A Source Book in Astronomy and Astrophysics, 1900–1975 [Reprint 2014 ed.] 9780674366688, 9780674366671

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Source Books in the History of the Sciences Edward H. Madden, General Editor From Frege to Godei : A Source Book in Mathematical Logic, 1879-1931 Jean van Heijenoort A Source Book in Animal Biology Thomas S. Hall

A Source Book in Astronomy and Astrophysics, 1900-1975 Kenneth R. Lang and Owen Gingerich A Source Book in Chemistry, 1900-1950 Henry M. Leicester

A Source Book in Classical Analysis Garrett Birkhoff A Source Book in Geography George Kish A Source Book in Geology, 1400-1900 Kirtley F. Mather and Shirley L. Mason A Source Book in Greek Science Morris R. Cohen and I. E. Drabkin A Source Book in Mathematics, 1200-1800 Dirk J. Streik

A Source Book in Medieval Science Edward Grant A Source Book in the History of Psychology Richard J. Herrnstein and Edwin G. Boring

A Source Book in Astronomy and Astrophysics, 1900-1975 Edited by Kenneth R. Lang and Owen Gingerich

H A R V A R D UNIVERSITY PRESS

Cambridge, Massachusetts, and London, England 1979

Copyright © 1979 by the President and Fellows of Harvard College All rights reserved Printed in the United States of America Library of Congress Cataloging in Publication Data Main entry under title: A Source book in astronomy and astrophysics, 1900-1975. (Source books in the history of the sciences) Includes bibliographical references and index. 1. Astronomy—Addresses, essays, lectures. 2. Astrophysics—Addresses, essays, lectures. I. Lang, Kenneth R. II. Gingerich, Owen. QB51.S67 520'.8 78-9463 ISBN 0-674-82200-5

III. Series.

General Editor's Preface

The Source Books in this series are collections of classical papers that have shaped the structure of the various sciences. Some of these classics are not readily available and many of them have never been translated into English, thus being lost to the general reader and in many cases to the scientist himself. The point of this series is to make these texts easily accessible and to provide good translations of the ones that have not been translated at all, or only poorly. The series was planned to include volumes in all the major sciences from the Renaissance through the nineteenth century. It has been extended to include ancient and medieval science and the development of the sciences in the twentieth century. Many of these books have been published already and others are in various stages of preparation. The Carnegie Corporation originally financed the series by a grant to the American Philosophical Association. The History of Science Society and the American Association for the Advancement of Science have approved the project and are represented on the Editorial Advisory Board. This Board at present consists of the following members : Marshall Clagett, History of Science, Institute for Advanced Study, Princeton I. Bernard Cohen, History of Science, Harvard University Thomas A. Goudge, Philosophy, University of Toronto Gerald Holton, Physics, Harvard University Ernst Mayr, Zoology, Harvard University Ernest Nagel, Philosophy, Columbia University Dorothy Needham, Chemistry, Cambridge University Harry Woolf, History of Science, Institute for Advanced Study, Princeton I am indebted to the members of the Advisory Board for their indispensable aid in guiding the course of the Source Books. Edward H. Madden Department of Philosophy State University of New York at Buffalo

V

Preface

The 132 selections that make up this Source Book represent the seminal contributions to twentiethcentury astronomy and astrophysics through the year 1975. One of the first things that will strike an observant browser is the wide variation of level, ranging from Hale's popular article on a proposed giant telescope in Harper's Magazine to the tensor calculus of Schwarzschild and Einstein. Our book is not necessarily made for general reading, although we hope that readers will find numerous inviting and informative articles. Rather, we have tried to illuminate the vigorous development of our celestial science by giving something of its entire fabric—from Forest Moulton's account of the collision hypothesis in his elementary textbook and Edwin Hubble's identification of the Crab nebula with the supernova of 1054 in a Leaflet of the Astronomical Society of the Pacific, to Ralph Fowler's application of degenerate gas statistics to white dwarfs in the Monthly Notices of the Royal Astronomical Society, and Jan Oort's demonstration of galactic rotation in the Bulletin of the Astronomical Institutes of the Netherlands. It is more disappointing than astonishing that astronomers so quickly forget the exciting moments of yesterday's science. Today it seems obvious that the universe is predominantly hydrogen ; yet this was a landmark discovery made less than fifty years ago, when astronomers still believed that the universe, like the earth, was predominantly composed of the heavier metallic elements. For better or for worse, it is all too easy to accept well-formulated theories without remembering the conflicting, embryonic ideas from which they arose. We hope that our selections will stimulate a renewed interest in the sources and history of twentieth-century astronomy. In this volume readers can rediscover A. S. Eddington's elegant proof of the virial theorem for star clusters, Walter Baade and Fritz Zwicky's 1932 proposal for the existence of neutron stars, and Thomas Gold's forecast of collapsed stars as radio sources. They can reread, in the words of the original observers, the discoveries of the cosmic rays, the Van Allen belts, Martian volcanoes and canyons, pulsars, interstellar hydrogen, cosmic magnetic fields, quasars, and the remnant background radiation of the primeval big bang. Here will also be found a dozen new translations of articles originally in German, French, or Dutch. For example, we include the first English translations of Heinrich Vogt's article on stellar interiors; C. F. von Weizsäckers discovery of the CNO cycle; Karl Schwarzschild's original derivation of the space-time metric of a static, spherical mass ; Albert Einstein's explanation of the motion of Mercury ; and Aleksandr Friedmann's description of the curvature of space. vii

PREFACE

We have grouped together sequences of key articles to tell the story of the development of basic ideas. Readers can, for example, find the changing answers to such fundamental questions as: How did the solar system originate? What makes the stars shine? What lies in the vacuous space between the stars? Are the spiral nebulae distant "island universes"? Do the intense radio sources lie outside our galaxy? Will the universe continue to expand forever? Within each chapter we have adopted a generally chronological arrangement, although occasionally certain thematic subgroups have their own characteristic ordering. We fully realize that the selections presented here cannot take the place of the hundreds of important articles on which modern astronomy and astrophysics are based. Instead, we have tried to choose articles that present an important idea or a significant new observation. A concept or measurement need not be right to be influential, and our selections document a few erroneous observations and theories. Nevertheless, being right generally helps make an article memorable. Being first also increased the likelihood of an article's inclusion, but we have not felt bound to include the first appearance of each idea. Often the second or third presentation is so much more lucid, complete, or recognizably significant that the clear choice for an original "source" rests with the later publication. We have resisted the temptation to document the progress of twentieth-century astronomy through review articles, excellent though they may be. In this respect our selections differ from those of Harlow Shapley's earlier Source Book of Astronomy, 1900-1950 ; our articles present the fundamental ideas in their original statements, rather than in more polished or popularized forms. We have, with each selection, tried to state why it is important. When related material has appeared earlier, we have attempted to cite the relevant references. For example, Karl Jansky's pioneering work on extraterrestrial radio sources appeared in three articles between 1930 and 1935, but the astronomical connection is only just hinted at in the first two, and therefore we have chosen part of the third. Again, Carl Wirtz anticipated Edwin Hubblè in finding the redshift-magnitude relation for galaxies, but Hubble's results were so much more thorough that they must be considered the primary source. In each case, however, our introductory note mentions the earlier research. Similarly, we have also attempted to reference closely related subsequent work. Our original goal was to present these papers in uncut versions, but as we sifted through the 600 articles specifically considered for inclusion, we quickly realized that such a policy would force us to exclude many important contributions. Even with substantial abridgment of the longer selections, our original selection of 180 articles or groups of articles was simply too expensive to print. In order to keep this volume within the price range of the individual consumer, we were forced to eliminate 48 more selections. During this final cutting we tended to eliminate mathematical works available in other sources, together with those articles whose recent publication makes their long-range importance difficult to assess. Thus, for example, Karl Schwarzschild's article on the equilibrium of the stellar atmosphere and Albert Einstein's article on the cosmological implications of his relativity theory were excluded because they appear, respectively, in Selected Papers on the Transfer of Radiation and The Principle of Relativity (both Dover paperbacks). At least one extremely important type of astronomical publication has, by its nature, been excluded from this compendium. These are the catalogues—of stars and galaxies and radio sources, of positions and magnitudes and spectral types—basic for the progress of astronomy yet impossible to present here. HD, FK3, GC, 3C—these are but four of the household codes in the astronomical vocabulary, indicating the Henry Draper Catalogue of stellar spectral types, the Dritter Fundamental Katalog of stellar positions, the Boss General Catalogue of positions and proper motions of some 33,342 stars, and the third Cambridge survey of radio sources. These laborious compilations are viii

PREFACE

the truly fundamental sources for our contemporary science. We saw no way to include them, yet we realize that with their omission a significant slice of twentieth-century astronomy passes unheralded in our selections. In collecting important contributions from a variety of sources and putting them within the covers of a single book, the editors and Press found that a number of compromises had to be made in matters of editorial consistency and style. In some cases these decisions can perhaps be criticized, but in the interest of economy and expeditiousness they have been accepted even when a better solution later presented itself. We have retained the original numbering of equations, but both figures and tables have been renumbered, and we have added identifying captions or titles when these were absent in the original sources. Figures have frequently been redrawn for clearer reproduction, and the Press has edited the tables to attain greater aesthetic unity. W e have renumbered the references and added citations to articles originally published as "in press," but we have made no attempt to force the references into a uniform style. The systems of units and their abbreviations reproduce those of the original articles, including the early labeling of radial velocities in " k m " rather than "km/sec." We hope that none of the editorial changes made by ourselves or by the Press have affected the information contained in the selections. Incidentally, although we have both carefully proofread the entire volume, we know from the comparative statistics that errors undoubtedly remain ; we console ourselves with the conviction that we have proofread more accurately than many of the original authors ! We have profited from the advice of many persons in assembling this collection. A m o n g those who contributed useful suggestions are Ludwig Biermann, Alastair Cameron, David Dewhirst, William Fowler, Jesse Greenstein, Icko Iben, William McCrea, Donald Menzel, Jan Oort, Carl Sagan, and Sydney van den Bergh. A s our editing reached its final stages, we asked a number of our colleagues to check one or more of the introductions. Foremost among these friendly critics have been Bart J. Bok and Cecilia Payne-Gaposchkin, and the list includes Hannes Alfvén, Ralph Alpher, Viktor Ambartsumian, Robert d'E. Atkinson, John Bahcall, Alan Barrett, Eric Becklin, Hans Bethe, Ludwig Biermann, Adriaan Blaauw, John Bolton, Hermann Bondi, Geoffrey Burbidge, Margaret Burbidge, Bernard Burke, Alastair Cameron, George Clark, Theodore Dunham, Bengt Edlén, Farouk El-Baz, Harold Ewen, George Field, William Fowler, Kenneth Franklin, Riccardo Giacconi, Vitaliii Ginzburg, Thomas Gold, Jesse Greenstein, Herbert Gursky, Robert Hanbury Brown, Chushiro Hayashi, George Herbig, Robert Herman, Antony Hewish, James Hey, Gerald Holton, Michael Hoskin, James Kemp, Frank Kerr, Robert Kraft, C. C. Lin, Per Olof Lindblad, Ursula Marvin, Thomas Matthews, Cornell Mayer, Stanley Miller, William Morgan, Philip Morrison, Gerry Neugebauer, Jan H. Oort, Ernst Öpik, Franco Pacini, Arno Penzias, Edward Purcell, Grote Reber, John Rogerson, Martin Ryle, Carl Sagan, Edwin Salpeter, Maarten Schmidt, Martin Schwarzschild, Irwin Shapiro, Iosif Shklovskii, Lyman Spitzer, Albrecht Unsold, Hendrik van de Hülst, Carl F. von Weizsäcker, Gart Westerhout, John Wheeler, Fred Whipple, Charles Whitney, Gerald Whitrow, and Robert Wilson. We gratefully acknowledge the generous permission to reprint selections from the following sources: Arkiv für matematik, astronomi ochfysik; Astronomical Journal (American Astronomical Society); Astronomicheskii zhurnal (Soviet Astronomy—American Institute of Physics); Astronomische Nachrichten ; Bulletin of the Astronomical Institutes of the Netherlands', Doklady akademii na.uk USSR (Soviet Physics Doklady—American Institute of Physics); Dover Publications; Harper's Magazine·, Icarus (Academic Press); International Astronomical Union; Journal of Atmospheric Sciences (American Meteorological Society); Journal of Geophysical Research (American ix

PREFACE

Geophysical Union) ; Lick Observatory Bulletin ; Monthly Notices of the Royal Astronomical Society (Blackwell Scientific Publications); Nature (Macmillan Journals); Naturwissenschaften·, Nuovo cimento (Società Italiana di fisica); Observatory·, Philosophical Transactions of the Royal Society·, Physical Review and Physical Review Letters (American Institute of Physics); Physikalische Zeitschrift', Physikalishe Zeitschrift der Sowjetunion; Proceedings of the Institute of Radio Engineers·, Proceedings of the National Academy of Sciences; Proceedings of the Royal Society; Publications de l'Observatoire de Tartu; Publications of the Astronomical Society of Japan; Publications of the Astronomical Society of the Pacific (California Academy of Sciences) ; Reviews of Modern Physics (American Physical Society); Science (American Association for the Advancement of Science); Space Research (North Holland Publishing Company); Sky and Telescope; Vistas in Astronomy (Pergamon Press); Zeitschrift für Astrophysik and Zeitschrift für Physik (Springer-Verlag). We also wish to acknowledge permission to reprint a number of articles from the Astrophysical Journal (American Astronomical Society and the University of Chicago Press). We are especially indebted to the staff of the scientific periodicals library of the Cambridge Philosophical Society and the library of the Harvard-Smithsonian Center for Astrophysics for their aid in locating references. We would like to thank the American Philosophical Society and the Tufts University Faculty Research Committee for financial aid. Kenneth R. Lang Tufts University Owen Gingerich Harvard-Smithsonian Center for Astrophysics

χ

Contents

CHAPTER I.

N E W W I N D O W S ON THE UNIVERSE

1. On the Application of Interference Methods to Astronomical Measurements Albert Abraham Michelson

2. A Test of a New Type of Stellar Interferometer on Sirius Robert Hanbury Brown and Richard Q. Twiss

3. Concerning Observations of Penetrating Radiation on Seven Free Balloon Flights Victor Franz Hess

4. The Possibilities of Large Telescopes George Ellery Hale

5. A Rapid Coma-Free Mirror System Bernhard V. Schmidt

6. On the Discovery of Extraterrestrial Radio Waves A Note on the Source of Interstellar Interference Karl G. Jansky

Cosmic Static Grote Reber

7. Searching for Interstellar Communications Giuseppe Cocconi and Philip Morrison

8. The Photoelectric Photometry of the Stars The Measurement of the Light of Stars with a Selenium Photometer, with an Application to the Variations of Algol Joel Stebbins

Fundamental Stellar Photometry for Standards of Spectral Type in the Revised System of the Yerkes Spectral Atlas Harold L. Johnson and William W. Morgan

xi

CONTENTS

9. The First Results Obtained from Photographs of the Invisible Side of the Moon N. P. Barabashov and Yu. N. Lipskii

10. X-Ray and Extreme Ultraviolet Observations of the Sun Herbert Friedman

11. Evidence for X-Rays from Sources outside the Solar System Riccardo Giacconi, Herbert Gursky, Frank R. Paolini and Bruno B. Rossi

12. Infrared Observations of the Galactic Center Eric E. Becklin and Gerry Neugebauer

13. Interstellar Deuterium Abundance in the Direction of Beta Centauri John B. Rogerson and Donald G. York

CHAPTER I I .

T H E SOLAR SYSTEM

14. The Moon's Face: A Study of the Origin of Its Features Grove Karl Gilbert

15. Of Atmospheres upon Planets and Satellites George Johnstone Stoney

16. On the Probable Existence of a Magnetic Field in Sun-Spots George Ellery Hale

17. How Could a Rotating Body such as the Sun Become a Magnet? Joseph Larmor

18. Polarization of the Moon and of the Planets Mars and Mercury Bernard Lyot

19. The Theory of Continental Drift Alfred Wegener

20. The Spectra of Venus, Mars, Jupiter, and Saturn under High Dispersion Theodore Dunham

21. The Mystery of Coronium and the Million-Degree Solar Corona On the Question of the Significance of the Lines in the Spectrum of the Solar Corona Walter Grotrian An Attempt to Identify the Emission Lines in the Spectrum of the Solar Corona Bengt Edlén

22. Corpuscular Influences upon the Upper Atmosphere Sydney Chapman

23. The Origin and Nature of Comets The Structure of the Cloud of Comets Surrounding the Solar System and a Hypothesis Concerning Its Origin Jan H. Oort A Comet Model I : The Acceleration of Comet Encke Fred L. Whipple xii

24. Observations of a Variable Radio Source Associated with the Planet Jupiter Bernard F. Burke and Kenneth L. Franklin

25. Solar Corpuscular Radiation and the Interplanetary Gas Ludwig F. Biermann

26. Radiation Observations with Satellite 1958e James A. Van Allen, Carl E. Mcllwain and George H. Ludwig

27. The Hot Surface Temperature of Venus Observations of Venus at 3.15 cm Wave Length Cornell H. Mayer, Timothy P. McCullough and Russell M. Sloanaker

Soft Landing of Venera 7 on the Venus Surface V. S. Avduevsky, M. Ya. Marov, M. K. Rozhdestvensky, N. F. Borodin, and V. V. Kerzhanovich

28. Radar Determinations of the Rotations of Venus and Mercury Rolf B. Dyce, Gordon H. Pettengill, and Irwin I. Shapiro

29. Mars as Viewed from Mariner 9 Harold Masursky, Bradford A. Smith, Carl Sagan, Conway B. Leovy, Bruce C. Murray, John F. McCauley and James B. Pollack

30. The Moon after Apollo Farouk El-Baz

31. The Encounter Theory of the Origin of the Solar System The Spiral Nebula Hypothesis Forest Ray Moulton

The Dissipation of Planetary Filaments Lyman Spitzer

32. The Nebular Theory of the Origin of the Solar System The Stability of a Spherical Nebula James H. Jeans

The Origin of the Solar System Carl Friedrich von Weizsäcker

33. A Production of Amino Acids under Possible Primitive Earth Conditions Stanley L. Miller CHAPTER I I I .

STELLAR ATMOSPHERES AND STELLAR SPECTRA

34. On the Radiation of Stars Ejnar Hertzsprung

35. Relations between the Spectra and Other Characteristics of Stars Henry Norris Russell

36. Some Spectral Criteria for the Determination of Absolute Stellar Magnitudes Walter S. Adams and Arnold Kohlschütter

37. On the Radiative Equilibrium of Stars Arthur Stanley Eddington

CONTENTS

38. Ionization in the Solar Chromosphere

236

Meghnad Saha

39. The Abundances of Chemical Elements in Stellar Atmospheres

243

The Relative Abundances of the Elements Cecilia H. Payne

On the Composition of the Sun's Atmosphere Henry Norris Russell

40. On the Rotation of Stars

254

Grigory Ambramovich Shajn and Otto Struve

41. Intensity Measurements of the Fraunhofer Lines in the Wavelength Region 5,150 to 5,270 Â

259

Marcel Minnaert and Gerard Mulders

42. Electron Affinity in Astrophysics

264

Rupert Wildt

43. The Quantitative Analysis of the B0-Star τ Scorpii, Part II

268

Albrecht Unsold

CHAPTER I V .

STELLAR EVOLUTION A N D NUCLEOSYNTHESIS

44. The Equivalence of Mass and Energy

275

276

Does the Inertia of a Body Depend upon Its Energy Content? Albert Einstein

The Principle of the Conservation of the Motion of the Center of Gravity and the Inertia of Energy Albert Einstein

45. The Internal Constitution of the Stars

281

Arthur Stanley Eddington

46. The Mass-Luminosity Relation for Stars

291

On the Relation between the Masses and Luminosities of the Stars Arthur Stanley Eddington

The Relationship between the Masses and Luminosities of the Stars Heinrich Vogt

47. Atomic Synthesis and Stellar Energy I, II

303

Robert d'Escourt Atkinson

48. Element Transformation inside Stars

309

Carl Friedrich von Weizsäcker

49. Energy Production in Stars

320

Hans Albrecht Bethe

50. Nuclear Reactions in Stellar Evolution

339

George Gamow

51: Stellar Structure, Source of Energy, and Evolution Ernst Opik

342

52. Nuclear Reactions in Stars without Hydrogen Edwin E. Salpeter

53. Inhomogeneous Stellar Models II : Models with Exhausted Cores in Gravitational Contraction Allan R. Sandage and Martin Schwarzschild

54. Studies of Young Clusters and Stellar Evolution in the Early Phases of Gravitational Contraction Studies of Extremely Young Clusters I: NGC 2264 Merle F. Walker

Stellar Evolution in Early Phases of Gravitational Contraction Chushiro Hayashi

55. Synthesis of the Elements in Stars E. Margaret Burbidge, Geoffrey R. Burbidge, William A. Fowler and Fred Hoyle

56. Neutrinos from the Sun Solar Neutrinos I : Theoretical John N. Bahcall

Search for Neutrinos from the Sun Raymond Davis, Don S. Harmer, and Kenneth C. Hoffman CHAPTER V .

VARIABLE STARS A N D D Y I N G STARS

57. Periods of Twenty-five Variable Stars in the Small Magellanic Cloud Henrietta S. Leavitt

58. On the Pulsations of a Gaseous Star and the Problem of the Cepheid Variables Arthur Stanley Eddington

59. Τ Tauri Variable Stars Alfred H. Joy

60. Binary Stars among Cataclysmic Variables III : Ten Old Novae Robert P. Kraft

61. The Discovery of White Dwarf Stars An A-Type Star of Very Low Luminosity Walter S. Adams

The Spectrum of the Companion of Sirius Walter S. Adams

62. On Dense Stars Ralph Howard Fowler

63. Neutrino Theory of Stellar Collapse George Gamow and Mario Schönberg

64. Discovery of Circularly Polarized Light from a White Dwarf Star James C. Kemp, John B. Swedlund, John D. Landstreet, and James R. P. Angel

65. On the Gravitational Field of a Point Mass according to the Einsteinian Theory Karl Schwarzschild

CONTENTS

66. On the Theory of Stars

456

Lev Davidovich Landau

67. The Discovery of a Candidate Black Hole

460

Cygnus X-l : A Spectroscopic Binary with a Heavy Companion? B. Louise Webster and Paul Murdin

Dimensions of the Binary System HDE 226868 : Cygnus X-l Charles Thomas Bolton

68. Novae or Temporary Stars

466

Edwin P. Hubble

69. On the Possible Existence of Neutron Stars

469

On Super-Novae Walter Baade and Fritz Zwicky

On Massive Neutron Cores J. Robert Oppenheimer and George M. Volkoff

70. Spectra of Supernovae

478

Rudolph Minkowski

71. The Crab Nebula

481

Rudolph Minkowski

72. On the Nature of the Luminescence of the Crab Nebula

488

Iosif S. Shklovskii

73. Energy Emission from a Neutron Star

494

Franco Pacini

74. Observation of a Rapidly Pulsating Radio Source

498

Antony Hewish, S. Jocelyn Bell, John D. H. Pilkington, Paul Frederick Scott, and Robin Ashley Collins

75. Rotating Neutron Stars as the Origin of the Pulsating Radio Sources

505

Thomas Gold

CHAPTER V I .

T H E DISTRIBUTION OF STARS A N D THE SPACE BETWEEN THEM

76. Investigations of the Spectrum and Orbit of Delta Orionis

509

510

Johannes Franz Hartmann

77. Star Streaming

514

Jacobus Cornelius Kapteyn

78. The Kinetic Energy of a Star Cluster

520

Arthur Stanley Eddington

79. The Scale of the Universe

523

Harlow Shapley and Heber D. Curtis

80. First Attempt at a Theory of the Arrangement and Motion of the Sidereal System Jacobus Cornelius Kapteyn

542

81. Star-Streaming and the Structure of the Stellar System Bertil Lindblad

82. Observational Evidence for the Rotation of Our Galaxy Observational Evidence Confirming Lindblad's Hypothesis of a Rotation of the Galactic System Jan H. Oort

Rotation Effects, Interstellar Absorption, and Certain Dynamical Constants of the Galaxy Determined from Cepheid Variables Alfred H. Joy

83. On the Dark Nebula N G C 6960 Max Wolf

84. The Source of Luminosity of Gaseous Nebulae The Planetary Nebulae Donald H. Menzel

An Application of the Quantum Theory to the Luminosity of Diffuse Nebulae Herman Zanstra

85. The Origin of the Nebular Lines and the Structure of the Planetary Nebulae Ira Sprague Bowen

86. The Physical State of Interstellar Hydrogen Bengt Strömgren

87. Preliminary Results on the Distances, Dimensions, and Space Distribution of Open Star Clusters Robert J. Trumpler

88. The Solid Particles of Interstellar Space Hendrik C. van de Hulst

89. The Polarization of Starlight Polarization of Light from Distant Stars by the Interstellar Medium William Albert Hiltner

Observations of the Polarized Light from Stars John Scoville Hall

90. The Temperature of Interstellar Matter Lyman Spitzer and Malcolm P. Savedoff

91. Radio Waves from Space: Origin of Radiowaves Hendrik C. van de Hulst

92. The Radio Frequency Detection of Interstellar Hydrogen Radiation from Galactic Hydrogen at 1,420 MHz Harold I. Ewen and Edward M. Purcell

The Interstellar Hydrogen Line at 1,420 MHz and an Estimate of Galactic Rotation C. Alex Muller and Jan H. Oort

93. Some Features of Galactic Structure in the Neighborhood of the Sun William W. Morgan, Stewart Sharpless, and Donald Osterbrock

CONTENTS

94. The Galactic System as a Spiral Nebula

643

Jan H. Oort, Frank J. Kerr, and Gart Westerhout

95. Density Waves in Disk Galaxies

652

Chia C. Lin and Frank H. Shu

96. The Discovery of Protostars (?)

656

Small Dark Nebulae Bart J. Bok and Edith F. Reilly The Spectra of Two Nebulous Objects near N G C 1999 George H. Herbig Herbig's Nebulous Objects near N G C 1999 Guillermo Haro Observations of an Infrared Star in the Orion Nebula Eric E. Becklin and Gerry Neugebauer

97. Radio Observations of OH in the Interstellar Medium

666

Sander Weinreb, Alan H. Barrett, M. Littleton Meeks, and John C. Henry

98. Galactic Magnetic Fields and the Origin of Cosmic Radiation

671

Enrico Fermi

99. Cosmic Rays and Radio Emission from Our Galaxy

677

Cosmic Rays as the Source of General Galactic Radio Emission Karl Otto Kiepenheuer The Nature of Cosmic Radio Emission and the Origin of Cosmic Rays Vitallii Lazarevich Ginzburg

100. Expanding Stellar Associations and the Origin of the Runaway O and Β Stars

685

Expanding Stellar Associations Viktor Amazaspovich Ambartsumian On the Origin of the O- and B- Type Stars with High Velocities (The "Run-Away Stars") and Some Related Problems Adriaan Blaauw CHAPTER V I I .

NORMAL GALAXIES, RADIO GALAXIES, AND QUASARS

101. Novae in Spiral Nebulae

697

698

Heber D. Curtis, George W. Ritchey, and Harlow Shapley

102. A Spectrographic Investigation of Spiral Nebulae

704

Vesto M. Slipher

103. Modern Theories of the Spiral Nebulae

708

Heber D. Curtis

104. Cepheids in Spiral Nebulae

713

Edwin P. Hubble

105. Extra-Galactic Nebulae Edwin P. Hubble

xviii

716

106. A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae Edwin P. Hubble

107. On the Masses of Nebulae and of Clusters of Nebulae Fritz Zwicky

108. Nuclear Emission in Spiral Nebulae Carl K. Seyfert

109. The Resolution of Messier 32, NGC 205, and the Central Region of the Andromeda Nebula Walter Baade

110. A Revision of the Extra-Galactic Distance Scale Walter Baade

111. Redshifts and Magnitudes of Extra-Galactic Nebulae Milton L. Humason, Nicholas U. Mayall, and Allan R. Sandage

112. On the Evolution of Galaxies Viktor Amazaspovich Ambartsumian

113. Fluctuations in Cosmic Radiation at Radio Frequencies James S. Hey, S. J. Parsons, and J. W. Phillips

114. Positions of Three Discrete Sources of Galactic Radio-Frequency Radiation John G. Bolton, Gordon J. Stanley, and O. B. Slee

115. Cosmic Radiation and Radio Stars Hannes Alfvén and Nicolai Herlofson

116. The Origin of Cosmic Radio Noise Thomas Gold, Fred Hoyle, George McVittie, and Martin Ryle

117. Identification of the Radio Sources in Cassiopeia, Cygnus A, and Puppis A Walter Baade and Rudolph Minkowski

118. The Nature of Cosmic Radio Sources Martin Ryle

119. First True Radio Star? Thomas A. Matthews, John G. Bolton, Jesse L. Greenstein, Guido Münch, and Allan R. Sandage

120. The Discovery of Quasars Investigation of the R a d i o Source 3 C 273 b y the M e t h o d o f Lunar Occultations Cyril Hazard, M. B. Mackey, and A. J. Shimmins 3 C 273: A Star-like Object with Large Red-Shift Maarten Schmidt Red-Shift of the U n u s u a l R a d i o Source: 3C 48 Jesse L. Greenstein and Thomas A. Matthews A b s o l u t e Energy Distribution in the Optical Spectrum of 3 C 273 J. Beverly Oke

121. The Quasi-Stellar Radio Sources 3C 48 and 3C 273 Jesse L. Greenstein and Maarten Schmidt

CONTENTS

CHAPTER V I I I .

RELATIVITY A N D COSMOLOGY

122. Explanation of the Perihelion Motion of Mercury by Means of the General Theory of Relativity

8i9

820

Albert Einstein

123. A Determination of the Deflection of Light by the Sun's Gravitational Field, from Observations Made at the Total Eclipse of May 29, 1919

826

Frank Watson Dyson, Arthur Stanley Eddington, and Charles Davidson

124. Fourth Test of General Relativity: New Radar Result

833

Irwin I. Shapiro, Michael E. Ash, Richard P. Ingalls, William B. Smith, Donald B. Campbell, Rolf B. Dyce, Raymond F. Jürgens, and Gordon H. Pettengill

125. On the Curvature of Space

838

Aleksandr Friedmann

126. A Homogeneous Universe of Constant Mass and Increasing Radius accounting for the Radial Velocity of Extra-Galactic Nebulae

844

Georges Lemaître

127. On the Relation between the Expansion and the Mean Density of the Universe

849

Albert Einstein and Wilhelm de Sitter

128. The Cosmical Constants

851

Paul Adrien Maurice Dirac

129. The Steady-State Theory of the Expanding Universe

853

Hermann Bondi and Thomas Gold

130. The Origin of Chemical Elements

864

Ralph A. Alpher, Hans Bethe, and George G a m o w

131. The Evolution and Physics of the Expanding Universe

866

Evolution of the Universe Ralph A. Alpher and Robert C. Herman

The Physics of the Expanding Universe George Gamow

132. A Measurement of Excess Antenna Temperature at 4080 M H z

873

A r n o A. Penzias and Robert W. Wilson

XX

Author Index

879

Subject Index

889

CHAPTER I

New Windows on the Universe

1. On the Application of Interference Methods to Astronomical Measurements Albert Abraham Michelson (Philosophical Magazine 30, 1-21 [1890])

In this paper Albert Abraham Michelson describes the use of interference methods in measuring the angular size and the one-dimensional brightness distribution of sources that are too small to be resolved by a single telescope. This fundamental technique has found widespread application both in optical and in radio astronomy. At optical wavelengths, radiation from a source is received by two mirrors and combined on the focal plane of a telescope. If the separation of the mirrors, D, is not too great, the source is unresolved and the coherent light will produce interference fringes of alternating light and dark bands. As the two mirrors are gradually separated, the fringes will disappear when the source is resolved. In this case, the angular diameter of a source is 1.22 A/D, where λ is the wavelength of the observation. The first successful measurement of the angular diameter of a star was made by Michelson and Francis G. Pease on December 13,1920, by using two mirrors separated by 20 ft and placed at the end of the open tube of the 100-in Hooker telescope. 1 They found that the angular diameter of the supergiant star α Orionis (Betelgeuse) was 47 χ 10 3 sec of arc. Although the angular diameters of six giant stars were measured with this instrument, 2 subsequent efforts to extend the measurements to the smaller main-sequence stars failed. This was due partly to the effects of atmospheric scattering and partly to the practical difficulty of constructing large mirror separations. These problems were finally overcome with the development of the intensity interferometer described in our next selection. When extraterrestrial radiation was found at radio wavelengths, Michelson interferometers were constructed to measure the angular sizes and the brightness distributions of the sources. In this case, radio frequency signals received at two telescopes were transmitted to some central location for correlation, and coherence was maintained by the transmission of a common local oscillator signal to the two telescopes. In this way, the angular sizes of several bright radio sources were found to be a few minutes of arc; later detailed maps showed that many of the radio objects consisted of two components. 3 The Michelson type of interferometer has been since used to give angular resolutions u p to 0.1 sec of arc at a radio wavelength of 6 cm; 4 and the very long baseline (V.L.B.I.) adaptation using signals recorded simultaneously at two independent radio telescopes has given angular resolutions up to 0.001 sec of arc. R. H a n b u r y Brown and his colleagues suggested that mutually coherent local oscillators were not necessary at the two telescopes, and showed that post-detection correlation of signals recorded at two

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1. INTERFERENCE METHODS A N D ASTRONOMICAL MEASUREMENTS

different telescopes could be used to measure the angular sizes of radio sources (see selection 2). This method has vastly extended the interferometer baselines at both optical and radio wavelengths. 1. Astrophysical Journal 53, 249 (1921). 2. Ergebnisse der exakten Naturwissenschaften 10, 84 (1931). 3. M. Ryle, Proceedings of the Royal Society (London) A211, 351 (1952); Β. Y. Mills, Australian Journal of Physics 6,452 (1953). 4. A historical account of the use of interferometry in radio astronomy is given by J. S. Hey in The Evolution of Radio Astronomy (New York : Neale Watson Academic Publications, 1973).

I . N E W W I N D O W S ON T H E U N I V E R S E

I

N A R E C E N T P A P E R on "Measurement by LightWaves" 1 it was shown that the limitation of the effective portions of an objective to the extreme ends of a diameter converted the instrument into a refractometer; and although definition and resolution are thereby sacrificed, the accuracy may be increased ten to fifty fold. The simplest way of effecting this in the case of a telescope is to provide the cap of the objective with two slits adjustable in width and distance apart. If such a combination be focused on a star, then, instead of an image of the star, there will be a series of coloured interference-bands with white centre, the bands being arranged at equal distances apart and parallel to the two slits. The position of the central white fringe can be marked from ten to fifty times as accurately as can the centre of the telescopic image of the star.

It was found that the first indication of indistinctness occurred when a was 0.08 millim. wide, and at 0.14 millim. the fringes almost vanished. But on continuing to widen the slit they again became clearly visible, to disappear and reappear at regular intervals. Now, though it might with truth be urged that the observation of the indefinite vanishing of interference-fringes depends so much on the attendant circumstances, and especially on the condition of the observer, that it can scarcely be called a precise measurement, yet the statement applies no longer when the disappearance depends on the existence of wellmarked minima of distinctness; and, as will appear below, it is possible to measure, with accuracy, by the observation of these minima the width of a source of light, which in a telescope can with difficulty be ascertained to have an appreciable size.

One of the most promising applications of the method is the measurement of the angular magnitudes of small sources of light. This may be accomplished by taking advantage of the wellknown principle that in order to obtain clear interferencebands from two pencils diverging from the same source (width a) at an angle β, it is necessary that either β or a be very small. Thus let us take

The theory of these successive appearances and disappearances is as follows : Let χ be the distance of any element of the source from the axis of the telescope, dx the width of the element, and y = φ{χ) the length. Then the difference in the two paths xS and x S , P terminating at the wave-front P, which makes an angle y with the plane perpendicular to the axis of the telescope, will be ßx - yb, and the resulting intensity in the direction y for the whole source will therefore be

a = ee± = width of the source. d = distance of source from the objective. fc = SSj = distance between the slits.

2π : = J > ( x ) 1 + cos — (ßx — yb) dx. λ CASE I.

Also put S i P = Δ, a/d = a, b/d = β.

The distance b = SS ^ is the distance between the two slits that are placed in a plane perpendicular to the telescope axis. The distance Δ = SjP is the extra distance between S, and the wavefront that lies along the line extending from point S to point P. Then the usual statement is that the interference-fringes vanish when Sej — Se = βα/2 = fea/2 = λβ, or when

UNIFORMLY ILLUMINATED SLIT

If the source be a slit whose centre is in the axis, and whose length is parallel to the slits SSj, and whose width is a, then π

Λ Π II = a Η, — • -— Ρ α cos 2— ?- I,b. - sin

πβ

λ

λ

Ii - I

2

But

α = α0. Or, in words, the fringes disappear when the source subtends an angle which can just be resolved by the telescope. The experiment was first tried with an objective of 45 millim. effective diameter (distance between the slits) at a distance of ten metres from an adjustable slit which served as the source.

4

(2)

If I [ be the intensity at the centre of a bright fringe, and I 2 that at the centre of a dark fringe, then the visibility of the fringes may be expressed by

d i + I2

But λ/b is the "limit of resolution" of the telescope of aperture b, and if this be denoted by a 0 we have

(1)

λ . πβ 11 = a + -— sin — a, πβ λ , 12 = α

Α πβ

. πβ sin —- α; λ

πβ sin—-α Α V = «β

τα

(3)

1 . INTERFERENCE M E T H O D S A N D A S T R O N O M I C A L

or finally,

V =

α sin π —