–80; symposium on structure of galaxy at, 166 University of Mu¨ nster, 3 University of Texas, 82. See also McDonald Observatory University of Virginia, 190 University of Wisconsin, 53, 107 ¨ nsold, Albrecht, 18, 42, 82–83, 215, U 219 Ursa Major cluster of galaxies, 35, 42 Ursa Major dwarf galaxy, 222 van Agt, Stephen, 222 van Biesbroeck, George, 50 van de Hulst, Hendrik, 116, 132; identifies 21-cm H I line, 147–50 van de Kamp, Peter, 177 variable stars, 8–12, 27, 110, 127. See also specific types Vashikadze, Mikhail A., 157 W Virginis variables, 114, 163–64, 166–67 Virgo A. See M 87 von Laue, Max, 189 von Seeliger, Hugo, 7 von Weizsa¨ cker, Carl F., 139, 219 Wachmann, Arno A., 190 Walraven, Theodor, polarization observations of, 158–59, 161–62 Warner and Swasey Company, 144 Warner and Swasey Observatory, 144–45 Weaver, Harold F., 142, 180 Weichert, Emil, 3 Wesselink, A. J., 23, 168, 170, 175 Wheeler, John A., 196 Whipple, Fred L., 135–36

Whitford, Albert E., 54, 83, 107, 120, 142, 149–50 Wigner, Eugene, 189 Wilson, Olin C., 99, 182–83 Wilson, Ralph E., 99, 160, 218 Wolf-Lundmark-Melotte “nebula,” 22 Wolf, Max, 8, 9, 32, 62 Woltjer, Lo, 225; Crab nebula research of, 159, 162; receives Baade’s unfinished research papers, 221–22 Woolley, Richard v.d.r., 200, 204 World War I: Baade during, 3, 213; end of, 5; Hubble during, 6–7 World War II: America’s entrance into, 98; Baade during, 98–101, 214; beginning of, 88, 90; family and friends of Baade during, 213–14; Mount Wilson staff during, 98–101; Netherlands during, 90–91, 113–14; Oliphant during, 200; Oort during, 113–14; period preceding, 42, 75, 80–81, 105–6, 107–8; radar in, 64, 156; Schwarzschild during, 138; Swope during, 127 Wright, Frances, 223 Wright, Thomas, 34, 180, 215 Wright, William H., 21, 83, 89, 103 Yale University, 113 Yerkes Observatory, 19–20, 50, 82 Zanstra, Herman, 42 Zanstra method, 42 Zwicky, Fritz, 217, 160; break with Baade, 61, 95, 161, 217; collaboration with Baade, 58–61; dwarf galaxy search of, 95–96; polarization work of, 161; supernova research of, 58–62, 70, 85–87