Deep-Sky Companions: Southern Gems: Southern Gems 9781139844826, 9781107015012

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Deep-Sky Companions

Southern Gems In Southern Gems, Stephen James O’Meara makes a detour beneath the southern skies, presenting a fresh list of 120 deep-sky objects for Southern Hemisphere stargazers to observe. Showcasing many exceptional objects catalogued by the pioneering observer James Dunlop, known as the “Messier of the southern skies,” all are visible through small- to moderate-sized telescopes under dark skies. The list features a rich assortment of bright and dark nebulae, planetary nebulae, open and globular star clusters, and galaxies of all types. Each object’s profile includes beautiful photographs and sketches, original finder charts, visual histories, and up-to-date astrophysical background information. Whether you live in the Southern Hemisphere or are just visiting, this new Deep-Sky Companion will make a perfect observing partner, whatever your background. There is no other southern sky guide like it on the market. Author of several highly acclaimed books, including others in the celebrated Deep-Sky Companions

series, Stephen James O’Meara is well known among the astronomical community for his engaging and informative writing style and his remarkable skills as a visual observer. O’Meara spent much of his early career on the editorial staff of Sky & Telescope before joining Astronomy magazine as its Secret Sky columnist and a contributing editor. An award-winning visual observer, he was the first person to sight Halley’s comet on its return in 1985 and the first to determine visually the rotation period of Uranus. One of his most distinguished feats was the visual detection of the mysterious spokes in Saturn’s B-ring before spacecraft imaged them. Among his achievements, O’Meara has received the prestigious Lone Stargazer Award, the Omega Centauri Award, and the Caroline Herschel Award. Asteroid 3637 was named O’Meara in his honor by the International Astronomical Union. In his spare time, he travels the world to document volcanic eruptions. He is a contract videographer for National Geographic Digital Motion and a contract photographer for the National Geographic Image Collection.

Deep-Sky Companions

Southern Gems Stephen James O’Meara

CAMBRIDGE UNIVERSITY PRESS Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo, Delhi, Mexico City Cambridge University Press 32 Avenue of the Americas, New York, NY 10013-2473, USA www.cambridge.org Information on this title: www.cambridge.org/9781107015012 © Stephen James O’Meara 2013 This publication is in copyright. Subject to statutory exception and to the provisions of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published 2013 Printed in China by Everbest A catalog record for this publication is available from the British Library. Library of Congress Cataloging in Publication Data O’Meara, Stephen James, 1956– Southern gems / Stephen James O’Meara. p.  cm. – (Deep-sky companions) Includes bibliographical references and index. ISBN 978-1-107-01501-2 (hardback) 1.  Astronomy – Southern Hemisphere – Amateur’s manuals.  I.  Title. QB63.O639  2012 520.9181′4–dc23    2012019714 ISBN 978-1-107-01501-2 Hardback Cambridge University Press has no responsibility for the persistence or accuracy of URLs for external or third-party Internet Web sites referred to in this publication and does not guarantee that any content on such Web sites is, or will remain, accurate or appropriate.



To Donna, My love To Daisy Duke, My joy And in memory of Milky Way, Miranda-Pywackett, and Pele, My angels.

Contents Preface Acknowledgments 1 About this book 2 The Southern Gems

page ix xiii 1 11

Appendix A Southern Gems: basic data

433

Appendix B Forty-two additional Southern Gems

436

Appendix C James Dunlop and a brief history of the early telescopic exploration of the far-southern skies

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Appendix D Illustration credits

447

Wide-field star charts Index The Southern Gems checklist

Contents

449 457 464

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Preface

I l ove e x pl o ri ng t he sou t he rn night sky. For much of my life, this large and lush celestial wilderness was a secret garden to which I hadn’t a key. I grew up observing the stars from a mid-northern latitude, in Cambridge, Massachusetts. Back then, I could only imagine the beauty of objects such as globular cluster Omega Centauri, 47 Tucanae, the Jewel Box open cluster, the Eta Carinae nebula, and, of course, the Large and Small Magellanic Clouds, to name but a few. These objects, though familiar to me by name, were among the many “invisible wonders” of the universe. I finally obtained a key to a large part of the garden when I temporarily resided in Hawaii for several months in 1981 and 1982 – before moving to Volcano on the Big Island of Hawaii in 1994. Actually, I have been traveling to the Southern Hemisphere since 1982, and over the years I’ve accumulated impressions of

Preface

some of the more prominent southern-sky objects through a variety of instruments. In August 1997, I traveled to New Zealand, where I used the Auckland Observatory’s 20-inch f/13.5 Zeiss reflector and 4.5-inch finderscope to observe some of these objects. I also used the 9-inch refractor at Carter Observatory (in Wellington), as well as a Celestron 8-inch Schmidt-Cassegrain telescope set up in the backyard of a friend’s house in Wellington, to view others. I’ve also observed deep-southern objects from the Australian Outback, from the Altiplano in Bolivia, from the plains of South Africa, and with friends in Central America. For my observations at home, I used several instruments, principally a Tele Vue 4-inch f/5 Genesis refractor (from 1994 to 2007) and a Tele Vue NP-127 Nagler–Petzval 5-inch (660-millimeter) f/5.2 apochromatic refractor (from 2007 to the present). I also used 7 × 35 and 10 × 50 binoculars, and, on occasion, a nineteenth-century brass telescope made by Ross of London, which I refer to simply as “the antique telescope” in Chapter 2; its tube measures 17 1/8 inches when open and 7 1/4 inches when closed. But that’s only part of the story behind the making of this book. How the 120 Southern Gems Were Selected The foundation for the Southern Gems list was laid in 1996, when I started work on my book titled Deep-Sky Companions: The Caldwell Objects. While researching the histories of some of the southern objects in the Caldwell list, I became aware of discoveries made by James Dunlop, who in 1826 surveyed the southern skies for nebulae and star clusters from Australia with a 9-inch f/12 reflector. This led to the creation of his Catalogue of Nebulae and Clusters of Stars in the Southern

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Hemisphere observed at Parramatta in New South Wales, which he presented to the Royal Society of London in 1827. The Society published the catalogue of 629 objects in its Philosophical Transactions for 1828. When my friend Brent Archinal (then of the U.S. Naval Observatory) learned of my interest in Dunlop’s work, he presented me with a copy of Dunlop’s catalogue, which I began to read with fascination. My interest in Dunlop swelled when I discovered that John Herschel, during his stay in South Africa in the 1830s, had failed to identify about two-thirds of the objects catalogued by Dunlop with an 18 1/2inch speculum-metal-mirror reflector (see Appendix C). Nevertheless, I found many of Dunlop’s des­criptions of the identifiable objects well suited for amateur astronomers using smallto ­moderate-sized telescopes – like the ones I used in the field. And that in itself attracted me. My respect for Dunlop was further magnified by a 2010 paper titled “James Dunlop’s Historical Catalogue of Southern Nebulae and Clusters” in the Journal of Astronomical History and Heritage (http://eprints.jcu .edu.au/10919/1/10919_Cozens_et_al_2010 .pdf ). In it, authors Glen Cozens, Andrew Walsh, and Wayne Orchiston (of the Centre for Astronomy, James Cook University, Townsville, Queensland, Australia) make the following conclusions about Dunlop’s work: Dunlop’s catalogue contains most of the bright star clusters, nebulae and galaxies south of declination –30°, and therefore is the southern equivalent of Messier’s famous northern catalogue, as suggested by Cozens and White in the June 2001 edition of Sky [&] Telescope. It unfortunately also contains a large number of entries which are probably faint double stars or asterisms, because Dunlop was unable to resolve them. Omitting the double stars and asterisms

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gives rise to an impressive catalogue. We therefore believe that the Dunlop catalogue should be a useful resource for southern amateur astronomers viewing galaxies, nebulae and clusters.

Cozens promotes the same argument in his Ph.D. thesis titled “Nicolas-Louis de La Caille, James Dunlop and John Herschel: An Analysis of the First Three Catalogues of Southern Star Clusters and Nebulae”: In defense of Dunlop’s work his catalogues are actually better suited to amateur astronomers with smaller telescopes. His descriptions are of interest to those with 6- to 10-inch telescopes because Dunlop’s telescope was of a similar size and had a similar limiting magnitude. The descriptions are also important because they were the first made of many bright far southern objects, including genuine objects that John Herschel failed to see.

Thanks to Cozens et al., my quest to create an observing guide for Southern Hemisphere observers had a clear focus. To help me in my selection of objects for this book, Cozens graciously supplied me with several different lists of identifiable Dunlop objects that he felt were well suited for today’s amateur astronomers using telescopes of 6- to 10-inch aperture (or greater), mainly those listed in the Dunlop 100, which appeared in Sky & Telescope (June 2001); the Dunlop 244 (the Dunlop 100 plus 144 other objects identified from John Herschel’s 1864 General Catalogue and Arthur Auwers’ 1862 catalogue); and a list of 150 select Dunlop objects. My task, then, was to review these lists and create a new one that would provide a balanced observing program. I wanted a mix of bright targets, as well as a few challenging objects (those that Dunlop found near the vision limit in his sweeps). I found the latter objects quite remarkable, as they are testaments to the visual prowess of Dunlop, who

Preface

used a speculum-metal-mirror telescope that performed perhaps as well as a modern 6-inch reflector. In the end, I selected 120 Dunlop objects: 43 galaxies, 36 globular star clusters, 30 open star clusters, 6 planetary nebulae, 3 bright nebulae, and 2 dark nebulae. Given that Dunlop has been called the Messier of the southern skies, I tried to balance the list so that the number of each individual type of object in the Messier and Southern Gems lists was approximately equal (the Southern Gems catalogue includes 10 more objects than the Messier catalogue). Galaxies comprise about 35 percent of both the Messier and Southern Gems catalogues; globular star clusters comprise about 25 percent of the Messier catalogue and about 30 percent of the Southern Gems catalogue. And in both the Messier catalogue and the Southern Gems catalogue open star clusters comprise 25 percent of the objects. You can compare the other types of objects in the following table: Number of various types of objects from Messier and Southern Gems catalogues

Object type

Messier catalogue

Southern Gems catalogue

Galaxies Globular star clusters Open star clusters Bright nebulae Planetary nebulae Dark nebulae Supernova remnants Star clouds Double stars Asterisms Unidentified objects Total

39 28 27 7 4 0 1 1 1 1 1 (M102) 110

43 36 30 3 6 2 0 0 0 0 0 120

Preface

To ease the selection process, I did not include objects in the Large or Small Magellanic Clouds, which could be the bulk of another book. However, I did include a selection of these in Appendix B, which lists 42 additional Southern Gems (all Dunlop objects) appended with Dunlop’s catalogue entries. Furthermore, because I had researched, observed, and included many deep-sky objects in my book Deep-Sky Companions: The Caldwell Objects, it was only natural to include in the Southern Gems list those objects in the Caldwell list that also appear in Dunlop’s catalogue. I then updated most of the Caldwell essays, which I had written more than a decade ago. Otherwise, for galaxies, I selected those of magnitude 11.0 or brighter – with two exceptions: NGC 7590 (magnitude 11.3) and NGC 7599 (11.1). These two galaxies belong to the famous Grus Triplet, the other member being NGC 7582 (magnitude 10.1). The only fainter object in the list is planetary nebula NGC 2818 (magnitude 11.6). Not only is NGC 2818’s light concentrated into a disk only 38″ across (making it a decent target), but we see it projected on the sky against the pretty open star cluster Melotte 96 in Pyxis, another Dunlop discovery. The Southern Gems list is replete with celestial wonders, some superlative in their nature. There’s the globular cluster dynamo Omega Centauri (Southern Gem 62)  – one of the oldest objects in the Milky Way (its age being comparable to that of the universe itself ), which may be the nucleus of a cannibalized dwarf galaxy (a former companion to the Milky Way). Centaurus A (Southern Gem 61) is a superlative in virtually every region of the electromagnetic spectrum. It is one of the brightest naked-eye galaxies; by far the nearest and most violent Seyfert-type galaxy

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known; one of the most intense radio sources in the heavens; and a wellspring of infrared, x-ray, and gamma-ray radiation. Eta Carinae (Southern Gem 48) had the largest explosion that any star is known to have survived. At radio wavelengths, Eta Carinae produces the brightest known stellar wind. What’s more, through a telescope, the star is surrounded by an elaborate tapestry of stellar energy fused into luminous folds of gases strung together with threads of dust. And NGC 1365 (Southern Gem 16) is one of the most stunning barred spiral galaxies in the night sky – an impressive system with a linear diameter of 160,000 light-years and a luminosity 200 billion times that of the Sun, making it a good match for our Milky Way. And there’s so much more: some of the blackest dark nebulae known, icy blue planetary nebulae, rich open star clusters, ringed and lenticular galaxies, luminous Seyfert galaxies (extraordinary energy engines that emit about a thousand times more energy than the radio nucleus of our galaxy), starburst galaxies, interacting galaxies, grand-design spirals, extragalactic systems with supermassive black holes, and more. The 120 objects in the Southern Gems list are here for your enjoyment. All can be seen with a 5-inch telescope

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under a dark sky. Many are visible with binoculars, and some can be seen with the unaided eye. Although the title of this Deep-Sky Companions volume is Southern Gems, all the objects in it were either discovered by Dunlop or appear in his catalogue of nebulae and clusters. With 156 discoveries Dunlop, Cozens determined, is the third greatest discoverer of bright NGC/IC objects after William and John Herschel. By sharing these wonders with you, I hope to keep Dunlop’s name alive well into the future, especially because, as Cozens, White, and Orchiston point out at the end of their 2010 paper, Dunlop is “virtually unknown today, a forgotten pioneer of the southern sky.” My tacit desire in creating this special edition of the Deep-Sky Companions series is to help both Southern Hemisphere observers and Northern Hemisphere observers with the desire to travel and explore the southern skies on their own to grow as observers and expand their visual envelope, and make the act of observing a fun and memorable experience – one that will inspire the collective you to share this exciting adventure with others and continually perpetuate the love, joy, and romance of observing.

Preface

Acknowledgments Before we journey on, I would like to thank Vince Higgs, Lindsay Barnes, Helen Wheeler, and the editorial staff at Cambridge University Press for their encouragement, help, and support with this book and the Deep-Sky Companions series. I bow low to Al and David Nagler of Tele Vue Optics in Chester, New York, for making my nights under the stars so pleasurable with their refracting telescopes. And I thank Sue Tritton at the Plate Library of the Royal Observatory Edinburgh for granting me permission to use the Digitized Sky Survey images taken with the UK Schmidt Telescope; your contribution was invaluable. I am grateful for the help of the professional astronomers who reviewed the astrophysics on the objects covered in this book; most I list as the authors or the principal investigators of the professional papers mentioned in the text. I am honored that South African amateur astronomer and lover of the stars Magda

Acknowledgments

Streicher contributed her observations of many of the Southern Gems as seen through her 12- and 16-inch Schmidt-Cassegrain telescopes. I am also extremely grateful for the kind assistance of Glen Cozens in Australia, who inspired me to spotlight the works of James Dunlop in this book by sending me his Ph.D. thesis. He also supplied me with various lists of Dunlop objects from which I could make my object selections, reviewed my brief history of early southern sky explorations in Appendix C, and made invaluable corrections to my proofs. Of course, and to paraphrase Cassius in Shakespeare’s Julius Caesar, “Any fault, dear readers, is not in the stars, But in myself, as I am an underling.” Finally, I would like to express my love for my beautiful wife, Donna, for helping me on this journey with her love, support, and understanding, and for Daisy Duke, our faithful papillon.

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Chapter 1

About this book I d e ta il e ach sou t he rn Gem in Chapter 2 – the heart of the book. Each object essay opens with a photograph of the target object (oriented with north up and east to the left) and a table of basic data. The data table includes the following: the Southern Gem number; common name(s), if any (note that many of these names are not officially sanctioned but rather fun monikers that reflect imaginings by me and/or other observers); NGC, IC, or other catalogue name; object type; constellation (Con); right ascension (RA) and declination (Dec) in equinox 2000.0 coordinates; apparent magnitude (Mag); surface brightness (SB) in magnitudes per square arcminute (for most objects); angular size or dimensions (Diam/Dim); distance (Dist); and the object’s discoverer (Disc) and date of discovery. Beneath the table, you’ll find the discoverer’s catalogue description of the object, with its original catalogue number in parentheses at the end. In most cases, James Dunlop made the discovery, otherwise Dunlop’s description follows that of the discoverer(s). John Herschel’s catalogue description and catalogue number always follow Dunlop’s. The catalogue number in parentheses at the end of each object description is a code – sometimes simple, sometimes a bit more complex. For instance, following Dunlop’s description of NGC 2546 (Southern Gem 37) is the code (D 563), which means that it is the 563rd object in Dunlop’s catalogue;

About this book

the capital letter D stands for Dunlop. The code for NGC 2546 used by John Herschel is (h 3116), meaning it’s the 3116th object in John Herschel’s Cape Catalogue. (John Herschel uses a small “h” to differentiate himself from his father William Herschel, whose catalogue code is “H.”) In the case of Lacaille, the code is a little more involved. For example, Lacaille’s discovery code for NGC 2456 is (II-4). Lacaille placed his objects into three “classes”  – Class I: “nebulosities not accompanied by stars”; Class II: “nebulosities due to clusters”; and Class III: “stars accompanied by nebulosity.” NGC 2456 is the fourth object in his Class II. William Herschel’s code is the most complex. For instance, NGC 6624 (Southern Gem 105) is Herschel’s (H I-50). The Roman numeral in William Herschel’s system

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identifies the class into which Herschel placed each object: I. Bright nebulae II. Faint nebulae III. Very faint nebulae IV. Planetary nebulae: Stars with burrs, with milky chevelure, with short rays, remarkable shapes, etc. V. Very large nebulae VI. Very compressed and rich clusters of stars VII. Pretty much compressed clusters of large or small stars VIII. Coarsely scattered clusters of stars So H I-50 is the 50th object in Herschel Class I (bright nebulae). The original 1888 New General Catalogue (NGC) description or a description from the supplemental Index Catalogues follows these entries. If the object is not from one or the other of these catalogues, it is left blank. Most object essays start with a general description of the object, followed by where it lies in the sky (in what constellation and near which stars) and how easy or difficult it will be to see through a small- to moderate-sized telescope from the city or country. I usually then follow this opener with historical anecdotes about the constellation in which it lies, the object’s discovery, and/or early telescopic impressions of the object. As with Messier’s catalogue, Dunlop’s includes not only objects discovered by him but also those who preceded him in the search for southern nebulae and clusters: most notably Giovanni Batista Hodierna (1597–1660), Edmond Halley (1656–1742), Abbé Nicolas Louis de Lacaille (1713–1762), Charles Messier (1730–1781), and William Herschel (1738–1822). John Herschel (1792– 1871) appears prominently in many of the

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historical accounts because he investigated Dunlop’s catalogue during his monumental search for southern nebulae and clusters from South Africa in the 1830s. The essays also include references to other notable historical figures who played prominent roles in the early astrophysical studies of these objects. In many cases, the essays describe recent observations from the Hubble Space Telescope, the world’s largest ground-based telescopes, and a fleet of spacecraft that now peer (or have peered) into the universe with x-ray and infrared-sensitive “eyes.” I then tell you how to find the object using the book’s wide-field and detailed star charts. I conclude each essay by describing how the target object appears through the telescopes I used. And many of the essays include additional observations by some avid Southern Hemisphere observers using larger telescopes. Most notable are the contributions of Magda Streicher (see photo), who lives in Polokwane/Pietersburg, in the far northern part of South Africa. Of her interest in the heavens, she says: My interest in the stars goes right back to childhood and I can clearly recall the fascination those twinkling, silvery lights in the dark skies above held for me as a youngster. Since then I have become an active amateur astronomer, and over many years I have become steadily more involved in advanced participation in South Africa. Using 12-inch and 16-inch Schmidt-Cassegrain telescopes enables me to do useful deep-sky observing [from Polokwane] as well as on our farm close to the Zimbabwe border with excellent dark skies. I have done that with great passion and satisfaction over the past years, searching out deep sky objects, sketching them to add to a wonderful interesting world with certain exceptional and unique characteristics. I strive to share my interest with others through regular radio talks, and articles

About this book

years later to see how you are progressing as an observer.

for the local newspaper. I hope that my humble contribution can help reveal some of the wonders of the universe.

Several appendices complete the work. Appendix A tabulates and identifies each Southern Gem’s NGC, IC, or other modern designation, its right ascension and declination, the constellation in which it appears, type (galaxy, nebula, open cluster, etc.), apparent magnitude, angular size, and Dunlop (D) catalogue number. Appendix B does the same for the 42 additional Southern Gem objects, but with the addition of a Dunlop catalogue description for each object. Appendix C is a brief history of the early telescopic exploration of the southern skies, highlighting the contributions of James Dunlop. At the end of the book is a Southern Gem checklist – a place for you to make personal notations on each object you find; it includes spaces for you to write down important information, such as the date observed, your location, the telescope and magnification used, atmospheric seeing and transparency, and any other special notes you want to record. It is a personal log that you can return to weeks, months, or

About this book

Sources of Data and Information The data and information in this book were drawn from a variety of modern sources. Generally speaking, I gleaned recent research findings on the physical nature of each object from the Astronomical Journal or the Astrophysical Journal, and citations are given. From each object’s apparent diameter and distance, I calculated its physical dimensions using the formulas that appear on page 35 of the first edition of Deep-Sky Companions: The Messier Objects. The Dunlop catalogue descriptions are from Dunlop’s Catalogue of Nebulae and Clusters of Stars in the Southern Hemisphere observed at Parramatta in New South Wales, published in 1828 (provided by Brent Archinal). William Herschel’s quotations come from his original observing notes (provided by Larry Mitchell of Houston, Texas). The Messier quotations were gleaned from my Deep-Sky Companions: The Messier Objects. Aside from John Herschel’s quotations, all others are from the late Kenneth Glyn Jones’s The Search for the Nebulae (Chalfont St. Giles: Alpha Academic, 1975). Most of John Herschel’s quotations have been gleaned from his original observations, published in 1847 as Results of Astronomical Observations made during the years 1834, 5, 6, 7 & 8, at the Cape of Good Hope. During his stay in South Africa, John Herschel often made several observations of each object. The quotations used in this book’s tables, however, refer only to his first observation; a date is given only if he discovered the object. Other information, such as constellation lore, properties of stars, and each object’s position, apparent magnitude, angular size, and surface brightness, come from the

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following excellent sources (primary sources are listed first). Star names, constellations, and mythology Allen, Richard Hinckley. Star Names: Their Lore and Meaning. New York: Dover Publications, 1963. Ridpath, Ian. Star Tales. New York: Universe Books, 1988. See also http://www.ianridpath.com/startales/contents.htm. Staal, Julius D. W. The New Patterns in the Sky: Myths and Legends of the Stars. Blacksburg, VA: McDonald and Woodward, 1988. Stellar magnitudes and spectra Stars, http://stars.astro.illinois.edu/sow/ sowlist.html. Hirshfeld, Alan, Roger W. Sinnott, and Francois Ochsenbein, eds. Sky Catalogue 2000.0, vol. 1, 2nd ed. Cambridge: Cambridge University Press, and Cambridge, MA: Sky Publishing Corp., 1991. Stellar data Stars, http://stars.astro.illinois.edu/sow/ sowlist.html. ESA. The Hipparcos and Tycho Catalogues. Noordwijk: European Space Agency, 1997. Double stars Luginbuhl, Christian B., and Brian A. Skiff. Observing Handbook and Catalogue of Deep-Sky Objects, Cambridge: Cambridge University Press, 1989. USNO. The Washington Double Star Catalog. Washington, DC: Astrometry Department, U.S. Naval Observatory. http://ad.usno. navy.mil/ad/wds/wds.html.

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Variable stars American Association of Variable Star Observers, http://www.aavso.org/. Open star clusters Archinal, Brent A., and Steven J. Hynes. Star Clusters. Richmond, VA: Willmann-Bell, Inc., 2000. Open star cluster distances generally were gleaned from the professional literature. Globular star clusters Harris, William E. Catalog of Parameters for Milky Way Globular Clusters. Hamilton: McMaster University. http://www.physics. mcmaster.ca/~harris/mwgc.dat. Skiff, Brian A. “Observational Data for Galactic Globular Clusters.” Webb Society Quarterly Journal 99:7 (1995), updated May 2, 1999. Globular star cluster distances generally were obtained from the professional literature. Planetary nebulae Skiff, Brian A. “Precise Positions for the NGC/IC Planetary Nebulae.” Webb Society Quarterly Journal 105:15 (1996). (Positions.) Luginbuhl, Christian B. and Brian A. Skiff. Observing Handbook and Catalogue of Deep-Sky Objects. (Dimensions and ­central star magnitudes.) Cragin, Murray, James Lucyk, and Barry Rappaport. The Deep-Sky Field Guide to Uranometria 2000.0. Richmond, VA: Willmann-Bell, Inc., 1993. Planetary nebula distances generally were taken from the professional literature or from the Web page of the Space Telescope Science Institute (www.stsci.edu).

About this book

Diffuse nebulae Cragin, Murray, James Lucyk, and Barry Rappaport. The Deep-Sky Field Guide to Uranometria 2000.0. Diffuse nebula distances were gleaned from the professional literature. Galaxies The Deep-Sky Field Guide to Uranometria 2000.0. (Position, angular size, apparent magnitude, and surface brightness.) NASA. The Extragalactic Database. Pasadena, CA: Infrared Processing and Analysis Center, http://nedwww. ipac.caltech.edu/. (Types, mean distance, radial velocity, and all detailed ­descriptions of galaxy structures as seen in photographs have been taken from the accompanying notes.) Tully, R. Brent. Nearby Galaxies Catalog. Cambridge: Cambridge University Press, 1988. (Inclination, total mass, and total luminosity.) Some galaxy distances were obtained from the professional literature. Extragalactic supernovae “List of Supernovae.” Cambridge, MA: Central Bureau for Astronomical Telegrams. http://cfa-www.harvard.edu/ iau/lists/Supernovae.html. Other details of discovery were taken from individual IAU Circulars. Historical objects Smyth, Captain William Henry. A Cycle of Celestial Objects. Richmond, VA: Willmann-Bell, Inc., 1986. Glyn Jones, Kenneth. The Search for the Nebulae. Chalfont St. Giles: Alpha Academic, 1975.

About this book

Other historical anecdotes in this book were gleaned from various individual and professional papers from the nineteenth and early twentieth centuries. General notes Note that the Web Uniform Reference Locators, or URLs, are subject to change. The dimensions, magnitudes, and positions of all other additional deep-sky objects in this book were taken from The Deep-Sky Field Guide to Uranometria 2000.0. This book contains the most up-to-date astronomical data and more accurate historical and observational information about each object in the Southern Gems catalogue than you’ll find in any other book in the popular literature. The Finder Charts In each object essay in Chapter 2, I suggest a fast and efficient way to find a Southern Gem object. First I direct you to one of the seven wide-field finder charts that appear at the end of this book. The charts are of my own design and are simply intended to guide you to the general region that includes your target. Each wide-field chart shows the positions of numerous Southern Gem objects within a few hours of right ascension between  –10º and  –70º (at times, a bit farther south). It also shows the brightest stars in each constellation. For instance, wide-field chart 7 shows the Southern Gem objects visible within a few hours of right ascension high above the southern horizon around December 1 at 9 p.m., November 1 at 11 p.m., October 1 at 1 a.m., and September 1 at 3 a.m. The cross near the map’s center marks the point directly overhead from a latitude of –45º south.

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About this book

Each chart shows stars roughly to magnitude 4 or 5 but generally only in the region near the Southern Gem object, and only if I feel they will help in the naked-eye or binocular search. To help you in your search, I’ve traced out the “stick figure” forms of the main constellations. I’ve also labeled them and their brightest stars using the traditional Bayer (Greek) letters or Flamsteed numbers. Sometimes I’ve included a popular star name, such as Achernar (Alpha [α] Eridani). In special cases, you’ll find a nontraditional, italicized, lowercase letter, such as a or b. These are additional unnamed or numbered guide stars, which you’ll find in the text described as Star a or Star b, for example. One symbol, a circle, is used to mark the location of each Southern Gem object on the wide-field charts. Where there are several objects in a small region of sky, I use a line to point out the target(s). I then ask you to find the brightest star near the object, from which you can start your search using the detailed star chart accompanying the object’s text. (Both the wide-field finder charts and the detailed finder charts are oriented with north up and east to the left.) Once you locate that bright star in the sky, I ask you to switch to the detailed part

About this book

of the text that describes how to locate the object and simply follow the directions. In creating these detailed charts, my philosophy was to simplify the view to help you focus on your target by removing peripheral “noise.” The purpose of these charts is to help you hone in on your target, and I’ve done the fieldwork to help you get there the fastest and most efficient way possible. The detailed finder charts have the same orientation as

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the wide-field charts, but they show a much smaller area of sky in more detail. The constellation name is given. A scale bar appears at the bottom of each chart. Stars near the target are shown to magnitude 10 or 11. I’ve labeled each Southern Gem target with its full proper name (such as NGC 55) in black. A faint gray symbol is used to mark the location of other interesting deep-sky objects nearby, which are labeled (also in gray) with the NGC prefix omitted. Note, too, that on these charts the italicized letters may also refer to an asterism (a triangle, arc, pair of stars, or line), as described in the text. I use the traditional symbols that follow to represent the different classes of deep-sky objects. In the case of galaxies, I also show their apparent orientations. Again, to find a Southern Gem object, first locate the object’s position on the wide-field finder chart. Next, note the brightest star near the object and locate it on the detailed finder chart. Then read the accompanying text on how to locate the object, and simply follow the directions. Note that you don’t have to use the charts in this book. Because I provide the object’s equinox 2000.0 coordinates, you can instead employ your favorite detailed sky atlas (such

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as Wil Tirion’s Sky Atlas 2000.0) to locate the object. The choice is yours. The charts I created have a simple appearance. I’ve tried not to clutter them up with unnecessary details. They are designed to help you make the simplest and fastest sweep to each object, based on my personal experience. You might think otherwise, and I encourage you to pursue whatever venue you find suitable to your needs. The Images All the object images in this book are reproduced in black and white, with north up and east to the left. All are 15×15 arcminutes unless otherwise noted. If the images are larger, a scale bar accompanies the photo. All the main object images are from the Digitized Sky Survey and are reproduced here with permission, except for that of Barnard 283 (Southern Gem 97), which I took. Some object essays also include images from the Hubble Space Telescope and other large telescopes. These images were used for the sole purpose of inspiring you to use your imagination. You certainly will not see anything like these images when you look through your telescope, but how else can you fully appreciate what it is you are seeing? So do not be discouraged, be enlightened.

About this book

The Drawings All the sketches in Chapter 2 are composites of field drawings I made at various magnifications. They are shown with north up and east to the left. The orientation matches that of the corresponding photograph. Each Southern Gem object in the drawing has a scale bar to help you size up each object in your own telescope.

About this book

The composite drawings (such as that of the dim globular cluster NGC 6362 [Southern Gem 91]) show details visible at low, medium, and high powers. This is a technique I’ve employed for decades and that enables me to share with you the overall grandeur of the object in one portrait. It’s important to note, however, that I’ve boosted the contrast greatly for two reasons: (1) to hold the detail for reproduction and (2) to enhance the beauty of the subtle view. If I were to try and reproduce the delicate details of some deep-sky objects, you probably wouldn’t recognize them, or see them for that matter, in the drawings because they would be too faint. In the text, I do break down what details I could see (and could not see) at different magnifications, so use the verbal description as your explanatory guide. The time has come now for us to begin our adventure, to survey some of the wonders catalogued by one of the pioneering deep-sky explorers in the Southern Hemisphere: James Dunlop, the “gentleman astronomer” and the Messier of the southern skies.

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Chapter 2

Southern Gems

1 1 String of Pearls NGC 55 Type: Barred Spiral Galaxy (SBm) Con: Sculptor RA: 00h14.9m Dec: −39°11′ Mag: 8.1; 7.5 (O’Meara) Dim: 30.0′ × 6.3′ SB: 13.6 Dist: ~6.2 million light-years Disc: James Dunlop, 1826 james dunlop [July 7, 1826]: A beautiful long nebula, about 25[′] in length; position [northwest] and [southeast], a little brighter towards the middle, but extremely faint and diluted to the extremities. I see several minute points or stars in it, as it were through the nebula: the nebulous matter of the south extremity is extremely rare, and of a delicate bluish hue. This is a beautiful object. (D 507) john herschel : Bright; very large; very much elongated in a long irregular train, the preceding end being much the brightest. Whole length = 1.5 diameters of the field, or 22′. The nucleus is either a double star or a much more sharply terminated nebulous mass, elongated in a different position (146.5[°]) from that of the nebula (109.8[°]). (h 2315) NGC: Very bright, very large, very much extended, triple nucleus.

N G C 55 is one of t he ni ght sk y’s finest telescopic wonders. It shines two magnitudes brighter than the sky’s most famous edge-on spiral, NGC 4565 in Coma Berenices, and its form is visible in 10 × 50 binoculars.

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NGC 55 is high on my list of galaxies that show inordinate amounts of detail in small telescopes. James Dunlop discovered this magnificent system in July 1826 and catalogued it

Deep-Sky Companions

1 as the 507th object in his catalogue of nebulae and clusters. Through his 9-inch reflector, he found it a “beautiful object,” though “extremely faint and diluted to the extremities,” with several faint “points or stars.” John Herschel first observed NGC 55 on May 3, 1834, seeing it as “very much elongated in a long irregular train, the preceding end being much the brightest.” A year later, he called it “a strange object.” But his October 4, 1836, log entry truly captures the view: “a very long irregular crooked ray with 3 nuclei, the second of which appears to consist of stars.” In modern images, NGC 55 appears most unusual. A dense, dusty lens of light dominates the western end of its 1/2º-long spindle. A lagoon of dark material separates this mass from its faint, splotchy eastern extension, which has a separate IC catalogue number. Apparently, this dark lagoon caused some problems years ago, as Harold G. Corwin (Infrared Processing and Analysis Center) explains on the NGC/IC Project Web page: IC 1537 is the east-southeast[ern] arm of NGC 55. It was first seen, described, and sketched by James Dunlop in the 1820s. [John Herschel] provided a more detailed description and sketch a decade later. Both clearly noted that the [east-southeastern] end of the nebula was much fainter than the [north-northwestern], and their estimated sizes (note the typo in Dunlop’s paper: in place of 25 arcsec, read 25 arcmin) include the whole galaxy, not just the brighter portion. Furthermore, the fainter following part is clearly shown in both published sketches. In spite of these published observations, Lewis Swift claimed this part of [NGC 55] as his own discovery: “[That] Sir John Herschel does not mark it [NGC 55] with a sign as being a remarkable object lends plausibility to the idea that it [IC 1537] was not seen even by him.” And this after implying that Dunlop had certainly not seen the fainter eastern end.

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We see NGC 55 perfectly edge-on, meaning it’s inclined 90º from face-on. This late-type barred spiral belongs to the Sculptor (or South Polar) group of galaxies – along with NGC 300 (Southern Gem 4), NGC 247 in Cetus, and NGC 253 in Sculptor. If we accept a mean distance of ~6.2 million light-years, the galaxy has a true linear diameter of some 55,000 light-years, so it is some 30 percent smaller than NGC 253, the Great Sculptor Galaxy, which is about 10.4 million light-years distant. Thus, the two galaxies have nearly the same apparent length. But as Griet C. van de Steene (Royal Observatory of Belgium) and colleagues describe in a 2006 paper in Astronomy and Astrophysics (vol. 455, pp. 891–896), their study of the planetary nebula continuum in NGC 55, obtained with the Wide Field Instrument at the European Southern Observatory’s 2.2-meter telescope, led them to determine a likely distance to the galaxy of about 17 million light-years. If this distance is true, they say, then the Sculptor Group is farther away from the Local Group than previously thought. They also note that their planetary nebula luminosity function distance to NGC 55 is comparable to that of NGC 300 (Southern Gem 4), supporting the suggestion that these galaxies form a bound pair. In deep images, NGC 55 is replete with ionized gas features, such as giant H ii complexes, supergiant filaments, and very faint diffuse emission nebulae. Many of these features protrude well above the plane of the galaxy, as does one very faint fragmented shell extending 8,500 light-years above the disk. All these features are believed to be “chimneys” where hot gas in star-forming regions in the disk is allowed to funnel into the halo. Several of the identified chimneys are “capped” with clumps of ionized gas, one of which, located

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1 nearly 5,000 light-years above the plane of the disk, appears to be the site of recent star formation. The ROSAT satellite found extended x-ray emission along the optical bar as well as 22 point-like sources, seven of which probably belong to NGC 55. None of the brightest sources appear to be variable. As with NGC 300, deep infrared images confirmed that its disk contains an underlying old population of stars. Shell structures have also been observed, and the corresponding velocities support the idea of diffuse gas being pushed into the galaxy’s halo by supernova explosions and stellar winds. None of this, of course, can be seen with the eye. Although NGC 55 belongs to Sculptor the Sculptor’s Workshop, it touches the constellation’s southern border, just “across the tracks” from Phoenix the Peacock. In fact, the best way to find NGC 55 is to use wide-field chart 1 or chart 7 to locate 2.4-magnitude Alpha (α) Phoenicis (Ankaa), which lies roughly midway along (and a little east of ) an imaginary line between Alpha Piscis Austrini (Fomalhaut) and 1st-magnitude Alpha Eridani (Achernar). Ankaa is the brightest star in that region; confirm it by looking for 4th-magnitude Kappa (κ) Phoenicis, nearly 1 1/2° to its south. The galaxy lies nearly 4° to Ankaa’s northwest. If you’re under dark skies, you might want to try and locate it first with binoculars. Otherwise, to starhop to it with your telescope, begin by centering Ankaa in your scope

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at low power and then refer to the accompanying chart. From Ankaa, move a little more than 1 1/2° northeast to 6th-magnitude Star a and then hop about 1° north-northwest to 5.5-magnitude Star b. Next, look about 1 1/2° northwest of Star b for a nearly 1°-long chain (c) of three roughly 7th-magnitude stars ­(oriented east to west). NGC 55 is a little more than 1° due west of the westernmost star in Chain c. Lists often refer to NGC 55’s photographic blue magnitude (7.9), which also matches a 1991 photometric visual-band (V-band) estimate by the late Gérard de Vaucouleurs and his collaborators. My binocular estimate places the galaxy’s visual magnitude a bit higher, at 7.5. Through 10 × 50 binoculars, I could see its spindle oriented more or less in the same direction as the stars in Chain c to its east. I really needed averted vision to make

Deep-Sky Companions

1 out the spindle in binoculars. With a semidirect gaze, the galaxy appears to have a slightly elliptical center with a hint of tapered edges. At 23×, through the 4-inch, NGC 55 looked like a beautiful comet, with a graceful dust tail extending to the east-southeast and a small antitail to the west-northwest. The view became three-dimensional when I imagined it as a comet racing toward me at a slightly inclined angle. Make sure you spend time with this galaxy, allowing at least a few minutes for your eye to adapt to the scene. The longer you look at it, the more you should see. I found that, when I relaxed, the nuclear region gradually began to appear more and more irregular, as faint stars appeared along its length. It wasn’t hard to imagine it now as a “squashed comet”  – like the one Carolyn Shoemaker discovered on photographic plates taken by her late husband Eugene and by David Levy. Carolyn called that a “string of pearls comet” after seeing images of it taken with the Spacewatch telescope. That comet, D/1993 F2 (Shoemaker-Levy 9), ultimately smashed into Jupiter. As I saw it, NGC 55 was an extragalactic string of pearls. Curiously, with averted

Southern Gems

vision, its eastern segment (IC 1537) seemed to pulsate with dim glittering lights. The effect was most dramatic when I looked toward the nucleus at 23× but kept my attention on IC 1537. It was like a flashing signal reminding me to use higher magnification. At medium magnification, NGC 55 was stunning! Ribbons of dark material laced the entire nuclear region; the comet had been snared in a black net. The scene recalled M82, the fabulous starburst galaxy in Ursa Major, which has shock waves of dark matter rifting through it. With concentration, the core of NGC 55 looked beaded with fuzzy clumps and stars. (The galaxy’s eastern segment was also highly fractured and dappled with clusters of faint stars.) One phenomenal rift of darkness sliced through the eastern end of the central bulge before it curled around the dense core like warped metal. It was as if tiny explosions had blown apart the hull of this extragalactic traveler, which was tipping to the northwest and sinking into a black sea; any turbulence in our atmosphere might cause the galaxy’s dimmest parts to ripple, as if waves of darkness were lapping against this “ship’s” hull. This ethereal beauty recalled the words of Strepsiades in Aristophanes’ The Clouds: “the impetuous tempests … with aerial wings, loaded with mists.” Despite NGC 55’s modest overall surface brightness, its bright nuclear region took high magnification well. In fact, I thought the nuclear region looked best at high power. Here I saw dark rifts weaving through the superimposed stars and knots like little rivulets of water covered with black ice flowing through banks of snow. NGC 55 is without a doubt one of the most spectacular galaxies available to amateurs.

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2 2 47 Tucanae; Xi (j) Tucanae NGC 104 Type: Globular Cluster Con: Tucana RA: 00h24.1m Dec: −72°05′ Mag: 3.95 Diam: 50′ SB: 12.5 Dist: ~14,700 light-years d i s c: Abbé Nicolas Louis de Lacaille, included in his 1755 catalogue, though probably noted throughout history as a star. j ames dunl o p: (47 Toucan, Bode.) This is a beautiful large round nebula, about 8′ diameter, very gradually condensed to the centre. This beautiful globe of light is easily resolvable into stars of a dusky colour. The compression to the centre is very great, and the stars are considerably scattered [southwest to northeast]. (D 18) j o hn hers che l : The great cluster preceding the Nubecula Minor. Estimated diameter of the denser portion 5′; of the whole (not, however, including loose stragglers) 8′. Stars from magnitude 14 to 16, and one of 12th magnitude [northwest] of the center. Excessively compressed. (h 2322) NGC: Globular cluster of stars, remarkable, very bright, very large, very much compressed in the middle.

O u r n e x t S ou t he rn Ge m l i e s wi t hi n the celestial boundaries of one of the sky’s most “flighty” constellations: Tucana the Toucan. With an area of 295 square degrees,

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Tucana is the sky’s 48th-largest constellation; it contains some 80 stars between magnitudes 2.9 and 7. Johann Bayer first introduced this colorful bird simply as “Toucan” in his 1603

Deep-Sky Companions

2 Uranometria; the name was later latinized. The Toucan flies around the South Celestial Pole with three of the heavens’ greatest wonders dangling from its claws: the Small Magellanic Cloud (a dwarf galaxy that orbits our Milky Way) and the globular star clusters NGC 362 (Southern Gem 5) and 47 Tucanae (our target), which lie to the north and west, respectively, of the Small Magellanic Cloud. Abbé Nicolas Louis de Lacaille swept up this spectacular object and kicked off his 1755 catalogue with it. After seeing it through a 1/2-inch 8× telescope, he said 47 Tucanae was “like the nucleus of a fairly bright comet.” (Ironically, he placed it in his Class I, “nebulae without stars.”) But 47 Tucanae does have a history that predates Lacaille’s telescopic observation. The cluster appears as a “star” just west of Nubecula Minor (the Small Magellanic Cloud) in Bayer’s Uranometria, which was first published in 1603. Bayer never did see that “star” himself, and his information on far-southern stars came not from astronomers but from explorers who journeyed south of the equator. Indeed, the first catalogues of southern stars were those of the Dutch navigators Pieter Dirckszoon Keyser (published around 1595 and now missing) and Frederick de Houtman (published in 1603). Had these explorers possessed a keener understanding of the heavens, they undoubtedly would have noticed that the “star” just west of the Small Magellanic Cloud appears slightly swollen to the attentive eye. (I have seen it this way from Africa, with the “star” only 10° above the horizon, and I’m certain many aboriginal skywatchers would have done the same.) Astronomers did not study the southern sky in any detail until a 20-year-old Edmond Halley went to the island of St. Helena in the South Atlantic in 1676 with the sole purpose

Southern Gems

of cataloguing the southern stars. Halley, who had “poor eyesight,” spent 18 months on that remote island, plagued by inclement weather, an arrogant governor, and harsh living conditions. In the end, Halley catalogued only 341 stars. He did discover the diffuse Omega Centauri, however, and given more clear nights he might have noticed the fuzzy nature of 47 Tucanae before Lacaille did. In the nineteenth century, Benjamin Apthorp Gould (1824–1896), the founder and first director of Cordoba Observatory in Argentina, bestowed the Greek letter Xi (ξ) on the cluster in his 1879 Uranometria Argentina. This moniker was not universally adopted, though  – which is unfortunate because “Xi Tucanae” would complement “Omega Centauri” nicely. To this day, Tucana does not have a star labeled Xi. The name 47 Tucanae refers to the Flamsteed number of the “star,” and NGC 104 is the only nonstellar naked-eye object to bear such a number. It’s a shame Lacaille didn’t spy 47 Tucanae with an instrument larger than his 1/2-inch 8× telescope. What a missed opportunity to look into the eye of God. Interestingly, James Dunlop included it as the 18th object in his 1828 catalogue. Dunlop’s drawing of it follows on page 18: Through his 18-inch speculum-mirror telescope, John Herschel estimated the diameter of the cluster’s denser portion as 5′ and called it “excessively compressed.” Herschel also saw the innermost stars shining with a rose-colored tint, while the outliers were pure white. Agnes Clerke in The Sphere of the Stars (1905) deemed Herschel’s color perception rather subjective. “To the present writer, in 1888,” she penned, “the sheeny radiance of this exquisite object appeared of uniform quality from centre to circumference. . . .

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Perhaps no other cluster exhibits an equal degree of compression. . . . [I]t was, indeed, for several nights after his arrival in Peru, mistaken by [Alexander von] Humboldt for a comet.” Deep images of dazzling 47 Tucanae reveal several million stars – as many as some minor galaxies – across its 50′-wide disk. (Of course, this begs the question: Are all globular clusters the product of merging minor galaxies?) Recent proper-motion studies of 47 Tucanae’s stars confirm that they belong to our Milky Way system and are merely projected against the Small Magellanic Cloud’s halo some 176,000 light-years away. By comparison, 47 Tucanae lies only about ~14,700 light-years distant, making it one of the closest globular star clusters to the Sun; it is only about 350 light-years farther from the Sun than globular cluster M10 in Ophiuchus, though M10 is

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about 9,000 light-years closer to the Galactic center. Next to Omega Centauri (Southern Gem 62), 47 Tucanae is the brightest globular star cluster in the heavens. But Omega Centauri tops 47 Tucanae in brightness by only 0.3 magnitude. The magnificent 47 Tucanae is also 1.15 magnitudes brighter than the brightest Messier globular, M22 in Sagittarius, and its diameter is about 25 percent larger than M22’s. The cluster’s true physical extent is 214 light-years, and its apparent diameter is some 60 percent larger than the full Moon’s. It’s a relatively metal-rich cluster, with 1/6 as much metal in each of its stars (on average) as the Sun. Its integrated spectral type is G4. The cluster is also receding from us at a paltry 19 kilometers per second. The accompanying Hubble Space Tele­ scope (HST) image shows but a portion of the many star fields in this magnificent cluster, one of the densest in the southern sky. Globular clusters harbor some of the oldest stars known in our galaxy. But when the Hubble Space Telescope peered into the heart of 47 Tucanae, it revealed that the cluster’s entire core is crowded with mysterious “blue stragglers” – stars that are bluer and brighter than the other cluster members, as if somehow they were “born” more recently than their siblings. Blue stragglers were discovered about half a century ago and were enigmatic until recently. However, in recent years, astrophysicists have become convinced that these stellar nonconformists are formed either

Deep-Sky Companions

2 when the stars in a double star system slowly merge or when two unrelated stars collide. Data from HST’s Faint Object Spectrograph established the temperature, size, and rotation rate of one massive blue straggler (BSS-19) in 47 Tucanae. And, for BSS-19 at least, astronomers favor the slow merger scenario. BSS-19 has a mass of 1.7 solar masses; it is the first blue straggler in a globular cluster whose mass has been measured directly. The cluster’s crowded central region also contains rapidly spinning pulsars, discovered recently by radio astronomers. In 2011, at the 217th American Astronomical So­­ cie­ty meeting in Seattle, Washington, Jason S. Kalirai (Lick Observatory) and his colleagues reported that they used the HST to create a high-resolution ultraviolet and near-infrared image of the complete stellar populations of 47 Tucanae over a 180° wide-field view (covering a true physical extent > 65 light-years). “This complete stellar picture of a globular cluster has revealed several new features of the color–magnitude diagram and represents a comprehensive data base to test stellar evolution models in exquisite detail,” they say. “In addition to the primary focus of studying 47 Tuc, our imaging penetrates through the star cluster to reveal a rich population of background giants and low mass dwarfs belonging to the distant Small Magellanic Cloud.” In a 2011 paper in Astrophysical Journal Supplement (vol. 193, p. 23), Iain McDonald (Jodrell Bank Centre for Astrophysics,

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Manchester, UK) and colleagues determined an age of about 12 billion years for the cluster. Interestingly, it’s possible that the cluster we see today is actually the result of a merger. As Richard R. Lane (Sydney Institute for Astronomy, New South Wales) and colleagues discuss in a 2010 issue of Astrophysical Journal Letters (vol. 711, pp. 122–126), “The globular cluster 47 Tucanae (47 Tuc) is well studied but it has many characteristics that are unexplained, including . . . the exciting possibility of two distinct kinematic populations [of stars].” After exploring the evaporation of low-mass stars, a hierarchical merger, extant remnants of two initially segregated populations, and multiple star-formation epochs, the researchers found that the “most compelling explanation” for this result is that “47 Tuc formed as two separate populations arising from the same proto-cluster cloud

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2 which merged 104 cm-3) in the nebula with significant amounts of associated molecular gas, which is in the form of clumpy ejecta and/or interstellar material,” Marston reported. Marston added that early suggestions of the nebula forming via a bow shock appear unlikely because “Hipparcos measurements show the proper motion of WR 18 is almost at right angles to the direction required for the bow shock model.” Thus, the material surrounding the star is not uniform but clumped and denser near the bright edge of windblown NGC 3199. While the northern area of the optical nebula suggests that a possible blowout of the Wolf-Rayet wind through surrounding ejecta may be responsible for some of the velocity features observed, Marston’s findings also

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suggest that the brightening of the nebula’s southeast rim was produced by the motion of the star in that direction. But this is now known not to be the case. To find this amazing nebula, use widefield chart 4 to find 4th-magnitude s Carinae, which lies about 2 1/4° west-northwest of the center of the Eta (η) Carinae nebula (Southern Gem 48). Center that star in your telescope at low power and then switch to the accompanying chart.

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From s Carinae, move about 25′ west-northwest to 6th-magnitude Star a and then move 30′ northwest to similarly bright Star b, which has a little triad of faint stars around it. NGC 3199 is just about 45′ farther to the west-northwest. Through the 5-inch at 33×, the nebula appears as an irregular patch of light about 10′ in extent, oriented northwest to southeast, in a field dappled with stellar groupings. With averted vision, the glow breaks up into three ghostly patches along a broken crescent. The patches appear more distinct at 60×, but they do not take further magnification well. If you’re using a small telescope like I am, be patient and let your eye become accustomed to the field. The nebula’s bifurcated middle region has the greatest intensity and seems elongated along a roughly north-south line. At no time did I suspect that the patches were connected. On occasion, I suspected an arc of nebulosity extending about 10′ eastward from the northern patch and one about 15′ long stretching to the east from the southern patch. While these features appear in photographs, I find them to be either at the limit of vision for a 5-inch or as star chains that look fuzzy. I can’t be sure. The late Ernst J. Hartung found NGC 3199 in a 6-inch “faintly but definitely.” In his book Astronomical Objects for Southern Telescopes, he describes the view through his 12-inch reflector as a “diffuse fairly bright crescent,” convex in shape with a well-defined dark bay to the northeast. He,

Deep-Sky Companions

44 too, noted the rich star field, “sown with small pairs and triplets in a striking manner.” And Magda Streicher notes that through a 12-inch the dark region east of the nebula

Southern Gems

has an “island of very faint stars. The double star is yellow in colour.” I wonder how large an instrument will show the complete ring or bubble.

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45 45 NGC 3201 Type: Globular Cluster Con: Vela RA: 10h17.6m Dec: −46°25′ Mag: 6.7 Diam: 20′ SB: 13.5 Dist: ~17,000 light-years Disc: James Dunlop, 1826 j ames dunl o p [May 1, 1826]: A pretty large, pretty bright round nebula, 4′ or 5′ diameter, very gradually condensed towards the centre, easily resolved into stars; the figure is rather irregular, and the stars are considerably scattered on the [southwestern] side: the stars are also of slightly mixed magnitudes. (D 445) j o hn hers che l : Globular cluster, irregularly round, gradually brighter in the middle, not very much compressed, 6′, resolved into stars of magnitude 13 to 15. (h 3238). ng c: Globular cluster, very large, irregularly round, little compressed in the middle, stars of magnitude 13 to 16.

Th e re i s no u p or d own i n space . The celestial equivalents of north, south, east, and west are useful conventions that help us to see the gears of our clockwork universe. Adopting these conventions has enabled astronomers to find some patterns in the cosmic scheme of things. For instance, most planets spin on their axes from west to east. But Venus has a “retrograde” (backward) rotation, as it spins from east to west. If we look “down” at the 174

Solar System with the Sun’s north pole pointing “up” at us, we can see that the planets (and many minor ones) orbit the Sun in a counterclockwise direction, while some minor bodies move around the Sun in retrograde (clockwise) orbits. Likewise, the satellite Triton travels “backward” around its parent planet Neptune. Retrograde renegades appear to be a part, albeit a small part, of the Solar System. But what about the Milky Way Galaxy? It, too,

Deep-Sky Companions

45 has a disk whose constituents (mostly stars) orbit a shared center in the same direction, like most skaters in an ice rink. Do we see any renegade objects going against the flow in the Milky Way? The answer is yes, and one of the most remarkable objects is our target: the globular cluster NGC 3201. NGC 3201 is a roughly 16-billion-year-old globular residing in the halo of the Galaxy about 17,000 light-years from the Sun and 30,000 light-years from the Galactic center. The cluster has an exceedingly fast radial (line-of-sight) velocity, approaching us at 490 kilometers per second  – the greatest value, in an absolute sense, evinced by any known cluster. If you remove the effect of the Sun’s motion through the Galaxy, what remains is an excess velocity of about 240 kilometers per second. This isn’t fast enough for the globular to escape the intense bond of our galaxy’s gravity. But it does move the cluster through space in a direction opposite that of the general direction of the Galaxy’s disk. Yet, just as a comet’s retrograde orbit doesn’t change its physical characteristics, NGC 3201 remains a bona fide globular cluster. As Guillermo Gonzalez (University of Washington) reported in a 1998 paper in Astrophysical Journal (vol. 116, p. 765): “Although NGC 3201 has an unusual Galactic orbit, we find no new evidence from our analysis that it is a captured cluster.” He and George Wallerstein also note in Bulletin of the American Astronomical Society (vol. 30,

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p. 933, 1998) that “the composition of the stars in NGC 3201 is similar to that of typical halo globulars despite its unusual Galactic orbit.” Typically a globular star cluster spends most of its time moving slowly through the halo, at the outer extreme of its orbit, like a comet orbiting the Sun; only briefly does it whip in and buzz the Galaxy’s core. It’s also possible that it may have been part of an ingested dwarf galaxy; some modern theories suggest that the Galaxy’s entire halo was built up of such remnants. To find NGC 3201, use wide-field chart 4 to locate the enormous naked-eye nebula Eta (η) Carinae halfway between the False Cross and the Southern Cross. The bright star due north of Eta Carinae by 10° (a fist held at arm’s length) is the magnitude 2.7 star Mu (μ) Velorum. Now look about 2° northwest for 4th-magnitude p Velorum. You want to use

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45 binoculars or your unaided eyes to scout out 5th-magnitude Star a, nearly 3° to the westnorthwest, and then switch to the chart on page 175. Star a is easy to identify through a telescope because it has a magnitude 6.5 companion immediately to the northeast. From Star a, move 1° north-northwest to equally bright Star b. NGC 3201 is just about 25′ north-northeast of Star b. From Hawaii, I find the cluster’s magnitude 6.7 haze just at the limit of naked-eye visibility. The challenge is to separate the cluster from its rival (Star b) with averted vision; the separation is 10 times greater than the separation between the components of the naked-eye double star Epsilon (ε) Lyrae. The difficulty is seeing the globular, whose light is spread across a much larger area (20′), though you’ll be looking for the light from its innermost region, just a few arcminutes across. Plan on spending some time looking. You must be comfortable and relaxed, with no neck strain. Be sure to breathe and take a series of quick looks rather than one prolonged stare – that’ll just “drain” your “eye batteries.” I found that seeing this object was a bit more difficult than seeing the similarly bright and large globular cluster M2 with the naked eye. If you fail, try again on another night. Through 7 × 35 binoculars, the cluster is a cinch, and at 23× in the 4-inch it is a fabulous globular with an intense core surrounded by a “wobbly” envelope of stars – the direction and extent of the envelope varying depending on how you hold your head or use your averted vision. This is probably what Herschel meant when he called it “irregularly round.” With averted vision, the halo is definitely mottled, and some of its outliers can be resolved at low power. The brightest members of the cluster shine at magnitude

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11.7, and its horizontal-branch magnitude of 14.8 means that anyone with an 8-inch Schmidt-Cassegrain should have no problem looking deep into the cluster’s maze of stars. Even in the 4-inch at low power, I can make out hints of “spiral” structure and an arc of detached stars to the west. But before moving on to higher powers, sweep the general area of the cluster at 23× and see if you don’t encounter a copse of double stars, many of which are so tight that they appear fuzzy. Many sources complain that the suggested size of the cluster is not realistic, but I disagree; when viewed under dark skies with low power, the cluster’s full 20′ can be visualized, which in true physical extent translates into nearly 100 light-years. At 72×, much of the cluster is immediately resolved. Most fantastic is a bright “V” of stars (like a drawing compass) whose pointed end starts at the northeast side of the main “cloud” of stars and gradually opens up to the southwest. The stars are remarkably obvious, and most are of similar brightness. It is one of the cleanest visual “structures” I’ve ever seen in a globular. A pompadour of dim stars curls off the main cloud to the northwest, and a tail of starlight sweeps from the south side to an

Deep-Sky Companions

45 outer halo to the northwest; it ends in a prominent 12th-magnitude double. The eastern section of the cluster is broken by what appears to be a large lane of darkness (faint stars?) before a wedge of starlight continues to the east. The longer I look, the more complicated the view becomes. As my eyes become accustomed to the view, all manner of star strings, dark halos, faint extensions, and tangles of “interacting” starlight take shape and follow seemingly random and haphazard paths. The cluster becomes even more dramatic at high power, when swarms of 13th-magnitude stars sparkle forth from the background haze. A series of at least three dark patches can be seen to the east, each separated by a narrow bar of starlight. A dark patch also spots the northwest section of the inner core.

Southern Gems

Well-resolved globulars can be visually maddening. The eye-brain combination tends to work overtime trying to connect streams of starlight that come and go with the seeing. It’s like watching a snowfall through heat rising in waves before a window. When I get overwhelmed in this way, I always gently place the cluster out of focus and look for the strongest concentrations of starlight. Doing this also helps the dark areas to blend or to stand out more clearly from the stellar quagmire. If you return to low power and look 1 1/2° to the south of NGC 3201, you’ll see a red ember glowing just west of a gentle curve of three northeast to south trending stars. That’s the light of the M3 semiregular variable WZ Velorum, whose brightness oscillates between photographic magnitudes 9.0 and 10.0 every 130 days.

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46 46 Spider Spit Cluster, Little Jewel Box NGC 3293 Type: Open Cluster Con: Carina RA: 10h35.8m Dec: −58°13′ Mag: 4.6 (O’Meara); 4.7 Diam: 5′ SB: 8.2 Dist: ~8,000 light-years Disc: Abbé Nicolas Louis de Lacaille, listed in his 1755 catalogue abbé ni co l as l ou is de l acail l e : Small heap of four small stars in a lozenge. (II-8) j ames dunl o p: A very small cluster of very small bright stars; round figure, about 4′ diameter; rich in extremely small stars resembling faint nebula. (D 321) j o hn hers che l : A fine bright rich not very large cluster. (h 3276) ng c: Cluster, bright, rich in stars, pretty large.

If there is one object in the sky more deserving of the Jewel Box moniker than Kappa (κ) Crucis (NGC 4755), it is open cluster NGC 3293 in Carina. This 4th-magnitude treasure chest packs 93 stars 8th magnitude and fainter in an area only 5′ wide – almost half the apparent diameter of the Trapezium in the great Orion Nebula (M42). And while the Jewel Box in Crux is twice as large as our target and contains three times as many members, its stars do not punch the eye the way those bright and

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tightly packed ones in NGC 3293 do, especially in small telescopes. Let’s face it, part of the Jewel Box’s attraction is its location. It shines like a diamond ring at the end of the eastern arm of the famous Southern Cross, and its beauty helps to illuminate the bleak shores of that Black Sea of nebulosity to its southeast known as the Coalsack Nebula. NGC 3293, on the other hand, suffers the misfortune of lying only about 2° north-northwest of the core of the Eta (η) Carinae complex, a vast and alluring

Deep-Sky Companions

46 network of glowing gas, dark dust, and hot O- and B-type stars, many of which lie in clusters. Not only is NGC 3293 some 24 times smaller than the Eta Carinae Nebula and its associated starlight, but it is also surrounded by more than a dozen other open clusters in Carina, including 2nd-magnitude IC 2391 (the Southern Pleiades), 3rd-magnitude NGC 3532 (the Pin Cushion Cluster [Southern Gem 49]), and 4th-magnitude NGC 3114 (Southern Gem 42) – all dynamic objects, all vying for one’s attention, and all deserving of one’s time. Yet NGC 3293 is arguably the most striking object in the entire region. Although Lacaille is credited with discovering NGC 3293 (it is the 8th “nebulous cluster” in his 1755 catalogue), the object is part of the naked-eye backdrop of the Milky Way and no doubt was spied as an innocuous star by numerous wonder-struck aboriginal stargazers. When James Dunlop observed it on April 29, 1826, he noted that it was “rich in extremely small stars resembling faint nebulosity.” Both Lacaille’s and Dunlop’s references to nebulosity are curious only because, as stated earlier, nebulosity is involved with the cluster at its northern edge. In their 1975 “Catalogue of Southern Stars Embedded in Nebulosity,” van den Bergh and Herbst list two reflection nebulae in the cluster region: BHe 42A and BHe 42B. The first nebula surrounds the illuminating star and measures 4′ across on red plates and 1.62′ across on blue plates. The second nebula lies just to the north and west and measures 1.6′ on the blue plates, where it is strongest. Colin S. Gum also lists both nebulae as Gum 30 in “A Survey of Southern H II Regions.” He gives a maximum diameter of 40′ and rates it as being moderately bright against the background Milky Way.

Southern Gems

NGC 3293 lies on the fringe of the Eta Carinae complex, which contributes some illumination to the clouds of gas in the region – as any photograph or wide-field telescope under a dark sky will show. Some of the cluster’s stars illuminate a river of reflection nebulosity in the northern part of NGC 3293, while immediately to the cluster’s southeast lies a dark nebula that appears to be part of the prominent dust lane that runs through the Eta Carinae Nebula. D. G. Turner (David Dunlop Observatory) and his colleagues have raised the possibility that NGC 3293 may be associated with other objects in the Carina nebula complex, which is the heart of the Carina OB1 association. NGC 3293 is very young (~10 million years) and contains a large number of B-type giants and supergiants, as well as an M-type supergiant. Its age is similar to that of the Double Cluster in Perseus. Turner and his colleagues suspect that NGC 3293 may be physically related to IC 2581, another 4th-magnitude cluster in Carina, about 1 1/4° to the northwest. Turner and his colleagues see them as a Southern Hemisphere analog to the Double Cluster, though more data need to be collected to confirm this theory. Based on their studies of the region, Turner and his colleagues suggest that the star-forming process in the Carina OB1 Association, which was initiated in the northwest section of the region, led to the creation of NGC 3293 and IC 2581. Later episodes of star formation followed in regions lying progressively eastward and southward of these two clusters. “This rather simple picture does not account for all the observations,” the researchers warn, especially because there are some anomalous clusters in the region. Nevertheless, it appears that NGC 3293 and IC 2581 represent one of the oldest areas of

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46 to the northwest and with some fainter stars forms a r true branch or wishbone patGum 30 Carina tern. The cluster’s core is very bright, with a slightly swollen NGC 3293 disk, and one star is clearly 3324 separated to the north. With time, the central disk breaks u TR 15 up into a swarm of extremely vdB-Ha 99 congested stars. With averted 1 E TR 14 W vision, the swarm seems to TR 16 tremble and swell erratically, as if it were made of moldable clay and something inside is Eta Carinae CR 228 Nebula trying to push its way out. In W the antique telescope, the core is more defined and looks stretched like taffy 1˚ from the northwest to the S southeast. The cluster looks similar through the 4-inch at 23×, star formation in the association. But as G. only now the cocoon breaks down into a sizBaume (University of Padova) and his colzling swarm of stars. It’s hard to judge which leagues report in a 2003 paper in Astronomy stars are members, but in any case some 40 to and Astrophysics (vol. 402, p. 549), 19 stars 50 stars burn into view with any increase in with signs of H-alpha emission in the region magnification. They are a dazzling assortment of NGC 3293 indicate that star formation is of colorful orbs, especially one golden star, the still active there. one type M member of the cluster, which has To find this stunning cluster and mysteriaged the fastest. This star is the southernmost ous nebulosity, use wide-field chart 4 to find of three, which form a northeast to southwest the nebula surrounding Eta Carinae. Now trending line from the bright star at the cluslook about 2° north-northwest for two 4thter’s core. It is around this bright star that the magnitude objects separated by some 40′. cluster’s fainter members swarm like bugs to The northern one is the star r Carinae; the a light. A long arm extends to the east, and a southern one is NGC 3293. You can confirm stubby one ends at a distinct triangle to the the view by using the accompanying chart. west. With its strong central axis, arms, and In 7 × 50 binoculars, NGC 3293 is a fuzzy sharp surrounding starlight, I can imagine it ellipse of light hanging on a 1/2°-long branch as a sterling silver crucifix wrapped in a crysof four 8th-magnitude stars like “spider tal bead rosary. spit” – that foamy white embryonic goo from I find that the nebula just described which all manner of insects evolve in the requires low power and averted vision, as wild. The branch arcs gracefully from the east N

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46

well as the ability to differentiate nebulosity from the Milky Way, which takes a discerning eye. Try first in binoculars. If you fail to see it, move your scope a little more than 30′ to the southeast, where you will find NGC 3324  – a 6th-magnitude star immersed in a tiny but bright reflection nebulosity. Dunlop

Southern Gems

discovered the nebulosity, which is described in the original NGC as “pretty bright, very large, irregularly faint, double star involved.” Note that there has been no mention of a cluster. Yet today most sources and star charts label NGC 3324 as a cluster. Finally, I think it’s important to note that although NGC 3293 has a true physical diameter of 12 light-years (about the same as the Jewel Box’s), it is nearly twice as far away as the Jewel Box. If we could haul in NGC 3293 and position it next to the Jewel Box, we’d find it has about the same apparent diameter but shines nearly a full magnitude brighter – almost as bright as Delta (δ) Crucis, the star that marks the western arm of the Southern Cross.

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47 47 Theta Carinae Cluster, Southern Pleiades IC 2602 Type: Open Cluster Con: Carina RA: 10h42.9m Dec: −64°24′ Mag: 1.9 Diam: 100′ SB: 11.6 Dist: ~492 light-years Disc: Abbé Nicolas Louis de Lacaille, listed in his 1755 catalogue abbé ni co l as l ou is de l acail l e : Theta Argus, surrounded by many 6th- to 8th-magnitude [stars], like the Pleiades. j ames dunl o p: A cluster of extremely small stars, resembling a faint nebula, about 6′ diameter, round figure. (D 258) i c: Cluster, coarse, includes θ Carinae.

D r o p a vi s ual plu mb l i ne 4 1/ 2° due south of the magnificent Eta (η) Carinae Nebula (Southern Gem 48), and it will collide with another brilliant deep-sky wonder, IC 2602, the dazzling Theta (θ) Carinae Cluster. Abbé Nicolas Louis de Lacaille discovered this mine of celestial diamonds during his great exploration of the southern skies from 1751 to 1753. The object is included in his 1755 catalogue and is the 9th listing under his Class II category, “nebulous star clusters.” He called the object “Theta Argus” (Carina is the Keel of the now defunct constellation Argo Navis) and estimated its brightness as 3rd

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magnitude. Through his diminutive 1/2-inch 8× telescope, Lacaille saw the Theta Carinae Cluster looking “like the Pleiades.” That is why on modern charts IC 2602 is labeled “The Southern Pleiades.” With a declination of  –64°23.7′, the cluster is only 5 1/2° above my Volcano, Hawaii, horizon when at upper culmination, yet it is clearly visible to the unaided eye. The cluster is one of three major glows in the rich Carina Milky Way, the two others being the Eta Carinae Nebula and NGC 3532 (Southern Gem 49). Together with the dense star clouds of the attendant Milky Way, the sight of these

Deep-Sky Companions

47 three objects is without question the most stunning sight in the entire heavens. Several of IC 2602’s stars are visible to the naked eye, but just how many will depend largely on the darkness of your site and your ability to resolve stars packed tightly together. Photometry of two 15′ fields in IC 2602, conducted in 1991 and made with the 1-meter Swope telescope at Las Campanas Observatory in Chile, revealed that half the stars visible to the telescope were background stars  – not surprising for an open cluster in a rich Milky Way field. Brent Archinal says IC 2602 harbors at least 60 definite members spread across 100′; some 30 of these are brighter than 9th magnitude. Of the latter, 23 have spectral types of B or A, while the others range from F0 to K5. Theta Carinae, of course, is the brightest, shining at magnitude 2.8 and having a spectral class of B0. The nearly 5th-magnitude star 6′ to the southwest of Theta Carinae is V518. Its variability was discovered by the Hipparcos satellite, which recorded apparent visual magnitudes ranging from 4.60 to 4.76. IC 2602 is very young. Its stars burned through their dusty cocoons some 30 million years ago, which makes this cluster less than half the age of the Pleiades. Further evidence of the cluster’s relative youth can be found in the spin rate of its stars. Younger stars generally spin faster than older stars. However, there can be a wide range of rotation periods even among a young cluster’s stars. Recent

Southern Gems

studies have found that the rotation periods for 29 cluster members range from 0.2 days (one of the shortest known rotation periods of any single open cluster star) to about 10 days (which is almost twice as long as the longest period previously known for a cluster of this age). Images of IC 2602 from the ROSAT x-ray satellite revealed a total of 110 objects within an area of 11 square degrees. Of these, 68 have been identified with at least one optical counterpart; 44 of these are new optical candidates for cluster membership. In IC 2602, ROSAT detected x-ray energy radiating from stars of all spectral types. The F, G, and early-K stars appear to be more x-ray luminous than those in the Pleiades, while no significant difference was seen among late-K and M dwarfs. This may be related to the different rates at

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which stars of different masses shed rotational energy. If we accept its distance as 492 light-years, IC 2602 is 85 light-years more distant than the Pleiades and 23 light-years closer than M44, the Beehive Cluster in Cancer. It spans 14 light-years of space, as does the Pleiades. Furthermore, IC 2602 has some 10 stars 6th magnitude or brighter, like the Pleiades, and it is only 0.4 magnitude (39 percent) fainter than the Pleiades. So the two clusters really are

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twins. Visually, however, IC 2602 resembles M6 (Southern Gem 94), the Butterfly Cluster in Scorpius, more than it does the Pleiades; IC 2602 is a member of the Scorpius-Centaurus OB Association – a widespread group of hot, young stars that appear to have a common origin. Like the Pleiades, the cluster is best observed in binoculars or a rich-field telescope. In the 4-inch at 23×, the cluster is a loose aggregation of stars with two distinct groupings. The first grouping includes Theta Carinae and is a gentle arc of stars, like a slightly upturned saber glinting with dew; the second group lies to the east, separated from the Theta grouping by a 30′-wide lagoon of relative darkness, and looks either like a butterfly, an hourglass, or a bow tie. Before leaving the field, take the opportunity to view yet another of this stunning region’s countless open clusters: Melotte 101 (RA:10h42.1m; Dec:  –65°06′), an ­8th-magnitude fog of stars measuring 13′ across that lies 45′ due south of Theta Carinae. It is 10′ east of V364, a roughly 5thmagnitude variable, that Lacaille, no doubt, saw as his southern “Pleione,” though unfortunately it is not a true cluster member (of either cluster). The cluster contains about 40 faint stars, the brightest of which shine around 11th magnitude.

Deep-Sky Companions

48 48 Eta Carinae Nebula, Keyhole Nebula NGC 3372 Type: Variable Emission Nebula Con: Carina RA: 10h43.8m Dec: −59°52′ Mag: 4.8 (variable), nebula; 4.5 (O’Meara) Diam: 120′ (variable) Dist: ~7,500 light-years Disc: Known since antiquity (?) a bbé ni co l as l ou is de l acail l e : Two small stars surrounded by nebulosity. (III-5) Large group of a great number of small stars, a little compressed and occupying the space of a semicircle of 15′ to 20′ diameter, with a slight nebulosity spreading in that space. (III-6) j a mes dunl o p : [See the text.] (D 309) j o hn hers chel : [See the text.] (h 3295). n g c: Remarkable, great nebula, η Carinae.

If we cou l d phot ograph hu man imagination, the result might look something like the Eta (η) Carinae Nebula – an elaborate tapestry of stellar energy fused into luminous folds of gases strung together with threads of dust. To the naked eye, the complex appears as a silver pendant of nebulous matter in the crisp collar of the Carina Milky Way. It is the brightest and most spectacular example of an emission nebula in the heavens, and it resides in the richest and most dramatic region of the Milky Way, a stormy swath of stars between the False Cross and the Southern Cross. Wide-field photographs show Eta Carinae as

Southern Gems

a symphony of visual splendor; my heart sees it as the Taj Mahal of galactic wonder. Abbé Nicolas Louis de Lacaille first documented the nebula. But southern stargazers probably have seen it throughout time as part of the fabric of the Milky Way. When Lacaille looked at the nebula through a 1/2-inch 8× telescope, he recorded it as simply “[t]wo small stars surrounded by nebulosity.” On another night he recorded it as a “[l]arge group of a great number of small stars, a little compressed and occupying the space of a semicircle of 15′ to 20′ diameter, with a slight nebulosity spreading in that

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48 space.” Note that he refers to the nebulosity as “slight.” Edmond Halley is said to have observed the bright star embedded in the nebula (Eta Carinae, thus the nebula’s name) from St. Helena in 1677 (nearly 80 years before Lacaille), but he failed to mention the presence of any nebulosity, which is also curious. Not until James Dunlop observed the object in the 1820s do we get a sense that a remarkable nebula can be found here: (η Roboris Caroli*, Bode) is a bright star of the 3rd magnitude, surrounded by a multitude of small stars, and pretty strong nebulosity; very similar to that in Orion, but not so bright. . . . I can count twelve or fourteen extremely minute stars surrounding η in the space of about 1′; several of them appear close to the disk: there is a pretty bright small star about the 10th magnitude north following the η, and distant about 1′. The nebulosity is pretty strongly marked; that on the south side is very unequal in brightness, and the different portions of the nebulosity are completely detached. . . . There is much nebulosity in this place, and very much extensive nebulosity throughout the Robur Caroli, which is also very rich in small stars.

Is it possible that the nebula has been gradually brightening over time? Today the brightest parts of the nebula are so intense that they can be glimpsed as such even in astronomical twilight. Today, the Eta Carinae Nebula is as much a part of the naked eye Milky Way as, say, the Double Cluster in Perseus, so maybe the “discovery” of Eta Carinae belongs to no one. Now let’s see what modern science has to say about the nebula and its source of radiant energy.

The Eta Carinae Nebula is a remarkable giant H ii region, the largest known in our galaxy. It measures some 260 light-years across, which is nearly seven times larger than the Orion Nebula in true physical extent. The nebula appears severely disjointed, with fantastic blossoms of glowing gas clinging to a twisted black branch of cold dark material. Indeed, no fewer than eight star clusters lie in or near the nebula, four of which lie very close to the nebula’s center. The Eta Carinae complex harbors one of the highest concentrations of early O-type stars known in the Galaxy, and it is these hot young cluster stars, perhaps just a few million years old, that energize the entire nebula. But it is in the bright northern fan that we find one of the most intriguing stars in the night sky, the enigmatic novalike variable Eta Carinae, one of the most powerful laboratories available for investigating the early evolution of the most massive stars. Eta Carinae has roughly 100 times the Sun’s mass and shines at 4 million times its luminosity. Over the years, though, its brightness has varied most spectacularly. Historical records show the star ranging from magnitude  –0.7 (outshining nearly all the night sky’s stars) to about magnitude 7.6 (well below naked-eye visibility). Little is known about its intensity prior to Halley’s 1677 observation. However, German orientalist Peter Jensen believes the fluctuations of this star have been known almost since prehistoric times. The star, he says, is mentioned in Babylonian inscriptions. Apparently Eta Carinae was a temple star associated with Ea, or Ia, otherwise known as Oannes, the mysterious human fish

* Robor Carolinium (Charles’ Oak) is a constellation Edmond Halley created during his stay on St. Helena. To form the Oak, he borrowed 12 stars from Carina and then published the creation in 1679. The constellation commemorated the Royal Oak that sheltered his patron, Charles II, after his defeat by Cromwell in 1657. But Halley’s action upset Lacaille, who complained that the Oak used some of the finest stars in the Ship. Alas, the Oak fell 50 years after that “storm” and no longer stands on modern star charts.

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Deep-Sky Companions

48 and greatest god in the kingdom of Eridhu (the Holy City). This implies that Eta Carinae shone magnificently near the dawn of human civilization. In his book Star Names: Their Lore and Meaning, Richard Hinkley Allen adds that in ancient China Eta Carinae was known as Tseen She, Heaven’s Altars  – yet another exalted moniker implying greatness upon high. But little else has been uncovered about the star or its nebula. Even Halley’s observation has an aura of mystery. John Herschel credited Halley with making the first observation of the star’s brightness (4th magnitude). But, as Allen notes, Eta Carinae does not appear in Halley’s Catalogues Stellarum Australium, the compendium of 341 stars he observed while at St. Helena. How then did knowledge of Halley’s observation become public knowledge? In the 150 years following Halley’s observation, Eta Carinae’s brightness was to fluctuate erratically, achieving a maximum, at 1st magnitude, in 1827. The star then dipped back to 2nd magnitude, where it remained for about the next five years. Writing in his 1847 “Results of Astronomical Observations,” John Herschel picks up the story from there: When first observed by myself in 1834, it appeared as a very large star of the second magnitude, or a very small one of the first, and so it remained without apparent increase or change up to nearly the end of 1837. . . . It was on the 16th December 1837 that . . . my astonishment was excited by the appearance of a new candidate for distinction among the very brightest stars of the first magnitude. . . . After a momentary hesitation, the natural consequence of a phenomenon so utterly unexpected . . . I became satisfied of its identity with my old acquaintance Eta Argus. Its light was however nearly tripled. While yet low it equaled Rigel, and when it had attained some altitude was decidedly greater.

Southern Gems

It was far superior to Achernar. Fomalhaut and Alpha Gruis were at the time not quite so high, and Alpha Crucis much lower, but all were fine and clear, and Eta Argus would not bear to be lowered to their standard. It very decidedly surpassed Procyon, which was about the same altitude, and was far superior to Aldebaran. . . . From this time its light continued to increase. On the 28th December it was far superior to Rigel, and could only be compared with Alpha Centauri which it equaled, having the advantage of altitude, but fell somewhat short of it as the altitudes approached equality. The maximum of brightness seems to have been obtained about the 2nd January 1838 . . . it was judged to be very nearly matched indeed with Alpha Centauri. . . . After this its light began to fade. . . . On the 20th, it was visibly diminished – now much less than Alpha Centauri, and not much greater than Rigel. The change is palpable.

An even more spectacular event occurred in 1843, when the star’s maximum light reached about  –0.7 magnitude, outshining every star in the sky with the exception of Sirius. Eta Carinae then slowly faded from naked-eye view, hitting a rock-bottom brightness of magnitude 7.6 in 1968. It then rebounded to 6th magnitude in 1978. The star brightened to 5th magnitude between December 1997 and February 1999. On June 12, 1999, I estimated Eta’s brightness as magnitude 4.2 – about as bright as when Halley saw it in 1677 – and, as of July 2012, the star hasn’t changed much. Kris Davidson (University of Minnesota) believes this dramatic increase in brightness suggests that Eta might be preparing itself for another major outburst. Today we know that the great 1843 outburst (which became known as Nova Carinae 1843) was the biggest explosion that any star is known to have survived. According to Davidson, several solar masses of matter were ejected at speeds around 1,000 kilometers per

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48 second. The explosion sent out two opposing expanding lobes, which squeezed out of a dense, 15-solar-mass torus of cold dust and gas perhaps 0.1 light-year in diameter  – the gaseous shell expanding like a balloon inflating in a napkin ring. Also called the Homunculus Nebula, that balloonlike portion of the nebula has been expanding at about 1.5 million miles per hour ever since and presently extends to about 0.8 light-year. Meanwhile, Eta Carinae and its eponymous nebula together have become the brightest object beyond the Solar System at infrared wavelengths. The Hubble Space Telescope has imaged the star’s expanding bipolar lobes in unprecedented detail, showing two polyps of gas pulse with veins and clots of dust (tiny condensations 100,000 times fainter than Eta) and strange radial streaks that stretch 16 billion kilometers from the central star. The HST’s high-resolution spectrograph has also resolved individual gas blobs, which can fade or disappear in days or weeks. As more data are gathered, a new picture of Eta Carinae is emerging from its shroud of stellar mystery. Eta Carinae is most likely a binary star, and the “napkin ring” is the outer mantle shed by one of those two stars some 1,000 or 2,000 years ago. The material was stripped from the star and flung away by the gravity of its stellar companion. Thus stripped of some of its mass, this “naked” star became unstable, leading to the powerful 1843 blast. The spectral signature of the torus supports that theory, for it is nitrogen-rich and oxygen- and carbon-poor, implying that it contains processed material from the star’s interior. At radio wavelengths, Eta Carinae produces the brightest known stellar wind, which might also explain the star’s unique x-ray-emitting sheet of scorching hot gas (60 million degrees Kelvin). The x-ray emission probably arises as

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stellar winds flowing away from each member in the binary pair collide or, if Eta Carinae is a solitary star, by fast and slow winds interacting. A binary system would also explain why the star displays highly unusual changes in its spectrum every 5 1/2 years. Augusto Damineli (University of Sao Paulo, Brazil) and colleagues observed Eta during a predicted 1998 episode of spectral change. From their data, the researchers deduced that the star is indeed a binary system that travels around a common center of mass in a highly elongated orbit every 5.53 years. In a 2012 Monthly Notices of the Royal Astronomical Society abstract (vol. 420, p. 2064), T. I. Madura (Max-Planck-Institut für Radioastronomie, Bonn, Germany) and colleagues note how they used the Hubble Space Telescope’s Imaging Spectrograph to study the binary system in three dimensions. Their model shows a system with an orbital axis that is closely aligned with the inferred polar axis of the Homunculus nebula in 3D. “The companion star, ηB, thus orbits clockwise on the sky, and is on the observer’s side of the system at apastron,” the researchers explain.

Deep-Sky Companions

48 “This orientation has important implications for theories for the formation of the Homunculus and helps lay the groundwork for orbital modelling to determine the stellar masses.” As astronomers continue to delve deeper into Eta Carinae’s spectral mysteries, most agree on one thing: Eta Carinae doesn’t have much longer to live. The star is one of only a few luminous blue variables in the Milky Way. These extremely massive stars live their lives in the “fast lane” of stellar evolution and could demolish themselves in a supernova explosion today, tomorrow, or in a few hundred thousand years. Doesn’t all this excitement want to make you just grab your telescope and dash out to look at this star? Back to our historical conundrum, considering Eta Carinae’s dramatic variability, isn’t it possible that the nebula itself changes in intensity over time? Of course it is – especially because erratic Eta Carinae is the most powerful star radiating in what is now the brightest section of the nebula. According to Allen, “The most brilliant portion [of the nebula], as drawn by Sir John Herschel, seems to have disappeared at some time between 1837 and 1871.” Impossible? Not really, especially if you consider how other variable nebulae such as Hubble’s Variable Nebula and the R Coronae Australis Nebula behave. (The Eta Carinae case also tells us something about the value of astronomical drawings.) I know of no catalogues that offer brightness estimates for the Eta Carinae Nebula. In June 2000, I estimated the nebula’s brightness as magnitude 4.5. Given the nebula’s probable variable nature, it’s possible that it was known to some civilizations but not to others. John Herschel wrote that it “would manifestly be impossible by verbal description to give any just idea of the capricious forms

Southern Gems

and irregular gradations of light affected by the different branches and appendages of this nebula.” But I’ll try. My first view of the Eta Carinae Nebula was with the naked eye in February 1982 while flying at 40,000 feet (12  kilometers) to New Zealand. The nebula was a magnificent presence, not an eye-squinting 6th-magnitude cough of light. And that’s the way it is now from Hawaii. The bright northern section was so intense in February 1998 that I saw it in diminishing twilight with the unaided eye, and in twilight with 7 × 35 binoculars, the object looked like a wedge-shaped asterism of two separate star clusters surrounded by gas. In fact, on the very face of it, my twilight view was very similar to Lacaille’s dark-sky view with a vaguely comparable instrument. In the twilight, another object, open cluster NGC 3293 (Southern Gem 46), is visible 2° to the north-northwest as a compressed 5th-magnitude comet-like glow. As darkness fell, more and more nebulosity appeared in patches, until the bright central chrysalis of stars and gas emerged as a celestial butterfly with impressively broad wings. To find it, use wide-field chart 4 to locate the naked-eye position of the nebula near u and r Carinae. Then switch to the chart on page 190 for a detailed outline of the scene. At 23× in the 4-inch, the Eta Carinae Nebula displays three tiers of intensity. The bright central part is comprised of two concentrations of light. The brighter northern half is teardrop shaped. The head of the teardrop is comprised of two open star clusters: 10′-wide Trumpler 16 (10 stars, including Eta), and 5′-wide Trumpler 14 (5 stars) 10′ to the northwest of Trumpler 16. The nebulosity surrounding these clusters tapers to the northwest, where 45′ away it beads up again at the 14′-wide cluster vdB-Ha 99, a

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TR 14

W TR 16

Eta Carinae Nebula

CR 228 W

1˚ S

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loose but rich aggregation of stars collected around a 5th-magnitude star. A narrow vapor trail of gas continues away from vdB-Ha 99 to the northwest like the thin ion tail of a comet. In fact, the overall impression of this brighter northern section is one of a remarkable comet like Hyakutake. The tip of the tail points to a bright patch of irregular nebulosity (NGC 3324) surrounding the 9th-magnitude double star h338 (6″ separation, oriented east to west), and NGC 3293 lies 30′ north-northwest of it. This section of the nebula is a virtual five-car train of clusters and gas. Now return to the head of the teardrop and study it carefully. The head should be bisected by two crisscrossing lanes of dust. The darkest lane runs from the southeastern rim and curves northeastward to the west of orange Eta. In photographs, this lane is a black S-shaped ribbon of opaque matter with a dark black knob at the northern end and a more bell-shaped bottom; the overall shape of this intensely dark segment is reminiscent of a keyhole, which is why the nebula is sometimes referred to as the Keyhole Nebula  – but

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48 remember, the keyhole refers to a tiny black smudge in the bright nebula and not the nebula itself. Try as I might with the 4-inch, I could not distinguish the “keyhole” in this lane of darkness. Another fainter lane of darkness crosses east to west through the teardrop, just south of Eta Carinae. It does not show well in photographs but is very apparent visually. Of course, when we look at Eta Carinae we are not seeing the star itself but rather the expanding lobes of gas – the Homunculus. E. Gaviola explained the Homunculus in a 1950 paper in Astrophysical Journal (vol. 111, p. 408), saying the lobes looked like a mannequin or homunculus, complete with “head pointing northwest, legs opposite and arms folded over a fat body.” And these globs of nebulosity should be monitored very carefully. (It’s amazing we can see them at all!) In the first edition of his book Astronomical Objects for Southern Telescopes (1984), Ernst Hartung writes that “bright orange η Car is surrounded by an orange red nebula about 3″ wide, just visible with a [4-inch], and the spectrum shows numerous bands with the red H shining like a tiny lamp at one end.” I could not see this feature (the famed Homunculus) as distinctly as Hartung makes it sound with my 4-inch, but Eta was also brighter when I was observing than when Hartung made his observations, so the star’s increased brilliance could have made this delicate feature less distinct. To me, Eta Carinae looked like two orange spots slightly overlapping. In 1982, New Zealand amateur Graham Blow took me to the 9-inch refractor at Carter Observatory in Wellington to view Eta Carinae, and it is there that I observed the Homunculus for the first time in amazing clarity. Through that scope, the Homunculus looked like two

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puffs of glowing orange light 1″ on side of an intense core of light. The gas was so obvious that I thought I was looking at the famous Keyhole Nebula (remember, I was a planetary observer at the time; I didn’t know the Keyhole was a dark nebula). Tom Clark (editor of Amateur Astronomy magazine) described the Homunculus through his 36-inch reflector, which he took to Australia, as two balloons covered with kudzu. A massive dark swath checkmark separates the brighter comet-like northern section from the more rectangular southern section. Allen notes that this vacancy was called the “Crooked Billet” by Francis Abbott. The southern section is a 20″-long dense aggregation of sparkling starlight immersed in a cotton coffin of uniformly bright glowing gas. The coffin is oriented north to south, and inside it are scattered “bones” of Collinder 228, a magnitude 4.4 cluster with an indeterminate number of stars. Collinder 228’s dominant stars are strung out, in fact like a skeleton’s groping hand and forearm. The hand with strings of stars radiates like fingers to the north and then narrows into a long and thick arm of starlight to the south. Thus run the Eta Carinae Nebula’s brightest components. The second tier of nebulosity can be found swirling or looping like four butterfly wings, with the densest sections being to the south. These features are delicate and in fact are better seen in binoculars, where their light is condensed. The third tier is an even more delicate fan of gas extending from the northern comet to the northwest and northeast and a dim lip of gas to the southwest. Although the visual diameter of the nebula is 2°, some photographs show nebulosity extending twice that distance, though it’s hard to tell if this outer nebulosity is physically related to the Eta Carinae Nebula

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48 complex. For instance, the bright patch of gas, NGC 3324, to the northwest of Eta Carinae apparently has no association, nor does the cluster NGC 3293 (Southern Gem 46). But NGC 3293 is a marvel unto itself. I call it the Little Jewel Box. It is diamond shaped, with a finely resolved Y-shaped cluster core of about a dozen colorful stars laid on a fainter background of crushed gems. Finally, I suppose I can never look at Eta Carinae the same way again after reading David Levy’s biography of Bart Bok, who spent a good part of his life studying this ­enigmatic star. On November 15, 1975, Bart and his ailing wife, Priscilla, were in the Flandrau Planetarium at the University of Arizona waiting for Senator Barry Goldwater. The Boks were in a room killing time and looking at a panorama of the entire Milky

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Way. The exhibit had tiny red lights that could be turned on and off to point out specific Milky Way features. Priscilla pressed one of the buttons, and a faint red light turned on to identify the Eta Carinae Nebula. They stood there quietly for a moment, remembering the stunning photographs they had taken of it during years of observing. Finally Priscilla spoke. “You know, Bart,” Priscilla said, “When I am gone, that is where I am going. I will ask St. Peter to give me a front row seat right at the center of the nebula. I’ll see stars forming right before my eyes!” Overwhelmed by this profound merging of his two greatest loves, Bart tried to hold back tears as he hugged Priscilla. “Eta Carinae,” Priscilla repeated as they walked into the planetarium theater, “that is where I want to be.”

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49 49 The Pin Cushion Cluster NGC 3532 Type: Open Cluster Con: Carina RA: 11h05.5m Dec: −58°44′ Mag: 3.0; 3.2 (O’Meara) Diam: 50′ SB: 11.5 Dist: ~1,600 light-years Disc: Abbé Nicolas Louis de Lacaille, included in his 1755 catalogue but probably known since antiquity. a bbé ni co l as l ou is de l acail l e : A prodigious number of faint stars forming a semi-circle of 20–25 minutes diameter. (II-10) j a mes dunl o p : (5 Centauri, Bode) is a very large cluster of stars about the 9th magnitude, with a red star of the 7–8th magnitude, [northeast of ] the centre of the cluster. Elliptical figure: the stars are pretty regularly scattered. (D 323) j o hn hers chel : Chief star of a very large, round, loosely scattered cluster of stars 8 to 12th magnitude, which fills 2 or 3 fields. A fine bright object. (h 3315) n g c: Remarkable cluster, extremely large, round, little compressed, stars of magnitude 8 to 12.

Mi dway b e t we e n t he Fal se Cr oss and the Southern Cross lies one of the heavens’ true stellar spectacles, the 3rd-magnitude open star cluster NGC 3532. It is a burst of visual splendor in the swirling tempest of the rich Carina Star Cloud. Together with the Eta Carinae Nebula (Southern Gem 48, just 3° to Southern Gems

the west-southwest) and IC 2602 (Southern Gem 47, the Southern Pleiades), NGC 3532 is part of the most magnificent tract of Milky Way in the entire heavens. Doubtless it has been a visual landmark ever since aboriginal people first began looking to the heavens. Yet its discovery is 193

49 credited to an eighteenth-century European explorer, Abbé Nicolas Louis de Lacaille, who included it in his 1755 catalogue as the 10th listing under his Class II objects (nebulous star clusters). It would seem almost blasphemous to consider this object an eighteenth-century European revelation, and I’m certain Lacaille would have agreed. But the fact is that the brightest stars of this cluster shine at magnitude 8, so they are too faint to resolve without optical aid. To the naked eye, the cluster looks like a supreme enhancement of the Milky Way, a fantastic knot in the fabric of space, but not a cluster. It took Lacaille’s meager 1/2-inch 8× telescope to reveal for the first time that this diffuse glow was actually “a prodigious number of faint stars forming a semi-circle of 20–25 minutes diameter.” Nearly three-quarters of a century later, James Dunlop encountered NGC 3532 and included it in his 1828 catalogue of nebulae and star clusters. But Dunlop’s description (at the beginning of this essay) falls far short of declaring the object’s true glory, which John Herschel, and later William Pickering, extolled. Herschel called it a “superb” cluster, “the most brilliant object of the kind I have ever seen.” And like Herschel before him, Pickering saw it as the finest irregular cluster in the sky. Today NGC 3532 is classified as a very rich intermediate-age open cluster. Recent age estimates place it at about 300 million years; that’s halfway between M37 in Auriga (200 million years old) and M44 in Cancer (400 million years old). NGC 3532 weighs in with at least 2,000 solar masses, and its brightest stars huddle in the central region. In the 1930s, Harvard astronomer Harlow Shapley found the cluster unusually rich in bright members of the A-type spectral class.

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Surprisingly, the cluster received little professional attention since then until interest picked up again in the 1980s. Today we recognize 677 stars as being known members of the cluster, the vast majority of which are A-type stars, though there are also eight K-type stars, but no M-class giants. As John Herschel suspected, the cluster is also binary-rich; Herschel found “several elegant double stars, and many orange-coloured ones.” NGC 3532’s metallicity is essentially the same as the Sun’s. In a 1993 Astronomy and Astrophysics paper (vol. 275, p. 479), Detlev Koester (Louisiana State University) and Dieter Reimers (University of Hamburg) discovered seven white dwarf candidates, three of which were spectroscopically confirmed with temperatures of about 28,000 K. Two of the white dwarfs have masses equal to 0.6 solar mass, while the third is a stellar cinder of 0.9 solar mass. White dwarfs are the evolutionary end points of relatively low-mass stars like the Sun; more massive stars explode as supernovae. Where exactly is the dividing line between white dwarf precursors and supernovae-to-be? According to Koester and Reimers, it lies somewhere between 6 and 8 solar masses. Interestingly, Koester and Reimers found no white dwarfs in the cluster’s center, where the brightest stars convene; their finds all lie 15′ to 60′ away. This strengthens the theory that low-mass and high-mass stars become segregated in clusters as time passes, a process that takes about 40 million years to complete. To find the cluster (which appears to the naked eye as an intense cloud 1 1/2 Moon diameters across), use wide-field chart 4 to find the 4th-magnitude Cepheid variable V382 Carinae), which is labeled as Star a on the chart. It lies about 1 1/2° east of similarly

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bright u Carinae near the Eta Carinae Nebula, just on the southeastern edge of the cluster, whose dense elliptical core of bright stars curves away from that star to the west like a tiny “comet tail.” The accompanying chart shows a magnified view. My naked-eye estimate of the entire mass agrees fairly well with the published value of 3.0, though I find the cluster 0.2 magnitude (20 percent) fainter. (Note that V382 Carinae isn’t included in my estimate of the cluster’s apparent magnitude.) Train a pair of 7 × 50 binoculars on that innocuous haze and bang! – five dozen stars come screaming out at you through a blizzard of stellar dandruff. The core itself looks like a 30′

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× 15′ pile of shattered glass smeared out to the east and west, as if a crystalline globe had just fallen victim to a steamroller. Another yellow star shining at 6th magnitude lies on the northeastern fringe of this glassy core. At 23× in the 4-inch, the cluster is a stunning agglomeration of stars packed into a diamond-shaped core crisscrossed by thick lanes of dust, giving it the appearance of a diamond brocade set in black gold. Filigreed arms radiate from this core in all directions, and some take on macabre “spiral” forms, like limp or fractured arms. Defocus the view slightly and some equally magnificent dark features materialize out of the background fog. Most noticeable is a dark “7” (seen with south up) that slices through the eastern side of the cluster; an ink-black well lies immediately north of it. Due west of the elongated core is a double pond of dark matter with a river of shadows pouring off to the south. I could go on, but it is useless. These dark features are best

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49 left for the playground of your imagination. Increased magnification does little to enhance the ­cluster’s beauty. Some black voids show up better at this power, as do the many doubles and tiny groupings of stars. But NGC 3532 is one of the few objects in the starlit night that is best admired with low power, a sense of wonder, and fluid thought. During my January 2000 trip to New Zealand, I spent some time with Ian Cooper, the country’s leading deep-sky wizard.

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Really we were just observing what we could with the naked eye and some binoculars. The Large Magellanic Cloud and the Carina Milky Way had just peeked above a thick band of clouds. Suddenly Ian thrust a pair of 11 × 80 binoculars my way and pointed excitedly to NGC 3532. “There ya go, Stephen,” he said with his delightful accent, “Take a look at that . . . it’s the Pin Cushion.” I had to laugh. What a grand and appropriate name. I don’t know who first “pinned” that name to the cluster, but I hope it sticks.

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50 50 The Rip-Torn Cluster IC 2714 Type: Open Cluster Con: Carina RA: 11h17.3m Dec: −62°43′ Mag: 8.2 Diam: 15′ SB: 14.1 Dist: ~4,300 light-years Disc: James Dunlop, 1826 j a mes dunl o p : A cluster of very [small] stars, a little elongated [west] and [east], about 10′ diameter; the stars are congregated towards the centre, a pretty bright star south and a double star [southeast of ] this. (D 281) ic: Cluster, pretty concentrated.

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50 & 52 52 Melotte 105 Type: Open Cluster Con: Carina RA: 11h19.7m Dec: −63°29′ Mag: 8.5 Diam: 5.0′ SB: 12.0 Dist: ~7,200 light-years Disc: James Dunlop, 1826 j ames dunl o p: A rather bright nebula, about 2 1/2′ or 3′ long and 1′ broad, in the form of a crescent, the convex side [west]; no condensation of the nebulous matter towards any point. This is easily resolvable into many stars of some considerable magnitude, arranged in pretty regular lines, with the nebula remaining, which is also resolvable into extremely minute stars. This is probably two clusters in the same line. (D 271)

Our next t w o S outhern Gems (open cluster IC 2714 and Melotte 105) will present a bit of a challenge for those using small telescopes under light-polluted skies. They’re two rather inconspicuous objects “lost” in the extraordinary Milky Way between the Southern Cross and the False Cross. Specifically, their elusive forms lie in a southern “gutter” of a rich star field between the much more stunning open clusters IC 2602 (Southern Gem 47, the Southern Pleiades) and Collinder 249 (the Lambda Centauri Cluster). Let’s look first at IC 2714. Dunlop’s description of the cluster describes it

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perfectly. Interestingly, while the second Index Catalogue (1895) agrees, calling it “pretty concentrated,” in 1930 R. J. Trumpler classified it as a detached cluster with no noticeable concentration. But his classification of II2r also implies that it’s a rich cluster of stars, with a medium range in brightness, thinly but nearly uniformly scattered. All these latter points are right on the money. To me, the photographic appearance of the cluster consists of two layers: a small, loosely concentrated core of mixed starlight surrounded by a detached halo of fainter starlight. Amateur astronomer Kos Coronaios calls it the “Rip-Torn cluster,”

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50 & 52 presumably because of its overall tattered appearance. IC 2714 is located in the Carina section of the Milky Way, in a region where we see the Carina spiral arm tangentially. Interestingly, in 1966 J. Ruprecht identified the Trumpler class as II3m, the distinction being that it identifies more correctly that the cluster is detached, with a slight concentration and a wide range in brightness. The cluster has been poorly studied. Early distance estimates (1970) placed it anywhere from 3,200 to 19,000 light-years away. A 1996 photometric and radial-velocity study of the cluster by J. J. Clariá (Astronomical Observatory at Cordoba, Argentia) and colleagues, however, determined a distance of about 4,300 light-years. If true, the cluster spans nearly 20 light-years. Among the more than 200 stars studied in the field, Clariá et al. found 132 probable members and 13 possible members, including one variable star and 11 red giant members, one of them a spectroscopic binary. Also, they say, two red giants are either binaries or nonmembers. The reddening across the cluster is slightly variable, with a mean value of 0.36 magnitude, and the mean radial velocity is 14 kilometers per second in recession. The cluster’s about 320 million years old and has a metal abundance slightly greater than that of the Sun. Melotte 105, on the other hand, is almost the visual antithesis of IC

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2174 – namely, it’s very small (5′) and highly compressed. Trumpler catalogued it as I2r, meaning a rich, detached cluster with strong central concentration and a range of stellar brightnesses. William Matthew Worsell of the Union Observatory in South Africa discovered the cluster, which to this day remains lightly studied. In 2001, A. E. Patti and Clariá found the cluster had reddened by nearly a magnitude, and they measured a distance of 7,200 light-years, making it some 1.5 times farther away than IC 2714 and only half its true physical extent. Together these facts make the cluster noticeably diminutive in apparent size. Melotte 105, also a main-sequence cluster, has an estimated age of 350 million years; further studies in 2011 showed no variance in any of these findings. So, despite the apparent visual differences, the two clusters are at least close in age. To find these neglected wonders, use widefield chart 4 to find the Southern Pleiades and 3.5-magnitude Lambda (λ) Centauri. Now use your unaided eyes or binoculars to find

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50 & 52

4.5-magnitude z Carinae, which is about 3° west-northwest of Lambda Centauri or about 3° northeast of the Southern Pleiades. Now use the accompanying chart to find IC 2714 a little less than 1 1/2° to the southeast – about 5′ northeast of 8th-magnitude Star a, which is 10′ northwest of a similarly bright double star (b), as noted by Dunlop. Melotte 105 is only about 45′ south-southeast of IC 2714 and just 12′ east of 8th-magnitude Star c. Both clusters fit well in the 5-inch at 33×, with plenty of room to spare. IC 2714 looks like a large but soft 15′ glow that bristles with hazy starlight. About a dozen stars can be seen scattered across its face, with a slight concentration at the center. The 8th-magnitude stars bordering it stand out well, and the cluster seems to be balancing atop them like a basketball on a finger. Melotte 105 is a tight little knot of stars 45′ to the south-southeast – a pinch of celestial salt. At 60×, IC 2714 is a jagged jumble of irregular starlight, somewhat in a starfish pattern. A noticeable triangle of stars lies at the center. IC 2714 is surrounded by a wobbly fainter ring of stars that appears separated from an outer ring of light. Dim stars are interspersed, so depending on how the eye roams the field, 200

these rings become more, then less, apparent. The ringed impression shows up well in widefield photographs. At a glance, some 60 stars appear to be popping in and out of view. The cluster does look orderly at times, then haphazard, as if its order is in the process of being shredded into chaos. With averted vision and concentration, a crescent of a half dozen stars appears at the core. Now, if you put the stars slightly out of focus and relax your gaze, can you see how the roughly concentric rings of starlight I mentioned transform into waves of starlight that seem to propagate from west to east? Melotte 105, on the other hand, is a gaggle of compact starlight. I had to pump up the power to 94× before getting a visual hold of the minute stars that comprise its tiny disk. A bright star or concentration of stars appears at the center, while a chevron of starlight bleeds to the south-southwest. The northwest side has a scattering of faint stars that loops away from the central star. This visual dual nature obviously led Dunlop to believe that two clusters were seen projected along the same line. The visual aspect is not clear in photos, which show the cluster rather regular in starlight. Clearly, however, a brighter Deep-Sky Companions

50 & 52 concentration of starlight to the southwest does occur, which is also probably Dunlop’s crescent and my chevron. Through her 12-inch Schmidt-Cassegrain, South African

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amateur Magda Streicher says that “the brighter stars in this cluster [take the shape of ] the letter ‘M’ on a bed of fainter stars and could well be indicative of Mr. Melotte.”

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51 51 Southern Cross Galaxy, Frame Galaxy NGC 3621 Type: Mixed Spiral Galaxy (Sd) Con: Hydra RA: 11h18.3m Dec: −32°49′ Mag: 8.9; 8.5 (O’Meara) Dim: 9.8′ × 4.6′ SB: 12.9 Dist: ~22 million light-years Disc: William Herschel, 1790 willi am hers c he l [February 17, 1790]: Considerably bright, extended, 70º in a direction from north preceding to south following, very gradually brighter middle, 7′ length, 4′ breadth, within a parallelogram. (H I-241) james dunlop: A very faint pretty large nebula, about 2′ broad and 4′ long, very faint at the edges. The brightest and most condensed part is near the [southeast] extremity; a small star is involved in the [northwest] extremity, and there are two small stars near the south extremity, but not involved. (D 617) jo hn hers chel : Pretty bright, very large, oval, very gradually very little brighter in the middle, 5′ long, 3′ broad. ngc: Considerably bright, considerably large, extended toward position angle 160°, among 4 stars.

Hydra is host to innumerable ­galaxies, many of which can be picked up in small amateur instruments. Its biggest deep-sky prizes, of course, are M83, a magnificent spiral galaxy, and NGC 3242, the Ghost of Jupiter planetary nebula. Spiral

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galaxy NGC 3621 is another bright denizen of Hydra, but very little attention has been paid to it. The galaxy is not included in many popular handbooks, though it is similar in size to M83 (albeit nearly 1 1/2 magnitudes fainter).

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51 In long-exposure images, NGC 3621 is a remarkable pinwheel just 25° from edge-on. The galaxy is large (93,000 light-years across) and has a total luminosity 13 billion times that of the Sun. Its small, bright nucleus is nestled in a bright ellipse of spiral arms (cut by rifts of dust and dappled with numerous young star clusters) that gradually unwind into a fainter outer halo. One prominent arm extends southward from the small bulge; it carries some of this galaxy’s most luminous H i i regions. The spiral pattern of NGC 3621 is similar to that of the bright galaxy NGC 2403 in Camelopardalis. H i i regions are present in the spiral arms, and the largest have halo diameters of 4″. NGC 3621, which belongs to the Leo spur of galaxies, was one of 18 galaxies observed as a part of the Hubble Space Telescope’s Key Project on the Extragalactic Distance Scale, whose goal was to measure the Hubble constant to an accuracy of 10 percent. NGC 3621 contributed 69 Cepheid variable stars, with periods ranging from 9 to 60 days, to the project, which yielded a distance to NGC 3621 of 18.3 million to 22.8 million light-years. In 2001, astronomers announced that some of NGC 3621’s brighter stars were also studied at the European Southern Observatory’s Very Large Telescope at Paranal Observatory in Chile. The scope resolved 10 supergiant stars, which were used as standard candles to help establish the galaxy’s distance at 21.8 million light-years, putting it well beyond the Local Group. The universe’s expansion is carrying it away at a speed of 730 kilometers per second in recession.

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The Chandra spacecraft also viewed NGC 3621. In a 2009 paper in Astrophysical Journal (vol. 700, pp. 1759–1767), Mario Gliozzi (George Mason University) and his colleagues report how Chandra had detected a weak x-ray point-source coincident with the galaxy’s active galactic nucleus (AGN). This result, combined with infrared and optical spectroscopic results, the authors say, “suggests that NGC 3621 harbors a heavily absorbed AGN, with a supermassive black hole of relatively small mass accreting at a high rate.” Chandra also detected two bright sources located almost symmetrically at 20″ from the galaxy’s nucleus, which might be intermediate mass black holes with masses of the order of a few thousand solar masses. The researchers note, however, that higher quality x-ray data combined with multiwavelength observations are necessary to confirm these conclusions. To find NGC 3621, use wide-field chart 4 to find magnitude 3.5 Xi (ξ) Hydrae. Center that star in your telescope at low power and then switch to the accompanying chart. From Xi Hydrae, sweep about 1° west (and slightly south) to 8th-magnitude Star a. About 40′ to the west is a fine double star (b) with a magnitude 8.0 primary and magnitude 9.8 secondary roughly to the south. Less than 1° to the west-southwest, you’ll find solitary

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51 7th-magnitude Star c. Just about 1/2° to the south-southwest is 8th-magnitude Star d. NGC 3621 is almost equidistant from that star to the south-southwest. From the dark skies of Hawaii, NGC 3621 is visible in 7 × 50 binoculars as a soft peach-like glow, even with the crescent Moon up. In my antique scope, it’s a dim, blotted haze; the challenge in this star-poor region is to know exactly where to look. Telescopically, the galaxy appears as a big diffuse glow with a bright core and a faint, elliptical halo. Its spiral arms are faint and will challenge small-telescope users. Still, Ernst J. Hartung noted that the spiral system is “quite easy, though faint,” in a 3-inch. He also found it “lying in a trapezium of four stars in good contrast with a scattered star field.” In the 4-inch at 23×, it’s a fabulous sight, with three dim stars forming a cap on the galaxy’s southeast edge, which seem to pose as supernova suspects. In fact, the galaxy looks more like an elongated globular cluster just starting to be resolved. Do not be fooled by the photographic dimensions given at the beginning of this essay. The galaxy is really segmented into two parts: a bright inner region that measures about 5′ × 3′, surrounded by a gentle outer halo, which is but a breath of moist air on a cold morning. That halo will vanish in small apertures with any increase in power. At 72×, the bright inner lens is fantastic. With any concentration, it splinters into a veritable ornament of dim lights. The northeastern side is darker than the southwestern side, which is indicative of the dark and dappled veins of dust that riddle the galaxy in photographs. By moving my attention from my retina’s periphery to its fovea, I can register several bright areas. First the nuclear

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region is a diffuse circular haze, like a fuzzy peach core. A bright nub of an arm extends to the south-southeast, and with time I can follow two spiral arms that form a backward S – one arm starts in the southeastern quadrant and loops around to the west, and the other starts in the northwestern quadrant and loops around to the east. The southwest portion of the former spiral appears blotchy. A thin arm extends from the northeast side of the central peach pit and arcs toward the southwest. With powers ranging from 101× to 168×, the galaxy offers yet another layer of wonders. The core takes magnification particularly well, so do not hesitate to push it. At 168×, the core is lost in what appears to be a bar oriented northwest to southeast. Thin, dark lanes of dust border the bar to the east and west and separate it from equally thin segments of inner spiral arms.

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51 Through her 12-inch f/10 SchmidtCassegrain telescope, South African ob­server Magda Streicher saw the galaxy as “just a snatch of light,” a little brighter toward the middle, and with a sharp edge. She named it the Southern Cross Galaxy because a trapezium of stars closely surrounds it, “as if it could be holding it.” At powers of 218× and 316×, she saw it “mottled and slowly getting brighter to a wide dense core.” Although it’s a “large smudge of light,” Streicher said that

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the galaxy is “very much pleasing on the eye. Appears like a soft cloud surrounded in mistiness on its periphery with faint splinter stars embedded on its surface.” By the way, David Levy of Arizona instead calls NGC 3621 the Frame Galaxy because it lies in a nice parallelogram of stars  – like a picture in a frame. Note, too, that William Herschel called attention to this parallelogram in his description of the object cited earlier.

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53 53 Rich Man’s Jewel Box NGC 3766 Type: Open Cluster Con: Centaurus RA: 11h36.2m Dec: −61°37′ Mag: 5.3 Diam: 15′ SB: 11.2 Dist: ~5,800 light-years Disc: Abbé Nicolas Louis de Lacaille, included in his 1755 catalogue abbé ni co l as de l acail l e : Three faint stars in nebulosity. (III-7) j ames dunl o p: A pretty large cluster of stars of mixed magnitudes, about 10′ diameter. The greater number of the stars are of a pale white colour. There is a red star near the [western] side; another of the same size and colour near the [eastern] side; another small red star near the centre; and a yellow star near the [southeast] extremity, all in the cluster. (D 289) j o hn hers che l : The [westernmost] of two chief stars of a fine, large, loose, round cluster of stars 8th- to 12th-magnitude, gradually pretty much brighter in the middle, fills the field; 150 to 200 stars. (h 3352). ngc: Cluster, pretty large, pretty rich, pretty compressed, stars of magnitude 8 to 13.

Halfway bet ween alpha (α) crucis (the foot of the Southern Cross) and the famous Eta (η) Carinae Nebula (Southern Gem 48), just 1 1/2° due north of the Lambda (λ)

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Centauri Cluster and smack-dab on the galactic equator, is yet another young naked-eye open cluster: NGC 3766 in Centaurus. This cluster resides in the Carina Complex, a giant

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53 collection of giant molecular clouds with an extremely bright southern H ii region embedded in a giant molecular-hydrogen cloud that is associated with some of the most massive stars in our galaxy (Eta Carinae among them). Molecular-cloud complexes are hotbeds of star formation, and the Carina Complex is no exception. It teems with hot young bodies, including numerous Be-type stars, which are believed to rotate so rapidly that they shed mass, forming an expanding shell and a disk around each star’s equator. NGC 3766 contains the largest number of Be stars known in any single cluster. Like their less-massive cousins, the T Tauri stars, Be stars are believed to represent the last phase in the pre–main-sequence evolution of young, hot protostars. In brightness, size, and shape, NGC 3766 appears strikingly similar to M37 in Auriga. Each cluster shines roughly at 5th magnitude, measures 15′ in diameter, and displays an elongate body flecked with bright stars. Spanning 25 light-years of space, NGC 3766 is 20 percent larger than M37, and though its distance is also greater, by about 1,400 light-years, NGC 3766 would outshine M37 were it not for interstellar dust, which diminishes its light by a half magnitude. The main differences between the two clusters lie with their ages and stellar populations. M37 is 200 million years old and contains some 1,890 members, while NGC 3766 is a mere 24 million years young, with 137 known members (though the larger number of M37 stars may result from that cluster having been studied more often). NGC 3766 is another one of Abbé Nicolas Louis de Lacaille’s finds and is listed in his 1755 catalogue as the seventh Class III object (stars accompanied by nebulosity). Through his 1/2-inch, 8× telescope, he saw “three faint neighbored stars surrounded by nebulosity,”

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but as with many of the objects in this class, the nebulosity does not exist, it being unresolved starlight. Although the cluster is visible to the naked eye under a dark sky, don’t be fooled into believing that it is obvious. The cluster lies in a patchy, star-rich section of Milky Way, and the cluster’s 5th-magnitude glow requires knowing precisely where to look. Particularly disturbing is a 30′-long, cluster-like, rectangular asterism (oriented north to south), whose southern base lies 10′ from the northern fringe of NGC 3766. It looks larger and fuzzier to the naked eye than NGC 3766, and could be mistaken for our tiny target. To find NGC 3766, use wide-field chart 4 to locate Lambda Centauri. Now use your binoculars to look about 5° northwest, where you’ll find a roughly 5°-long chain of four similarly bright and equally spaced stars (oriented southeast to northwest). NGC 3766 should stand out at the northwest end of that chain; that latter star in the chain is depicted in the accompanying

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53 chart. Through 7 × 35 binoculars, the cluster’s half-dozen brightest stars form a sideways T. At 23× in the 4-inch refractor, the cluster’s brightest stars, ranging from 8th to 10th magnitude, are true scintillating jewels of varying colors strewn across a carpet of dim white stars. James Dunlop first noted the colors in 1826, writing that, “The greater number of the stars are of a pale white colour. There is a red star near the preceding side; another of the same size and colour near the following side; another small red star near the centre; and a yellow star near the south following extremity, all in the cluster.” And modern observers still comment on them: In Volume 7 of the Webb Society Deep-Sky Observer’s Handbook, NGC 3766 is described as having “some orange and yellow stars.” The late Ernst Hartung calls it a “fine scattered cluster … containing orange, yellow, white, and bluish stars.” And Phillip Harrington saw “some appearing golden, others bluish.” The most obvious color can be seen in a pair of stars at the northwestern edge of the cluster; the brighter one shines at magnitude 7.2. This wide double marks the foot of the T. The brighter of this pair’s two stars is V910 Centauri, which shines at magnitude 7.2 and has a ruddy spectral class of M0Ib. Now, if you relax your gaze, the southeastern side of the cluster will take on a triangular shape, with a 6′-long stream of stars flowing to the north and a fainter one flowing equidistant to the south, so the asterism now looks like a great frigate bird with outstretched wings and a long forked tail. With 72× and averted vision, look for a fantastic 1°-wide pool of darkness abutting the southeastern corner of the triangle. It has a slightly elliptical shape that manifests itself more clearly the more you concentrate on

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it. If you suspect seeing it, try defocusing the image ever so slightly. Doing this will blend the bright stars around it and cause the pool to stand out. Focus the image again and look closely at the multitude of stars in the triangle. How many doubles do you see? There should be at least a half dozen bright pairs and an equal number of fainter ones. Immediately east of the triangle, a roughly 9th-magnitude star forms part of a minicluster of dim 12th- to 13th-magnitude stars. Sweep your eyes east and west, then north and south along the cluster, and see if you can detect all manner of loops and star chains. At one moment I saw a ring of 11th-magnitude stars within the triangle; it had a string of equally bright stars dropping to the south. At high power, the cluster begins to lose its luster, though magnification does help separate the many fine doubles. My drawing shows only the cluster’s brightest stars, and serves mainly to show the location and extent of the dark pool mentioned earlier. Use the accompanying photograph as your guide to the many star patterns visible in this wonderful cluster.

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53 The eclipsing binary star BF Centauri lies at NGC 3766’s extreme northern edge. Its light fluctuates between magnitudes 8.5 and 9.4 every 3.7 days; the star spends 20 percent of

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this period in eclipse. Following the variation of this star would make a good project for an eager observer with a keen eye, a photometer, or a CCD.

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54 54 Blue Planetary, The Southerner NGC 3918 Type: Planetary Nebula Con: Centaurus RA: 11h50.3m Dec: −57°11′ Mag: 8.1 Diam: 12″ Dist: ~4,900 light-years Disc: James Dunlop, a footnote to the Brisbane Star Catalogue and not included in his 1828 catalogue j ames dunl o p: Fine blue star. j o hn hers che l : A perfect planetary disc 6″ diameter; quite sharp, with not the least haziness. It is of a most decided independent blue colour when in the field by itself, and with no lamp light and no bright star. About 10′ north of it is an orange coloured star of 8th magnitude. When this is brought into view the blue colour of the planetary becomes intense. Shown to my attendant, John Stone, who, on being asked what colour, said at once “blue.” (h 3365) NGC: Planetary, remarkable, small, round, blue, equals a star of 7th magnitude, diameter equals 1.5″.

NG C 3918 is a remarkable and bright planetary nebula about 3.5° northwest of Delta (δ) Crucis, the western arm of the Southern Cross, just over the border in Centaurus. Users of handheld binoculars can see NGC 3918 as an 8th-magnitude star. But don’t be fooled. This deep-sky wonder pre­ sents a challenge to all observers, as I will soon explain. 210

When Dunlop made the Brisbane Star Catalogue of 1826, Cozens explains in a private communication, “he noted that star [Brisbane] 3807 was a ‘fine blue star,’ but he did not know it was a planetary nebula.” Unaware of this note, John Herschel went on to claim discovery in April 1834 from the Cape of Good Hope. Like Dunlop, Herschel also recognized the object’s blue coloration; unlike Dunlop, Deep-Sky Companions

54 Herschel resolved the object into a disk and was the first to call it a “planetary.” The blue coloration is indeed the nebula’s finest quality for those using moderate- to large-sized backyard telescopes. And while Dunlop should be credited for being the first to note this striking peculiarity, John Herschel did much to promote this aspect to the masses. In his extract of a letter to Francis Baily from the Cape of Good Hope, dated October 22, 1834 (Monthly Notices of the Royal Astronomical Society, vol. 33, p. 75, 1835), the younger Herschel recaps his landing, the telescope erection, and the onset of observations, including his discovery of NGC 3918. We landed on the 16th of January; and on the 22nd of February, the 20-feet [18-inch aperture] telescope being erected and the mirrors unpacked, I turned it, for the first time, on the southern circumpolar heavens.… On the 5th of March my sweeps commenced, and have continued, at the average rate of about 10 sweeps per month … in the course of which I have already accumulated a pretty extensive collection of both nebulae and double stars: … On the 3d of April I discovered another fine planetary nebula, having a perfectly sharp disc, without the least haziness, of about 6″ diameter. The most remarkable feature about this is its evident blue colour, which needs not the presence of lamp light, or that of any red star, to be very conspicuous, as it appears when the nebula stands alone in a dark field.

Note, too, that Herschel in his first observation of the object (described earlier) introduces the phenomenon of simultaneous contrast (when the perceived color of an object can be influenced or enhanced by the color of a nearby object): “About 10′ north of [NGC 3918],” Herschel explains, “is an orange coloured star of 8th magnitude. When this is

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brought into view the blue colour of the planetary becomes intense.” In a later observation, Herschel expounded on the planetary’s color, calling it a “beautiful rich blue, between prussian blue and verditter green.… A feeble lamp light gives it a deep indigo contrasted colour. Ditto if a red star [northwest], about 10′ distant, be brought into the field with it. My attendant saw it and declared proprio motu that the light has quite a green cast in it.” In his last observation of it, he mimicked Dunlop’s discovery observation, calling it a “fine blue,” adding “very like Uranus,” though he detected no “satellite stars” near it. As for its shape, Herschel noted that “[w]hen kept steadily at rest its outline is sharp and clean, and perhaps a very little elliptic.” As with all planetary nebulae, NGC 3918 represents the last stage of evolution for most stars in our Milky Way. When a star can no longer support itself by nuclear fusion and begins to die, the exhausted star swells to several times its normal size to become a red giant that pulses and puffs, periodically ejecting shells of matter outward into space. These shells surround the star like a cocoon, which we see as the planetary nebula. High-energy ultraviolet radiation streaming out from the star’s exposed core heats the expanding gas, causing it to glow and making it visible to our eyes. Young planetaries sometimes show evidence of bipolar flows. In a 1999 paper in Astrophysical Journal (vol. 523, pp. 721–733), Romano L. M. Corradi (Instituto de Astrofísica de Canarias, Spain) and colleagues analyzed optical images and high-resolution spectra of the roughly 3,000-year-young nebula surrounding NGC 3918. They found it composed of an inner, spindle-shaped shell mildly inclined with respect to the plane of

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54 the sky. “Departing from the polar regions of this shell, we find a two-sided jet expanding with velocities that increase linearly with distance from 50 to 100 [kilometers per second]. The jet is probably coeval with the inner shell (age ~1000D years, where D is the distance in kiloparsecs), suggesting that its formation should be ascribed to the same dynamical processes that also shaped the main nebula, and not to a more recent mass-loss episode.” In a 2003 paper in Monthly Notices of the Royal Astronomical Society (vol. 340, pp. 1153–1172), B. Ercolano (University College London) and colleagues constructed a realistic three-dimensional model of the planetary nebula NGC 3918 that describes the shape of its inner shell as consisting of an ellipsoid (spindle) embedded in a sphere. Their spindle-shape model, they say, best compares to recent HST images and ground-based spectra. As can be seen in the accompanying Hubble Space Telescope image, the planetary’s shells appear as concentric rings surrounding the hot white dwarf central star, which shines at a blue magnitude of 15.7. Note the distinct shapes of the shells. NGC 3918’s distinctive eye-like shape (the spindle) forms a bright inner shell of gas, and a more diffuse outer shell extends far from the nebula. Although one could easily surmise that the two shells represent two episodes of mass ejection, this doesn’t seem to be the case. As reported in an

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August 2010 press release from the Space Telescope Science Institute, studies of the object now suggest that they were formed at the same time but are being blown from the star at different speeds. “The powerful jets of gas emerging from the ends of the large structure,” the press release says, “are estimated to be shooting away from the star at speeds of up to 350,000 kilometers per hour.” Spectroscopy reveals NGC 3918 is approaching us at about 17 kilometers per second, while the nebulosity is expanding at around 24 kilometers per second. Planetaries like NGC 3918 are short lived and are expected to exist for only a few tens of thousands of years. To find this beautiful blue planetary, use wide-field chart 4 to find Delta Crucis. Then use your unaided eyes or binoculars to locate the little asterism of four roughly magnitude 5.5 stars 3° to the west-northwest. Then

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54 switch to the accompanying chart for better clarification of the field. From Star a, move about 18′ west-southwest to 8.5-magnitude Star b. NGC 3918 is a similarly bright “star” less than 10′ to the south-southeast. Through the 5-inch at 33× and 60×, the nebula appears simply as an 8th-magnitude star. I could see no color. The disk begins to appear at 94×, but still, try as I might, I could see no color. When I compared the view to the orange 8th-magnitude star to the north-northwest, I could imagine a smoky gray hue, but I’m certain that was a color contrast illusion. I have read that the color shows in apertures 8 inches and larger, so I’m not that surprised at my failure. Then again, the object is lower in my Hawaiian skies than from farther south. Perhaps if the nebula could magically climb higher in my sky, I might see some blue tint. Then again, in June 2001 from South Africa, Magda Streicher said that NGC 3918 “possesses a distinctive soft blue-green colour” through her 16-inch Schmidt-Cassegrain. On a further look, she added, “This is what I call a proper police light that patrols the night skies between the stars. The planetary nebula displays a real blue that changes into a rich turquoise colour. It’s a very bright and outstanding object. Higher magnification brings out a slightly hazy edge. And high magnifications are just what this object requires. As I said in the opening

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of this essay, NGC 3918 is an easy target to “see” but a difficult one to observe (if you don’t have a large scope with a decent clock drive). The disk appears only about 8″

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54 across  – about twice the apparent diameter of Uranus! Nevertheless, with time and patience, I studied the nebula with powers up to 330×. At the highest power, I spotted a tiny inner feature (probably the blending of bright inner rings) that mimicked a central star surrounded by a uniform disk much like Uranus’s.

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Streicher says that she had difficulty seeing the planetary’s 13th-magnitude central star because of the nebula’s high surface brightness. But this only means that those with large apertures should use as much magnification as possible to enlarge the nebula; only then might you have a chance to catch a glimpse of it. Good luck!

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55 55 The Longtail NGC 4103 Type: Open Cluster Con: Crux RA: 12h06.7m Dec: −61°15′ Mag: 7.4 Diam: 6′ SB: 11.3 Dist: ~6,800 light-years Disc: James Dunlop, 1826 j a mes dunl o p [April 30, 1826]: A cluster of small stars of mixed magnitudes, irregular figure, about 6′ long and 4′ broad. (D 291) j o hn hers chel : Rich, large, irregularly round cluster; poor VI or rich VII, stars 10 to 14th magnitude; diameter 5′, with stragglers. (h 3377) ngc: Cluster, pretty large, pretty concentrated, irregularly round, stars from 10th to 14th magnitude.

N G C 4103 is a s mal l bu t ve ry pre t t y open cluster about 2° west-southwest of Epsilon (ε) Crucis. It lies in a small (~1°-long) asterism of binocular stars that looks a bit like the Southern Cross, with NGC 4103 being in the location of Epsilon. The cluster should be a fine sight through telescopes from all locations, and its irregular shape can inspire those with imagination to see all manner of familiar patterns. For instance, I see a longtail bird in flight. When James Dunlop discovered the object in 1826, he, too, noted its irregular figure. So did

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John Herschel, who classified it as either a poor Class VI (very compressed and rich cluster of stars) or a rich Class VII (pretty much compressed cluster of large or small stars) object. Later he narrowed it down to a Class VII cluster, it being “pretty rich and compact.” What I also enjoyed reading in this second observation is the state of the atmosphere. “The whole field,” Herschel said, “is in a state of wavy fluctuation, owing to the southeast wind, and so bad that each star is dilated into a large puff ball.” In 1930, Robert J. Trumpler classified the system as a moderately rich cluster detached

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55 from the Milky Way, with a medium range in brightness for the stars and a strong central concentration. Modern studies reveal that the cluster has a mean reddening of about 0.3 magnitude and a distance of about 6,800 light-years, making its true physical extent some 12 light-years across. In a 1996 paper in the Journal of the Royal Astronomical Society of Canada (vol. 90, p. 329), Douglas Forbes (Memorial University) identified 115 likely cluster members within 6′ of the nucleus; though star counts extend the cluster’s corona out to 12′. He estimated a cluster age of about 25 million years. J. Sanner (Sternwarte der Universität Bonn) and colleagues arrived at a similar conclusion in 2001 (Astronomy and Astrophysics, vol. 369, p. 511). Of specific interest is that the spectroscopic eclipsing binary star AI Crucis is NGC 4103’s fifth-brightest star and is almost certainly a member. Professor Th. Oosterhoff discovered the variable on plates of the Crux region taken at the Southern Station of Leiden Observatory at Johannesburg. The star’s primary dip varies between 9.5 magnitude and 10.3 magnitude every 1.4 days. Today, we know that AI Crucis is a semi-detached B5 and B8 system in which one star in the system has grown to fill its Roche lobe and is passing gaseous material to its companion. Such a system is a good astrophysical laboratory for the formation and evolution of massive close binaries. It is shown that mass transfer between the components and mass loss from the system are two key astrophysical processes that are needed to understand this evolution. In a 2010 paper in Research in Astronomy and Astrophysics (vol. 10, p. 438), Er-Gang Zhao and colleagues discovered that the system’s long-term orbital period is increasing with a rate of 0.86 seconds per

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century. This, they say, may reveal a very cool stellar companion in the system. Although the system is presently undergoing a slow mass-transfer stage, Zhao et al. say, it is insufficient to cause the observed period increase. Thus, they suggest that a strong stellar wind from the hot component is a contributor. “The observed period increase,” they say, “is the result of the combination of mass transfer from the secondary to the primary and mass loss via stellar wind from the massive primary.” A model for the star system reveals that it initially had a 9 solar mass primary and a 6.6 solar mass secondary, with a period of about 1.3 days. Then, after 10 million years, a rapid mass-loss phase lasted on the order of 100,000 years. AI Crucis has now been in a slow phase of mass transfer for the last 150,000 years. “It is estimated that the hot component lost a total mass of 4.1 [solar masses] during the slow mass-transfer stage and, thus, the evolution of the binary system should be changed greatly by the mass loss,” Zhao et al. conclude. To find this dense open cluster and its fascinating variable, use wide-field chart 4 to find Epsilon Crucis. You can now use your binoculars to look 2° to the west-southwest for the four magnitude 6.5 stars that look like a miniature Southern Cross (which is oriented west-northwest to south-southeast) and NGC 4103 among them. Or you can center that star in your telescope at low power and then switch to the accompanying chart. From Epsilon Crucis, move 20′ northwest to 8th-magnitude Star a, then move 50′ west-northwest to the pair of 7th-magnitude stars (b), which are oriented ­northeast to southwest. Now drop almost 1° south-­ ­southwest to the 10′-long chain of three progressively fainter stars (c), which is oriented

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northwest to southeast. NGC 4103 is 30′ south-southwest of Chain c. Through the 5-inch at 33×, the cluster appears as a small and condensed (6′) patch of well-resolved starlight. I immediately counted about a dozen stars of irregular magnitude scattered across its hazy background

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of unresolved starlight, a very pretty sight. Increasing the magnification to 60×, twice as many stars popped into view. The brightest stars at the core form a broken “V” that opens to the south-southwest. The eastern segment of the “V” has more stars; at high power, it is a nice arc of double and multiple stars in three distinct patches. A prominent tail of three roughly 9thmagnitude stars extends west-southwest from the westernmost star in the “V.” And the westernmost star in that line of three is AI Crucis; another similarly bright star can be seen beyond AI Crucis, extending that line even farther. This line is also the “long tail” of the bird I see. The bird’s head is the triple star marking the apex of the “V,” and its body and tail are the northwestern side of the “V” and the three stars extending from it. AI Crucis, then, marks the tip of the tail. The bird’s left wing is formed by the patches of starlight along the eastern side of the “V.” The bird’s right wing is an extension of two stars northwest of the bird’s head. The bird asterism is surrounded by other rays and arcs of irregular starlight – again all packed in a tight little package. Be sure to survey the cluster with high power to see its wealth of pairs and other stellar groupings. It’s quite a pleasing playground of starlight. At low power … just let your imagination fly.

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56 56 NGC 4372 Type: Globular Cluster Con: Musca RA: 12h25.8m Dec: −72°40′ Mag: 7.2 Diam: 5′ SB: 10.7 Dist: ~18,900 light-years Disc: James Dunlop, 1826 james dunlop [April 30, 1826]: [See text.] (D 67) j o hn hers che l : Globular cluster, very faint, large, very gradually brighter in the middle, 6′ diameter, resolved into stars of 15th magnitude, rich in stars; a delicate and faint object; has a star 45′ [northwest], distance 5′ from centre. Almost perfectly insulated in a very large space almost entirely devoid of stars, being the smaller and southern lacuna below the great “coal sack.” (h 3390) ngc: Globular cluster, pretty faint, large, round, stars magnitude 12 to 16.

Bu z z ing a r ou nd t he foot of t he Southern Cross, a tongue’s length east of Chamaeleon, is one of the sky’s tiniest constellations, Musca the Fly. Originally the constellation was known as Apis the Bee, a name Bayer introduced in 1603. Abbé Nicolas Louis de Lacaille is often credited with substituting Musca for Apis in about 1752. But as Richard Hinckley Allen points out in his popular book Star Names: Their Lore and Meaning, Edmond Halley had called the constellation Musca Apis in 1679, 218

and before him Giovanni Riccioli catalogued it as Apis seu Musca. Actually, the constellation’s full Latin name is Musca Australis (Southern Fly) because a Musca Borealis also once buzzed around the northern sky, over the back of Aries the Ram. You also may find some star charts calling Musca Australis by its more taxonomical name, Musca Indica (Indian Fly). What a busy history for such a tiny bee (or fly, as the case may be)! Actually, with an area of 138 square degrees, Musca is deceivingly large, covering slightly more Deep-Sky Companions

56 than twice as much celestial real estate as far more famous Crux. Our target, 7th-magnitude NGC 4372, lies only 45′ southwest of 4th-magnitude Gamma (γ) Muscae and 5′ southeast of a 7th-magnitude star. The nineteenth-century “gentleman scientist” James Dunlop discovered the cluster while surveying the southern skies from Australia with a 9-inch f/12 reflector. It was the 67th of 629 objects he recorded in A Catalogue of Nebulae and Clusters of Stars in the Southern Hemisphere observed at Parramatta in New South Wales (1828). Here is his description: A star of the 6th magnitude, with a beautiful well-defined milky way proceeding from it southeast; the ray is conical, and the star appears in the point of the cone, and the broad or south following extremity is circular, or rounded off. The ray is about 7′ in length, and nearly 2′ in breadth at the broadest part, near the southern extremity. With the sweeping power this appears like a star with a very faint milky way southeast, the ray gradually spreading in breadth from the star, and rounded off at the broader end. But with a higher power it is not a star with a ray, but a very faint nebula, and the star is not involved or connected with it: I should call it a very faint nebula of a long oval shape, the smaller end towards the star; this is easily resolvable into extremely minute points or stars, but I cannot discover the slightest indications of attraction or condensation towards any part of it. I certainly had not the least suspicion of this object being resolvable when I discovered it with the sweeping power, nor even when I examined it a second time; it is a beautiful object, of a uniform faint light.

NGC 4372 is indeed a very open globular cluster. In fact, astronomers do not agree on its size or magnitude; in the literature it ranges from 5′ to 18.6′ in apparent diameter and

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from 7.2 to 8 in magnitude. Archinal summarizes the problem: This cluster is open enough, and has a bright enough star in the field, that it’s no wonder the diameters and total magnitudes are discordant. The Skiff value may correctly indicate that the 25 magnitude per square arcsecond profile lies at diameter of 5′, but this does not seem to represent the overall size of the cluster very well. The half light diameter of 7.79′ matches the obvious size of the cluster on the Digital Sky Survey, and there are outliers to at least 10′. On the other hand, the Deep Sky Field Guide size of 18.6′ would appear to be far too large. So as usual with deep-sky information (as in the cases with NGC 4833 [Southern Gem 59] and 4372), it apparently just depends on what your definition of “diameter” and “total magnitude” is. The diameter is going to depend quite a lot on the limiting magnitude and how one determines where the cluster merges into the background stars. And then given various diameters, of course the total magnitude enclosed varies quite a bit. So there appears to be no definitive sizes and magnitudes for these objects. These observations reveal one truth about deep-sky objects in general: we still have a lot to explore and learn!

Today, we know NGC 4372 is an extremely metal-poor globular cluster 18,900 light-years from the Sun and 23,000 light-years from the Galactic center. Its myriad stars span about 100 light-years of space. Over the years, astronomers have characterized the light curves of 19 variable stars in NGC 4372. Eight of these variables belong to the SX Phoenicis class (metal-poor analogs of pulsating Delta Scuti stars) and eight are contact binaries. The variables range in brightness from 14th to 19th magnitude, so all are within range of modest-sized telescopes when the stars are at maximum light. CCD owners should be able

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to monitor all these stars throughout their variability cycles. All the SX Phoenicis variables are concentrated toward the center of the globular and are blue stragglers. The contact binaries are distributed throughout the cluster. Two more variables that are possible binaries have also been detected. The cluster’s total lack of RR Lyrae variables confirms that it is extremely metal-poor, like NGC 4833. Each of NGC 4372’s stars has about 1/120 the metal content of the Sun. The cluster is approaching us at 72 kilometers per second and has an estimated age of 15 billion years, plus or minus 4 billion years. According to the recent “inside-out” picture of galaxy formation  – where the active star-forming regions gradually move toward the outer regions of the galaxy disk as gas with ever higher angular momentum is accreted – NGC 4372 is predicted to be one of the oldest globular clusters in the Milky Way Galaxy. To find NGC 4372, use wide-field chart 3 to locate Gamma (γ) Muscae. Center that

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star in your telescope at low power and then switch to the accompanying chart. The cluster lies only 45′ southwest of that star, just 5′ southeast of 7th-magnitude Star a. It also lies about 1/4º due east of the southern tip of a 2 1/2º narrow river of obscuring matter called the Dark Doodad. The Doodad starts just 1 1/4º due south of Alpha (α) Crucis, the southernmost star of the Cross, and flows southwest. The presence of this dark cloud also indicates that the cluster itself is probably being dimmed by dust. Telescope users under dark skies should try to sail “down” this river because it is almost free of starlight; one pair of stars (roughly 10th and 11th magnitude) lies nearly at the halfway mark; the brighter of them is 168 light-years distant. Can you see the Dark Doodad with binoculars? With the naked eye? Under dark skies, NGC 4372’s tiny puff of light is overpowered by the neighboring 7th-magnitude star. But see if you can’t pick it out with averted vision in binoculars. The view through large binoculars or a 60-millimeter refractor gives the best impression of Dunlop’s conical ray of light following from the star  – like the comet’s tail flowing from its stellar head. I could barely see it at 19× under moonlight through the 4 1/2-inch f/7 finderscope on the 20-inch f/13.5 Zeiss reflector at Auckland Observatory; it was only a weak glow. Through the 20-inch itself, the cluster displayed loose ribbons of starlight lying atop a

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semitransparent, wrinkled sheet. I could also imagine a slightly denser sheet punctuated by the light of two roughly 12th-magnitude stars.

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Each ribbon was part of a wavy row of 13thto 14th-magnitude stars that flowed more or less parallel to one another in a northeast to southwest direction. Dark troughs separated these waves, though they were not completely black, being lightly peppered with dim stars. These wavy features and their dark troughs reminded me of the “rippled dunes” of starlight I saw in the open cluster M25 in Sagittarius through my 4-inch refractor. If you defocus the cluster slightly and use averted vision, can you locate some very thin “Dark Doodads” marring the wrinkled sheet? My drawing of NGC 4372 is a composite of lowpower views with the Auckland Observatory 4 1/2-inch finderscope and higher-power views with the 20-inch.

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57 57 NGC 4696 Type: Peculiar Elliptical Galaxy (E1 Peculiar) Con: Centaurus RA: 12h48.8m Dec: −41°19′ Mag: 10.2 Dim: 5.4′ × 3.9′ SB: 13.4 Dist: ~150 million light-years Disc: James Dunlop, 1826 j ames dunl o p [May 7, 1826]: A faint nebula, about 12″ or 15″ diameter, a little brighter to the centre, very faint at the margin. (D 510, D511 = NGC 4709) j o hn hers che l : Pretty bright, large, round, gradually brighter in the middle, 2′ resolvable. (h 3424) ngc: Pretty bright, large, round, gradually brighter in the middle, resolvable (mottled not resolved).

N G C 4696 is a smal l , pe cu l i ar ­g al axy about 7 1/2° southwest of 3rd-magnitude Iota (ι) Centauri, or about 1 1/2° southwest of 4th-magnitude n Centauri. James Dunlop, its discoverer, found it faint. And John Herschel thought he could resolve it. Indeed, by the second decade of the twentieth century, it was suggested that the object was not a nebula but a very distant, unresolved globular cluster. This demonstrates how the visual and early photographic appearances of distant objects could easily deceive us. NGC 4696 is the brightest and most central galaxy of the rich, nearby Centaurus cluster of

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galaxies (Abell 3526). Abell 3526 is actually the main subgroup of the much larger Centaurus supercluster of galaxies  – the nearest large supercluster. It lies near the Hydra Cluster (Abell 1060) and the very heavily obscured Norma Cluster (Abell 3627). Other members of Abell 3526 include NGC 4645, NGC 4677, NGC 4683, NGC 4706, NGC 4709, NGC 4743, NGC 4744, and NGC 4767. NGC 4696 itself is a fascinating elliptical­like system seen 45° from face-on. Its continuum spectrum is typical of early-type galaxies. For years it was considered to be a cD system, meaning it is a diffuse supergiant similar to,

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but much larger than, a lenticular or an elliptical galaxy. But NGC 4696, which spans some 240,000 light-years in true physical extent and shines with a luminosity 10 billion times that of the Sun, is now argued to be an early elliptical but one, literally, with a “twist.” Ellipticals differ from spirals in that they are large, amorphous accumulations of stars with no spiral structure or active star-forming regions. They most likely formed during collisions between spiral galaxies. As the galaxies collide, their dust and gas smash into one another, triggering brief bursts of star formation. In time, however, star formation stops, leaving the new and large conglomerate system to grow older and fainter. But, as the accompanying 2010 Hubble Space Telescope image release shows, NGC 4696 has a dust feature that looks unlike those in other elliptical galaxies; this curious 30,000-light-year-long dust band curls around itself “like a question mark.” Viewed at certain wavelengths, the Space Telescope Science Institute release explained, ”strange thin filaments” of ionized hydrogen are visible within it. The HST image reveals these structures as a subtle marbling effect across the galaxy’s bright center. NGC 4696 also has a hidden mystery: As both Dunlop and Herschel noted, the object

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gets brighter in the middle, and modern deep images show it to have a very bright nucleus. Now we know that a supermassive black hole lurks inside this intense core, blowing out jets of matter at nearly the speed of light. Indeed, as reported in an April 2006 press release, NASA’s Chandra X-ray Obser­ vatory discovered a vast cloud of hot gas surrounding high-energy bubbles 10,000 light-years across on either side of the central disk that surrounds NGC 4696’s supermassive black hole. “Surprisingly,” the release explained, “the results indicate that most of the energy released by the infalling gas goes, not into an outpouring of light as is observed in many active galactic nuclei, but into jets of high-energy particles. Such jets can be launched from a magnetized gaseous disk around the central black hole, and blast away at near the speed of light to create huge bubbles. An important implication of this work is that the conversion of energy by matter falling toward a black hole is much more efficient than nuclear or fossil fuels. For example, it is estimated that if a car was as fuel-efficient as these black holes, it could theoretically travel more than a billion miles on a gallon of gas!” So, when it comes to energy efficiency, black holes really are “green.” To find this little patch of extragalactic splendor, use wide-field chart 3 to find Iota Centauri and then n Centauri. Center n Centauri in your telescope at low power and then use the chart on page 224 to confirm the field. From n, move 1° southwest to 7th-­ ­magni­tude Star a. Then drop about 20′

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south-southwest to 8.5-magnitude Star b. NGC 4696 is about 15′ south of Star b. Through the 5-inch at 33×, the galaxy is a small (2′), amorphous, elliptical glow. And

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that’s essentially how it remains at all powers, only that at 60× and 94× the galaxy reveals a small brightening in the center, with another star immediately to the northwest and a dimmer one to the southeast. But I have to admit that at times the galaxy appears mottled, and this, I believe, is a result of the eye scanning between these two stars in this small area, which tends to break up the illusion of uniform light across the galaxy’s face. Even through her 12-inch telescope, Magda Streicher says that this galaxy is “very difficult to detect. With averted vision the relatively large round glow can be seen, but with higher magnification it completely disintegrates into just about nothing. However, with a UHC filter I could barely see the nucleus embedded in the haze of the galaxy. I think the low surface brightness causes the galaxy to be a difficult target, but the more you gaze at the galaxy the clearer it becomes.” Users of large telescopes should again be aware that NGC 4696 lies at the heart of the Centaurus cluster of galaxies, and the field is replete with faint members, including 13thmagnitude NGC 4706 about 12′ to the east, 11.5-magnitude NGC 4709 about 15′ to the southeast, and 12.8-magnitude NGC 4696B about 15′ to the west-northwest.

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58 58 Kappa Crucis, Jewel Box NGC 4755 Type: Open Cluster Con: Crux RA: 12h53.6m Dec: −60°21.4′ Mag: 4.2 Diam: 10′ SB: 9.2 Dist: ~4,900 light-years Disc: Abbé Nicolas Louis de Lacaille, included in his 1755 catalogue a bbé ni co l as l ou is de l acail l e : Five to six stars between two mag. 6 stars. (II-12) j a mes dunl o p : (χ Crucis, Bode) is five stars of the 7th magnitude, forming a triangular figure, and a star of the 9th magnitude between the second and third, with a multitude of very small stars on the south side. (D 301) j o hn hers chel : The central star (extremely red) or a most vivid and beautiful cluster of from 50 to 100 stars. Among the larger there are one or two evidently greenish; south of the red star is one 13th magnitude, also red; and near it is one 12th magnitude, bluish. (h 3435). ngc: Cluster, very large, stars very bright (Kappa Crucis).

Th e Jewe l B ox, a t ru e t re asu re among open clusters, adorns the Holy Grail of constellations, novelist Edward Robert Bulwer-Lytton’s “great cross of the South.” If there is one constellation wholly symbolic of travel and adventure, it is the Southern Cross. Early sailors saw the Cross as a good omen

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sent to guide them across uncharted seas. But before I continue, a note of clarification is in order. Crux, the constellation’s formal name, means “Cross,” not the Southern Cross (which would be Crux Australis). Astronomers often refer to it as the “Southern Cross” because we need to differentiate it from the Northern

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58 Cross  – an asterism in Cygnus the Swan. “Southern Cross” has become so entrenched in our vocabulary, however, that most modern references equate Crux with it. But that’s as wrong as saying Corona means “Southern Crown” or that “Aurora” means the “Southern Lights.” The Cross first appeared as such on celestial globes in 1592. Before that its stars belonged to Centaurus and could be seen on Arabic globes dating to 1225. Somehow Dante Alighieri  – “the spokesman of ten silent centuries,” as nineteenth-century Scottish essayist and historian Thomas Carlyle called him  – learned of the stars’ existence, probably through conversations with the great adventurer Marco Polo or other equally well-traveled men, for he alluded to the stars in his Divine Comedy, which he completed in 1321. In that tale, after Dante and Beatrice Portiani ascend from Hades on the other side of the world, the first thing they see when looking skyward are four brilliant stars representing the four principal virtues: Justice, Prudence, Fortitude, and Temperance. Among the first true navigators to glimpse this “other side of the world” was Christopher Columbus. And it was on his maiden voyage of discovery in 1492 that Columbus glimpsed four stars in the form of a cross (Crux) over the West Indies. Surprisingly, he believed that these stars were the ones referred to in Dante’s allegory. Richard Hinckley Allen notes that on Amerigo Vespucci’s 1501 voyage, Vespucci “called to mind the passages from Dante, insisting that he himself was the first of Europeans to see Four Stars.” Alas, Vespucci did not see these stars in the form of a Cross but as Mandorla, “an almond.” Vespucci’s reference was not to the tasty treat but to the vescia piscis or “oblong glory”

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s­ urrounding the bodies of saints ascending to heaven in Italian art. I believe the romance of the Southern Cross lies not in its handful of brilliant stars but in the visual majesty of the Milky Way surrounding it. The Southern Cross marks the eastern end of a swath of swirling madness running between 10 and 13 hours of right ascension and from  –55° to  –65° in declination. This region contains about two dozen open star clusters (including IC 2602: Southern Gem 47: the Southern Pleiades), about a dozen nebulae (including the Eta Carinae Nebula: Southern Gem 48), thick star clouds, and eerie lagoons of darkness such as the Coalsack. No other part of the visible heavens draws more attention to itself than this. The Southern Cross simply happens to be the most obvious asterism of stars in this Van Goghian celestial landscape. Yet, of all the clusters that dapple this region, none has captured the imagination more than the Jewel Box. Early explorers saw it as a 4th-magnitude star and designated it Kappa (κ) Crucis. Many believe that John Herschel discovered the cluster, because his description of it led to the cluster’s popular Jewel Box nickname: “[T]his cluster, which though neither a large nor a rich one, is yet an extremely brilliant and beautiful object when viewed through an instrument of sufficient aperture to show distinctly the very different colour of its constituent stars, which give it the effect of a superb piece of fancy jewellery.” But discovery credit passes to Abbé Nicolas Louis de Lacaille, who catalogued Kappa Crucis as a “nebulous cluster” during his 1751–1753 exploration of the southern skies. It is listed as the 12th object in his 1755 catalogue under Class II, with the description “five to six stars between two mag. 6 stars.”

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58 That Lacaille saw the cluster as nebulous is not surprising; he used tiny telescopes of 1/2-inch aperture. Through such instruments, any background of faint stars would have appeared as a milky haze. James Dunlop split that haze with his 9-inch telescope in Parramatta, New South Wales. His notes say that Kappa Crucis “is five stars of the 7th magnitude, forming a triangular figure, and a star of the 9th magnitude between the second and third, with a multitude of very small stars on the south side.” At the heart of the Jewel Box, John Herschel noticed a bright “extremely red” ruby among a smattering of emeralds “of different shades of green.” In all, Herschel found eight stars in which the “colour is conspicuous.” Many sources identify this ruby star at the center of the cluster as being the star Kappa Crucis, but it is not. That central star (a type M2Iab red supergiant known as SAO 252073) is too faint, at magnitude 7.2, to have received a Bayer designation. According to the Yale Bright Star Catalogue, Kappa Crucis is the magnitude 5.9 star at the southeast end of a pyramid of four 6th- to 8th-magnitude stars. But one could argue that naked-eye observers who first charted Kappa Crucis saw the entire cluster as a naked-eye star, just as they did with Omega Centauri. On many nights, I have gone out and looked at the Jewel Box with the naked eye, and I have convinced myself that it takes too much effort to isolate HD 111973 from the rest of the compact cluster. So, just as no single star in the Omega Centauri cluster is Omega, perhaps no star in the Jewel Box is Kappa. The Jewel Box packs 281 known members into an area only 10′ (1,114 light-years) across. The hypothetical inhabitants of a planet orbiting any of the cluster’s dimmer stars would have one of the most glorious night

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skies imaginable: four Rigel-like stars blazing brighter than a quarter Moon, with hundreds of other stellar companions projected against luminous splashes of Milky Way and dark, dusty voids. The light we see shining from these stars today began its journey across space shortly before the Syrians slayed Ahab, King of Israel, at Ramoth-Gilead. In the May 1995 Astronomy and Astrophysics Supplement Series (vol. 111, p. 75), Ram Sagar and R. D. Cannon estimate the age of the Jewel Box to be about 10 million years, placing it in the same age group as the Double Cluster in Perseus and the Tau Canis Majoris Cluster; the Jewel Box first began to shine, then, about the time mammals began to proliferate. After sampling 813 stars, some as faint as magnitude 20, Sagar and Cannon determined that obscuring dust is not uniformly distributed across the face of the cluster, though the extinction values have a mean value of 0.4 magnitude (30 percent). The cluster’s stars formed nearly at the same time from a molecular cloud that might have existed for a minimum period of about 6 to 7 million years. To date, a total of nine Beta Cephei variables have been discovered in it. To find the Jewel Box, use wide-field chart 3 or 4 to locate 1st-magnitude Beta (β) Crucis, the eastern arm of the crosspiece. Center that star in your telescope at low power and then switch to the chart on page 228. From Beta Crucis, move about 45′ southeast to 6th-magnitude Star a. The Jewel Box is only about 12′ east of that star. I find the Jewel Box visible to the naked eye as a 4th-magnitude “star” even under the light of a half Moon. Binoculars will bring out its four bright stars (even under moonlight) against a background of misty light. The four bright stars form a tiny pyramid  – the triangular figure seen by

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Dunlop. Here’s a challenge: Can you resolve its two brightest members with the naked eye? Both shine at 6th magnitude and are separated by about 1.5′. I resolved these two stars with effort from Zambia in June 2001. At 23×, through the 4-inch (with south “up”) the cluster looks like an ice cream cone. The bright pyramid of stars is the cone, a faint sprinkling of stars to the south and west is the ice cream, and a roughly 11th-magnitude pair of stars topping them is the fuzzy cherry on top. The Jewel Box also looks like two superimposed clusters, one bright, the other faint. Under a direct telescopic gaze, its brightest stars seemed chiseled out of black marble. Look with averted vision at the 6thmagnitude star in the southeastern corner of the pyramid – the Yale Kappa, or HD 111973 – and see if the area around it doesn’t sparkle with faint stars. HD 111973 is the jewel at the center of a pendant-shaped asterism of 10thmagnitude stars that is better seen at high magnification. At 72×, the cluster fractures into smaller groupings. The pyramid remains bold and bright. The pendant containing HD 111973 reveals at least seven stars that form a smaller pyramid at right angles to the larger brighter 228

one. HD 111973 looks yellow at 72×, as does HD 111904, the magnitude 5.8 star at the northwestern corner of the large, bright pyramid. In February 1997 and again in April 2000, HD 111973 appeared at least 0.5 magnitude (60 percent) brighter than HD 111904 to my eyes  – even though, according to Sky Catalogue 2000.0 data, it should be 0.1 magnitude fainter. An undated drawing by Kenneth Glyn Jones in Volume 7 of the Webb Society Deep-Sky Observer’s Handbook (published in 1987) shows HD 111973 as significantly fainter than HD 111904, and this observation was based on a view through a 12-inch reflector. Most photographs I’ve seen show HD 111973 fainter than HD 111904, but that may be an artifact of the photographic emulsion. (Note that in both Ernst Hartung’s and Robert Burnham Jr.’s handbooks the photographs of the cluster have been flipped, with east and west reversed.) Clearly these stars warrant watching. At 72×, through the 4-inch, SAO 252073 looks more orange than red to me. It is the northeasternmost star in a tiny line of three similarly bright stars oriented northeast to southwest. SAO 252073, to my eyes, is also the faintest of these three stars. Glyn Jones, however, saw it as the brightest. My eyes tend to Deep-Sky Companions

58 be blue-sensitive, while other observers are more sensitive to red light, so this discrepancy in brightness might be related to how different observers perceive color and not to any real brightness variations. Yet Edward James Stone, Her Majesty’s Astronomer at the Cape from 1870 to 1879, suspected that this red star altered its brightness since Sir John Herschel observed it. And the Hipparcos satellite’s database seems to support the notion, for on its watch the star’s apparent visual magnitude appears to have ranged from 7.08 to 7.52. However, Hipparcos didn’t enable astronomers to determine a period of variability or to classify the star’s light curve. Clearly it would behoove someone with a photometer to monitor all the Jewel Box’s supergiant stars for variability. Moderate magnification reveals other treasures in the Jewel Box. The brightest stars to the southwest of the large pyramid form two semicircles, like wide crowns, facing in opposite directions: one opens to the

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north-northwest and the other to the southeast. Immediately south of these crowns is a long chain of fainter stars oriented in the same northwest-to-southeast direction as the two semicircles. The void between these groupings may be a long lane of dust. At high power, the cluster fractures completely and the Jewel Box loses its luster. Most prominent is HD 111973 and its pyramidal pendant. Look for a black cross at the southwestern edge of the pyramid. Look also for a tight double immediately to the northwest of HD 111973 and one about 2′ southeast. For a slightly greater challenge, see if you can detect a 13th-magnitude red star south of SAO 252073. It is one that Herschel first noticed, along with so many others of the Jewel Box’s sparkling gems (though he didn’t say how far south of SAO 252073 it is): “Among the larger,” he says, “there are one or two evidently greenish; south of the red star is one 13th magnitude, also red; and near it is one 12th magnitude, bluish.”

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59 59 The Southern Butterfly NGC 4833 Type: Globular Cluster Con: Musca RA: 12h59.6m Dec: −70°52.5′ Mag: 8.4 Diam: 13.5′ SB: 14.1 Dist: ~19,600 light-years Disc: Abbé Nicolas Louis de Lacaille, included in his 1755 catalogue abbé ni co l as l ou is de l acail l e : A small faint comet. (I-4) j ames dunl o p: (12 Muscae, Bode) This is a pretty bright round nebula, about 4′ diameter, moderately condensed to the centre. This, with the sweeping power, has the appearance of a globe of nebulous matter with very small stars in the [northeast] margin. But with a power sufficient to resolve it, the globular appearance vanishes in a very considerable degree; and the brightest and most condensed part is to the west side of the centre, with the stars considerably scattered on the [northeastern] side. Resolvable into stars of mixed small magnitudes. A small nebula [is west of ] this. (D 164) j o hn hers che l : Globular, bright, large, round, gradually brighter in the middle, stars 14th magnitude, and one 7th magnitude [northwest] of the centre; a fine object. (h 3444). ngc: Globular cluster, bright, large, round, gradually then suddenly brighter in the middle, stars of magnitude 12.

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Deep-Sky Companions

59 N G C 4 8 3 3 is a bri ght and pre t t y globular cluster near 3.6-magnitude Delta (δ) Muscae and 3° northeast of globular cluster NGC 4327 (Southern Gem 56). Abbé Nicolas Louis de Lacaille (1713–1762) discovered NGC 4833 in 1752 during his two-year exploration of the southern skies from the Cape of Good Hope in South Africa. It is included in his list of 42 southern nebulae and clusters, and he lists it as a Class I object: “nebulosities not accompanied by stars”; Lacaille also likened its appearance to that of a small, faint comet. John Herschel classified NGC 4833 as a Class VII object  – a very loose and sparse cluster. Although the cluster is bright, there’s a difference in opinion about its size and magnitude. The Deep Sky Field Guide lists NGC 4833’s magnitude as 7.0 and its diameter as 13.5′. The late Ernst Hartung doesn’t re­cord a magnitude in his book Astronomical Objects for Southern Telescopes but says it’s a “fairly compact globular cluster with small outliers to a diameter of about 4′.” Burnham’s Celestial Handbook offers a magnitude of 8.5 and a diameter of 6′. In his globular cluster database, Brian Skiff lists a magnitude of 8.5 but gives no diameter. The professional literature is also confusing. “The [cluster’s] total magnitude,” Brent Archinal explains, “apparently is poorly determined, given the range of values in the professional papers of 6.80 to 8.35. However, this is mostly dependent on what diameter was chosen for this total magnitude. The cluster has a fairly obvious ‘half-light’ core that extends to a diameter of about 5′ with outer halo stars out to at least about 10′ on the Digital Sky Survey. Thus the 13.5′ Deep Sky Field Guide value … seems a little large, especially for visual observers.” The values I’ve tabulated come from the Catalog of Parameters for Milky Way Globular

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Clusters that McMaster University astronomer William E. Harris compiled in 1999, and from the Deep-Field Guide to Uranometria 2000.0. Adopting an angular diameter of 13.5′ and a distance of 19,600 light-years implies that the cluster’s physical diameter is 77 light-years, making it about two-thirds the size of NGC 4372. NGC 4833’s color–magnitude diagram places it in a class of extremely metal-deficient globular clusters, like NGC 4372. The cluster’s stars are metal poor because they formed about 15 billion years ago from gas that contained only a sprinkling of heavy elements. Each star in NGC 4833 contains, on average, only about 1/60 as much metal as there is in the Sun. To find NGC 4833, use wide-field chart 3 to find Delta Muscae. The globular lies only 3/4° northwest of that star. I spotted NGC 4833 in 10 × 50 binoculars from city-bound Auckland Observatory, under the light of a waxing gibbous Moon. It lies just 15′ south of an 8th-magnitude star. Through binoculars, the cluster’s core appears to possess a stellar radiance. But through the 4 1/2-inch finderscope on the observatory’s 20-inch reflector, I discovered

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59 that this was an illusion created by the light of a 9th-magnitude star superimposed on the outlying members of the cluster, only about 2.5′ north of the compressed core. Through the 20-inch, the cluster’s core looks layered. The brightest inner part measures about 2′ and appears distinctly square. The square is lined by moderately bright stars, making the core look rather hollow (save for a small clustering of about four tightly packed stars  – in the very center). A dark lane running east to west separates the box from a thin, similarly sized bar of stars to the north. The rest of the inner core sweeps out from this main body in four magnificent loops, to the northeast, northwest, southwest, and southeast, like the wings of a butterfly (thus my nickname for the cluster). The “butterfly” appears to be resting in a field of stars, which may or may not be related to the cluster. The drawing here reflects the view through the 4 1/2-inch finderscope (at powers of 19×, 75×, and 150×) and through the Auckland Observatory 20-inch reflector. The cluster’s fainter, roughly 5′-wide outer halo of stars was not noticeable as such in the 20-inch. Perhaps if I had seen the cluster at

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low power at a dark site with no Moon I would have realized that there was more to it than first meets the eye. In any case, the outer shell is obvious in photographs, though it is hard to tell where the cluster ends and the star field of the Milky Way begins. (No wonder astronomers had trouble determining the cluster’s angular size and apparent magnitude.) By the way, NGC 4833’s brightest star shines at magnitude 12.4, and the horizontal-branch magnitude of the cluster is 15.5, meaning it can be well resolved in moderate-sized amateur telescopes.

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60 60 The Tweezers Galaxy NGC 4945 Type: Spiral Galaxy (SBcd) Con: Centaurus RA: 13h05.4m Dec: −49°28′ Mag: 8.8 Dim: 18.6′ × 3.5′ SB: 14.0 Dist: ~17 million light-years Disc: James Dunlop, 1826 j a mes dunl o p [April 29, 1826]: [See the text.] (D 411) j o hn hers chel : Bright; very large; very much elongated; very gradually a little brighter in the middle. Length much more than a diameter of the field, or than 15′. Its light extends to a star 14th magnitude beyond the parallel of Brisbane 4299. Position of elongation 38.7°. (h 3459) ngc: Bright, very large, very much extended toward position angle 39°.

Th e s k y i s f u ll of i l lu si ons. For instance, at times we can see the Milky Way encircling us, making us appear to be at the center of the Galaxy; in reality, we are well off-center but see several spiral arms meshed together into an apparent ring. Another common illusion relates to double stars. Not all double stars are pairs of physically related stars; the stars in some doubles are many light-years apart but happen to lie almost directly along the same line of sight. This same “coupling” phenomenon occurs

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with galaxies. Indeed, galaxies are master illusionists. Most curious are those bizarre, apparently disturbed galaxies whose photographic images were collected in the 1960s by one of Edwin Hubble’s students, Halton “Chip” Arp. Arp investigated relationships between these disturbed galaxies and quasars found nearby. As Dennis Overbee shares in his Lonely Hearts of the Cosmos, “He turned out to be a genius at finding mystery. Every funny galaxy he inspected turned out to have

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60 a quasar tucked under an arm or at the end of a tendril of gas, or lines of them nearby. He photographed luminous bridges of gas that appeared to link galaxies.” Arp also proposed physical relationships between several radio galaxies and other more “normal” galaxies that, in most astronomers’ views, simply happen to lie in roughly the same direction in the sky. One of the more exotic and intriguing radio galaxies he collected was NGC 5128 (Southern Gem 61), the peculiar radio galaxy Centaurus A. One of the two jets of radio-wave-emitting plasma streaming out of Centaurus A, Arp noted, pointed right to our next target  – the nearly edge-on barred spiral galaxy NGC 4945. Although NGC 4945 is about 7 1/2° due southwest of Centaurus A in the sky, the two belong to the Centaurus group of galaxies and lie at similar distances from Earth. Does the radio jet connect the two galaxies? Maybe, maybe not. “Most astronomers dismissed Arp’s diagrams and his computer-processed photographs as coincidences,” Overbee writes. But the notion of an “intergalactic pipeline” is not totally absurd, especially since the Hubble Space Telescope (HST) has imaged a dark string of material flowing between two battered galaxies that bumped into each other about 100 million years ago. That pipeline begins in NGC 1410 and wraps around NGC 1409 like a ribbon around a package. Astronomers used the HST to confirm that the 20,000-light-year-long pipeline is a continuous string of material linking these compact galaxies, and it probably was created during a tussle between them. In photographs, NGC 4945 is a similarly chaotic brew of stellar clumps and dusty patches seen through a scrim of foreground stars. The galaxy is quite large, spanning 77,000 light-years, and is receding at 560 kilometers

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per second. It has a luminosity 28 billion times that of the Sun and is the third-brightest galaxy in the IRAS Point Source Catalog (a list of objects whose brightnesses were measured at far-infrared wavelengths by the Infrared Astronomical Satellite). Most of the galaxy’s infrared radiation comes from a compact region around the nucleus, which is surrounded by a dense torus of dust roughly 3 light-years wide. Seyfert galaxies, such as NGC 4945, are by nature characterized by small, bright nuclei that emit intense and variable x-ray, infrared, and radio radiation while looking comparatively normal at visible wavelengths. Unlike the spectra of most spiral galaxies, which display a continuum and a few absorption lines, those of Seyferts are dominated by prominent emission lines. The emission lines probably come from interstellar gas being heated near the nucleus by x-rays streaming out of the nucleus. There are two types of Seyfert galaxies. Type 2 Seyferts (such as NGC 4945) have narrower emission lines than Type 1 Seyferts.The difference between the two Seyfert types is a consequence of the galaxy’s apparent tilt. Type 1 emissions are associated with face-on galaxies, which allow us a clear view of the rapidly moving gas near the core. Because the cores of edge-on galaxies such as NGC 4945 are obscured by dust, we get to see only the narrow emissions coming from slow-moving gas located farther away from the center. After NGC 4151 in Canes Venatici, NGC 4945 is one of the closest galaxies where an active galactic nucleus and starburst coexist. It is also the brightest Type 2 Seyfert galaxy known at high x-ray energies, leading astronomers to believe that its nuclear activity may be powered by a supermassive black hole. In a 2000 paper in Astronomy and Astrophysics

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60 (vol. 357, p. 24), Alessandro Marconi and his colleagues discuss how they used the HST to detect a roughly 300 light-year-wide starburst ring as well as a conical “super bubble” blown out by winds generated by supernova explosions. Yet not even the HST could penetrate the opaque central torus to reveal the active nucleus. With the currently available data, it is still not possible to establish whether the intense energy streaming from the nucleus is powered by the active galactic nucleus or by the starburst. If the active galactic nucleus dominates, it must be fully obscured. NGC 4945 was the focus of a 2009 European Space Agency press release that likened the galaxy’s swirling, luminous arms and a bar-shaped central region to our Milky Way seen edge-on. To find this extragalactic treasure with a supermassive black hole “devouring reams of matter and blasting energy out into space,” use wide-field chart 3 to find the 1 1/2°-long triangle of 4th- and 5th-magnitude stars including Xi1 (ξ1) and Xi2 (ξ2) Centauri. NGC 4945 is 20′ due east of Xi1. The galaxy is a 9th-magnitude, 19′­long needle of light oriented northeast to

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southwest. From Hawaii, I find it very challenging in 7×35 binoculars, but with time its silvery haze glints in and out of view. Seeing it requires keen averted vision and patience. Look for a fine thread, sharp but fuzzy, like a dew-laden strand of cobweb reflecting the first glimmer of dawn. In the 4-inch at 23×, the galaxy is very apparent as a slender and diffuse shaft of light, with some patchiness and hint of a broad central region. The view begs for more power. And at 72× the galaxy begins to crumble into tiny details, all requiring time and averted vision. Most prominent is the bright nub at the galaxy’s core, which has a faint skirt of light extending on either side of the major axis. The disk of the galaxy on both ends looks gutted, so that the northwestern and southeastern rims look sharply defined and bright, like the prongs of a pair of tweezers, thus my nickname for the galaxy. A fairly bright star or clump punctuates the northeast end of the galaxy, and a slightly fainter though prominent clump lies due

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60 east of the bright hub on the southeast rim. The galaxy appears to bulge in that direction, making the northeast section of the disk appear narrow. But photographs show this to be an illusion created by a dense ribbon of dust blocking the light. A series of clumps in an arc bowing to the southwest surround the nuclear region on that side and give it a slightly parabolic look. The northern “prong” on the southwest side of the disk appears kinked. The view at high power is quite interesting. The galaxy no longer appears uniform in brightness along its full extent, but the southwestern end is dramatically more obvious. Interestingly, this is exactly what the galaxy’s discoverer, James Dunlop, observed: A beautiful long nebula, about 10′ long, and 2′ broad, forming an angle with the meridian, about 30˚ [southwest] and [northeast]; the brightest and broadest part is rather nearer the [southwest] extremity than the centre, and it gradually diminishes in breadth and brightness towards the extremities, but the breadth is much better defined than the length. A small star near the north, and a smaller star near the south extremity, but neither of them is involved in the nebula. I have strong suspicions that the nebula is resolvable into stars, with very slight compression towards the centre. I have no doubt but it is resolvable. I can see the stars, they are merely points. This is [northeast] the first [Xi] Centauri.

You can sense the enthusiasm of the moment in that passage. Clearly Dunlop was picking out the clumps of starlight shining through the long dust lanes and patches that run the

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length of the galaxy, as well as a number of foreground stars in the Milky Way rich section of sky. In his 12-inch, the late Ernst Hartung observed the northwest side as slightly convex and fairly uniform in brightness except toward the fading ends. And in her 13.1-inch reflector at 105×, Barbara Wilson saw it as a “long wispy streak of light that filled [the] eyepiece [and] looks like a flying saucer.” Through her 16-inch f/10 Schmidt-­ ­Cassegrain telescope, South African observer Magda Streicher saw this “pencil-like galaxy, getting slowly brighter to the middle with a few foreground stars embedded. The southern part of the galaxy is not as bright when viewed in contrast with the more outstanding northern part (290×). It looks mottled in parts (462×). In a fine star-field, towards the west, runs a chain of small stars that appears to skip away from the galaxy and which is underlined with Xi1 Centauri to the southwest.” Overall the galaxy reminds me of M82, what with the interplay between its dark lanes and its illuminated portions. The real challenge I found at high power was to resolve the dark notch (“eyelash”) between the eastern clump and the nublike core. Much of the dark matter in this galaxy can be inferred from the patchiness of the luminous sections. Take your time and be patient. I would suggest spending several nights on this galaxy. Each time you observe it, the more you will see. Before moving on, try for NGC 4976, a 10thmagnitude spiral galaxy about 30′ due east of NGC 4945. It’s a nice compact (5.4′ × 3.3′) object that can be mistaken for a star. A star of 9th magnitude abuts it to the east.

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61 61 Centaurus A, The Hamburger Galaxy NGC 5128 Type: Galaxy (S0p) Con: Centaurus RA: 13h25.5m Dec: −43°01′ Mag: 6.7; 6.6 (O’Meara) Dim: 16.7′ × 13.0′ SB: 13.7 Dist: ~16 million light-years Disc: James Dunlop, 1826 j a mes dunl o p [April 29, 1826]: [See the text.] (D 482) j o hn hers chel : A most wonderful object; a nebula very bright; very large; little elongated, very gradually much brighter in the middle; of an elliptic figure, cut away in the middle by a perfectly definite straight cut 40 arcseconds broad; position angle 120.3°; dimensions of the nebula 5′ · 4′′. The internal edges have a gleaming light like the moonlight touching the outline in a transparency. (h 3501). ngc: Remarkable, very bright, very large, very much extended toward position angle 122°, bifurcated.

N G C 5128 in Ce ntau ru s i s t he most dynamic and intriguing galaxy in the heavens. It is virtually a superlative in every region of the electromagnetic spectrum. It is one of the brightest naked-eye galaxies; by far the nearest and most violent Seyfert-type galaxy known; one of the most intense radio sources in the heavens; and a wellspring of infrared, x-ray, and gamma-ray radiation. The very Southern Gems

sight of its photograph tells us it is the epitome of mystery, a rebel against extragalactic conformity. Its background glow resembles that of an elliptical (E0) galaxy, but its face is masked, Zorro-like, by a severely warped, highly opaque band of dust  – one much wider and more chaotic than that found in any edge-on spiral. Compare this celestial egg with any galaxy in the Messier catalogue

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61 and you will see why NGC 5128 has been the nerve center of astronomical debate ever since James Dunlop discovered it almost two centuries ago. Dunlop first encountered this visual oddity while observing from Parramatta, New South Wales. His description of it is rather lengthy: “A very singular double nebula.… [T]he northern and rather smaller nebula is faint in the middle, and has the appearance of a condensation of the nebulous matter near each extremity. These two nebulae are completely distinct from each other, and no connection of the nebulous matters between them.” Several years later, John Herschel would call NGC 5128 “a very problematic object, and must be regarded at present to form a genus apart, since it evidently differs from mere ‘double nebulae,’ not only in the singular relation of its two halves to each other (having each a well and an ill defined side, their sharply terminated edges being turned towards each other and exactly parallel) but also by the intervention of the delicate nebulous streak intermediate between them and lying in exactly the same general direction.” How bizarre this new nebula must have seemed to Herschel, who, like his father, believed the Milky Way and all nebulae were nothing but stars. What was the dark band that had seemingly ripped apart this cloud? In his 1849 “Outlines of Astronomy,” Herschel prophetically decided that it was “a similar but much more strongly marked case of parallel arrangement than that noticed by Mr. Bond (in the Andromeda Nebula) … one in which two semi-ovals of an elliptically formed nebula appear cut asunder and separated by a broad obscure band parallel to the larger axis of the nebula, in the midst of which a faint streak of light parallel to the sides of the cut appears.”

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Herschel’s reference to Bond was critical, for Bond had observed in 1847 two parallel dark “canals” in the Andromeda “Nebula,” one of the last strongholds of William Herschel’s nebular theory, which stated that these masses of nebulous matter were in the process of condensing into new solar systems. By 1888, Isaac Roberts had successfully photographed the dark lanes in the Andromeda “Nebula,” as did Edward Emerson Barnard two years later. Soon these features would be recognized as dust lanes, part of the visible framework not of spiral nebulae but spiral galaxies. Still, NGC 5128’s appearance and nature remained a mystery. In 1918, Heber D. Curtis (Lick Observatory) classified it as an “edgewise spiral with a dark lane.” Five-hour exposures taken of it from Arequipa, Peru, with the Harvard College Observatory’s 24-inch Bruce refractor, and a 10-hour exposure taken with Harvard’s 60-inch reflector in South Africa in 1934, led John S. Paraskevopoulos to conclude that NGC 5128 could not be an external galaxy. Indeed, Edwin Hubble himself detected gaseous emission lines emanating from the object, and for this reason he classified it in 1922 as a local nebulosity. But 10 years later Harlow Shapley and Adelaide Ames listed NGC 5128 as an irregular galaxy in their now famous catalogue. Shapley explained his reasoning in his 1947 work Galaxies. NGC 5128, he said, is “a ‘pathologic’ specimen – one of the external galaxies with [a] peculiar spectrum. Any fully successful theory of galactic structure must take into account such abnormal forms.” By then even Hubble had conceded that NGC 5128 was truly abnormal. Two years later, in 1949, it was suggested that NGC 5128 was identical with the strong radio source known as “Centaurus A,” which blasts out 1,000 times as much radio energy

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as our Milky Way emits. The galaxy’s strongest radio emission originated in the dust lane, while a weaker source discovered in 1954 radiated from a nearly circular area 2° in diameter centered on the galaxy’s core. Then, in 1958, during a Paris symposium on radio astronomy, C. A. Shain announced that this unusual radio galaxy had an x-ray jet and large radio lobes shooting out in opposite directions at large angles to the dust lane. Walter Baade and Rudolph Minkowski explained all these outstanding discoveries by proposing that NGC 5128 was the violent aftermath of a collision between an elliptical galaxy and an edge-on spiral galaxy. That theory fell in and out of favor over the years, with some arguing that the galaxy is but a single massive object – a link between elliptical and spiral galaxies. But recent Hubble Space Telescope images have helped astronomers zero in on a new solution. We now know that NGC 5128 is truly a giant, containing more than a trillion solar masses. R. Brent Tully determined that the galaxy lies about 16 million light-years distant and spans 80,000

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light-years, though a NASA press release places the galaxy 10 million light-years off, implying a reduced diameter of 50,000 light-years. Hubble Space Telescope (HST) data support the galactic cannibalism scenario, though with a twist. Apparently, a massive black hole lies hidden at the center of this Seyfert 2 galaxy, which is voraciously consuming a smaller spiral galaxy that it collided with between 160 and 500 million years ago. The spiral will be fully consumed in a few hundred million years. Whenever one galaxy plows into another, the interaction sets off a wave of star formation. Indeed, in NGC 5128, the HST has found 21 candidate globular clusters, half of which are old like those in our Milky Way. The other half are young, implying that they might have formed during a recent merger. The HST also resolved open clusters of young, hot, blue (newborn) stars along the fringes of the dust lanes, which Hubble astronomers have also penetrated with the HST’s infrared camera. Behind that ring of dust lies a twisted disk of hot gas swept up into what appears to be a gravitational vortex surrounding a black hole. The evidence suggests that the black hole contains the mass of perhaps a billion stars squeezed into a region about the size of the Solar System. Radio jets have also been detected streaming out of the black hole. These jets reveal the orientation of the black hole’s spin axis. They can be seen forming at about 16,000 light-years from the nucleus before they expand into plumes that extend for

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61 800,000 light-years into intergalactic space. Interestingly, the maelstrom of gas surrounding the black hole is not perpendicular to the jets. Astronomers conjecture that the gas disk might be so young that it hasn’t yet aligned with the black hole’s spin axis. Or it may simply be influenced more by the galaxy’s gravitational tug than by the black hole’s. The hot gas disk is perpendicular to the galaxy’s outer dust belt, while the black hole’s spin axis is tilted approximately diagonally to these axes. The origin of the black hole remains a mystery. Was it always a part of the giant elliptical? The small spiral? Or was it created during the merger? You won’t solve that puzzle when you visually explore this orb of cosmic wonder, but you can experience the thrill of gazing at it. To find it, use wide-field chart 3 to find 3rd-magnitude Mu (μ) Centauri, then 4th-magnitude d Centauri, about 5° to the northwest. Center d Centauri in your telescope at low power and then switch to the accompanying chart. From d Centauri, move 1° west-southwest to 5th-magnitude Star a and then drop 30′ south-southeast to 6.5-magnitude Star b. Now make a slow and careful 1 1/2° sweep due south to 6th-magnitude Star c. NGC 5128 lies another 1 1/2° south of Star c. Through 7 × 35 binoculars, NGC 5128 is a large, diffuse glow, nearly 60 percent the size of the full Moon. From dark skies, there’s no mistaking this remarkably bright (magnitude 6.6) glow, which can be seen with the unaided eye. Indeed, NGC 5128 is 0.3 magnitude (32 percent) brighter than M81, 0.9 magnitude (2.3 times) brighter than M83, and 0.5 magnitude (58 percent) brighter than NGC 253 – all galaxies that have been dimly detected with the naked eye. That brings to six the number of galaxies within reach of human vision. I believe there could be more.

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61 The standard of achievable naked-eye limits in amateur astronomy has progressed well beyond the rudimentary boundaries of M31 and M33. We have now journeyed beyond a safe 2.3 million light-years to a razor’s edge at 16 million light-years. If you succeed in seeing Centaurus A with the unaided eye – and I urge you to try the next time you’re under an exceptionally dark sky – you will join a club of fellow earthlings who have helped to expand our natural visual envelope. Countless millions of skywatchers witnessed a rare sight in mid-April 1986 as Halley’s comet brushed past NGC 5128. The view opened a window on the imagination, allowing us to “see” the awesome dimensions of space. Here was a Solar System snowball millions of miles away dwarfing a pair of colliding galaxies millions of light-years away. And with a pair of binoculars held just right, Omega Centauri’s glittering globe joined the view. Fantastic! Truly an amazing sight, one that anyone can still enjoy today, for photographs of that passage abound. An even greater spectacle occurred several days later. On May 3, 1986 (Universal Time), the Rev. Robert Evans of Hazelbrook, New South Wales – history’s greatest visual supernova discoverer – became the first to spy an incredible stellar explosion in NGC 5128. The visual blast achieved an impressive brightness of magnitude 12.5 at the time of discovery, and it appeared smackdab on the galaxy’s dust lane, 2′ east and 1′ south of the galaxy’s center. Supernova 1986G was the first supernova discovered in Centaurus A. While comet hunting from Hawaii one evening with the 4-inch at 23×, I encountered the galaxy (without knowing what it was) and my heart nearly stopped. My immediate gut feeling was that I had discovered an enormous comet with a parabolic hood and

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a dark “shadow” running along its broad tail. But once I looked up at the sky, I realized the truth. Still, I was utterly amazed at how dark and obvious the dust lane appeared with such a small instrument, so I grabbed a pair of 7 × 35 binoculars and tried to see it in those. With averted vision and a little time, the dust lane was indeed apparent in my binoculars. At 23× in the 4-inch, the galaxy looks like a cell undergoing mitosis. Both halves of the galaxy and the dust lane are immediately visible. The brightest section of the galaxy is oriented southeast to northwest, and the lane runs fully across its face in that direction. The long axis of the galaxy, however, extends northeast to southwest, but at low power this all blends into a slightly elliptical fog, much smaller than that of its listed dimensions, perhaps by twothirds. The glow from the elliptical extension is so dim that increasing magnification only diminishes its appearance, so in the 4-inch at 72× and higher, the galaxy looks as if it is oriented only southeast to northwest. With careful scrutiny, the dust lane appears wavy and more open on the eastern end. The southern half appears brighter than the northern half, and a 9th-magnitude star punctuates the southeastern rim of the outer envelope (again this is only visible at 23×).

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61 At higher power, the galaxy is gracefully defined, the dust lane looking like a silken veil. The reason the southern half looks brighter than the northern half also becomes clear – two prominent foreground stars are superimposed on it. One, shining at roughly 12th magnitude, lies dead center, while a dimmer (magnitude 13.5) star lies on the south side of the dust lane to the west. Together the 9th-, 12th-, and 13.5-magnitude stars form an arc of diminishing brightness. Two concentrations of light (or even dimmer stars) lie to the east and west of the 12th-magnitude star. Supernova hunters should take note of these stars because they do not appear in photographs, which tend to burn out the bright halves of the galaxy. These stars have been mistaken for supernovae in the past. The entire northern lip of the northern half of the dust lane appears bright. Interestingly, this is where the Hubble Space Telescope imaged

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many of the hot new star clusters. The galaxy’s uniformly bright northern hemisphere looks something like a German World War II helmet. The dust lane widens to the east and fades into a diminishing sea of delicate vapors, like a slightly submerged walkway. In a more comical vein, Barbara Wilson called the overall view like that of a hamburger. Through her 13.1-inch reflector, she saw the dust lane as mottled. If you own a medium- to large-sized telescope, try swinging it 1° to the southwest, where a gaggle of six dim galaxies lie in a field only 1/2° wide. These are NGC 5082, NGC 5086, NGC 5090, NGC 5090A, NGC 5090B, and NGC 5091. Most of them lie immediately southwest of a 7th-magnitude star. Small-telescope users should at least try for the brightest of these, NGC 5090, which shines at magnitude 11.6 from a distance of approximately 150 million lightyears. Good luck.

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62 62 Omega Centauri NGC 5139 Type: Globular Cluster Con: Centaurus RA: 13h26.8m Dec: −47°29′ Mag: 3.7; 3.9 (O’Meara) Diam: 55′ SB: 12.5 Dist: ~17,300 light-years Disc: Listed as a star in Ptolemy’s Almagest of 150 AD; probably known as a “star” to skywatchers throughout human history ed mo nd halley : [See the text.] a bbé ni co l as l ou is de l acail l e : Naked eye, a 3rd mag. star in a fog. Telescope, like a big diffuse comet. (I-5) j a mes dunl o p : [Omega] Centauri (Bode) is a beautiful large bright round nebula, about 10′ or 12′ diameter, easily resolvable to the very centre; it is a beautiful globe of stars very gradually and moderately compressed to the centre; the stars are rather scattered [west] and [east], and the greatest condensation is rather north of the centre: the stars are of slightly mixed magnitudes, of a white colour. This is the largest bright nebula in the southern hemisphere. (D 440) j o hn hers chel : [See the text.] (h 3504) ngc: Remarkable, globular cluster, ω Centauri.

D r o p a vi s ua l plu mb l i ne 36 1/2° due south of brilliant Spica in Virgo, and you will encounter a soft and solitary light, a 4th-magnitude glow that even with a direct gaze looks slightly “hairy,” like the fuzzy Southern Gems

patina of a fading comet. That’s Omega (ω) Centauri, which marks the northwestern corner of an isosceles triangle with the roughly 2nd-magnitude stars Zeta (ζ) and Epsilon (ε) Centauri, and it is 13° northeast of Gamma 243

62 (γ) Crucis, the northern tip of the Southern Cross. How is it that the Alpha of globular clusters became known as Omega Centauri? The answer lies with the way early astronomers assigned Greek-letter designations to the stars. The Bavarian lawyer and astronomer Johann Bayer (1572–1625) is usually credited with the idea of attributing letters to stars, and the Greek-letter star labels used today are known today as Bayer designations. However, as the late Sky & Telescope editor Joseph Ashbrook writes in his Astronomical Scrapbook, the “idea of using letters to designate stars was … not original with Bayer, but goes back to the Italian scientist Alessandro Piccolomni (1508–1578), who was Archbishop of Patras. His book De le Stelle Fisse, which went through several editions between 1540 and 1579, contains 48 woodcut constellation maps in which the stars are labeled with letters. … This parallelism between Bayer’s sequence of Greek letters and Piccolomini’s Roman ones suggests that the German cartographer sometimes borrowed from his Italian predecessor.” His lack of originality aside, Bayer included Omega Centauri in his Uranometria of 1603 because Ptolemy had catalogued it in his Almagest of 150 AD. Ironically, there is some question as to whether Ptolemy had even made the observations that appear in the Almagest; instead he may have “borrowed” from Hipparchus and others to advance his theories. In Histoire de L’astronomie ancienne (1817), Jean Baptiste Delambre writes, “Did Ptolemy himself make observations? … [W]e cannot see how to decide it.” That aside, Omega Centauri is bright and obvious enough that aboriginal skywatchers throughout time must have seen its luminance, though they

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presumably wouldn’t have known just what kind of celestial magnificence blazed before them. Omega remained a “star” until 1677, when Edmond Halley rediscovered it from St. Helena, recording it as a nonstellar object “in the horse’s back.” Halley later included Omega Centauri in a list of six “luminous spots or patches” published in the 1715 Philosophical Transactions of the Royal Society. Halley’s contribution was titled “An Account of several Nebulae or lucid spots like Clouds, lately discovered among the Fixed Stars by help of the Telescope.” Of this new object he writes (in the third person): The fourth [luminous spot or patch] was found by M.. Edm.. Halley in the year 1677, when he was making the Catalogue of the Southern Stars. It is in Centaur, that which Ptolemy calls [the one who emerges from the (horse’s) back] which he names in dorso Equino Nebulus [Nebula on the back of the horse] and is Bayer’s Omega. It is in appearance between the fourth and fifth Magnitude, and emits but a small Light for its Breadth, and is without a radiant Beam; this never rises in England, but at this time its Place is [Scorpius] 5 deg 3/4 with 35 deg 1/2 South Lat.

Many sources credit Halley with first discerning Omega Centauri’s cluster nature, but he never did so. Nor did the Swiss astronomer and mathematician Philippe Loys de Cheseaux (1718–1751). De Cheseaux included Halley’s observations in his 1746 list of 21 nebulae, which were presented to the French Academy of Sciences in 1746, investigated by Guillaume Bigoudan in 1884, and published as part of a review of nebulous objects by Bigoudan in the 1892 Annales de l’Observatoire de Paris. Of the first 14 objects in that list, de Cheseaux writes: “I begin with those which,

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62 viewed by telescope, are found to be simple clusters of stars.” But Omega does not appear in that section. Rather it is listed in the second part of Bigourdan’s list, which contained the “the nebulae properly so called … which, when observed with much larger telescopes, always appear like white clouds.” Bigourdan described Omega Centauri as “[that nebula] in Centaurus, discovered by Mr. Halley.” Although Bigourdan made an effort to observe most of the 21 nebulae in de Cheseaux’s list, he could not have scrutinized Omega because it was always below his horizon. Clearly then, Bigourdan’s description of Omega Centauri was based solely on Halley’s impression of it. Omega Centauri’s true nature also eluded the keen eye of Louis de Lacaille, who listed it in his 1755 catalogue under the category of “nebulae without stars.” Mind you, Lacaille observed with a simple 1/2-inch 8× telescope – an instrument inferior to most of today’s binoculars. He described it as a “[Nebula] in Centaurus; with simple view, it looks like a star of 3rd magnitude viewed through light mist, and through the telescope like a big comet badly bounded.” Some sources imply that Sir John Herschel first identified Omega Centauri as a star cluster, as per his famous account that makes the notion plausible: The noble globular cluster ω Centauri, beyond all comparison the richest and largest object of its kind in the heavens. The stars are literally innumerable, and as their total light when received by the naked eye affects it hardly more than a star of the 5th or [5th to 4th] magnitude, the minuteness of each may be imagined: it must however be recollected that as the total area over which the stars are diffused is very [considerable] (not less than a quarter of a square degree), the resultant impression on the sensorium is doubtless thereby much enfeebled, and

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that the same quantity of light concentrated on a single point of the retina would very probably exceed in effect a star of the 3rd magnitude.

But James Dunlop clearly had that notion before him. In his 1828 catalogue, Dunlop said Omega Centauri is: a beautiful large bright round nebula, about 10′ or 12′ diameter, easily resolvable to the very centre; it is a beautiful globe of stars very gradually and moderately compressed to the centre; the stars are rather scattered preceding and following, and the greatest condensation is rather north of the centre: the stars are of slightly mixed mags, of a white colour. This is the largest bright nebula in the southern hemisphere.

Omega Centauri is generally regarded as the preeminent globular star cluster in the entire heavens, but is it? Let’s look at the statistics. Omega is the brightest globular star cluster, but just barely so. In William Harris’s “Catalogue of Parameters of Globular Clusters” (2010), he lists Omega as magnitude 3.7, making it only 0.2 magnitude brighter than 47 Tucanae. So which is the greater globular? Skiff says, “Some prefer the sheer richness of Omega Centauri, and others the star-density and remarkable (indeed unique) structure of 47 Tuc. … Both are luminous objects with large numbers of stars, as anyone who has observed them can confirm, and their combined light sends them to the top of the total magnitude list. But because of their distance, the brightest stars are not as bright as nearer clusters.” Indeed, when ranked by the brightnesses of their constituent stars, both Omega Centauri and 47 Tucanae fall short of the top. Omega Centauri’s brightest stars shine at an impressive magnitude 11.5, but five other globular clusters beat it in that category, while seven clusters beat out 47 Tucanae. When ranked by horizontal-branch magnitude (which

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62 is a measure of the cluster’s resolvability), three clusters beat out 47 Tucanae, while seven clusters are more readily resolved than Omega Centauri in small telescopes. Of course, a cluster’s greatness is highly subjective; it’s all in the eye or the heart of the beholder. Observing is a highly personal ­experience, and we all react differently to what we see through a telescope. And even one observer’s reaction will differ with aperture. But if we were to magically place all the globulars at the same distance and look at them side by side, we would find that Omega Centauri is the largest and most massive of them all. It is in fact the biggest globular in our Milky Way. The cluster spans 182 light-years of space, contains several million stars, and has a mass of about 5 million solar masses, making it about 10 times more massive than other big globulars and a near match for some dwarf galaxies. Omega Centauri is also one of the oldest objects in the Milky Way; its age is comparable to that of the universe itself. On average, each of the cluster’s members contains about 1/40 as much metal as the Sun – a pretty typical metallicity for a globular cluster. Its overall spectral type is F5, and its radial (line-ofsight) velocity is 230 kilometers per second. In a 1999 article in Nature (vol. 402, p. 55), Young-Wook Lee (Yonsei University, South Korea) and colleagues studied 50,000 members of Omega Centauri with the 1-meter reflector atop Cerro Tololo in Chile. They found several stellar populations that had formed in distinct bursts over a 2-billion-year period. The scientists speculate that the prolonged starburst activity would make sense if Omega Centauri were the remnant nucleus of a small galaxy that merged with our Milky Way. If true, this would make Omega Centauri

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akin to the globular cluster M54, which appears to be the nucleus of the Sagittarius dwarf galaxy, now being consumed by the Milky Way. More recently, in a 2011 paper in Astrophysical Journal (vol. 728, p. 155), Andrea Dupree (Harvard-Smithsonian Center for Astrophysics) and colleagues announced that high-resolution spectra taken with the 8-meter (PHOENIX) Gemini South telescope and the 6.5-m Magellan Telescope/CLAY in Chile reveal direct evidence for helium-abundance variations in 12 red giant stars in the globular star cluster Omega Centauri (Southern Gem 62). The Gemini data, combined with optical data from the Magellan Clay Telescope, confirm the helium-abundance variations. This work is important in that years ago astronomers once believed that the stellar populations of globular star clusters formed at the same time. The direct evidence of variations in helium abundance, however, indicates the possibility that these multiple stellar populations formed at different times. To find Omega Centauri, use wide-field chart 3 to locate the naked-eye “star” that marks the northwestern corner of an isosceles triangle with the roughly 2nd-magnitude stars Zeta (ζ) and Epsilon (ε) Centauri. In 7 × 35 binoculars, Omega Centauri is magnificent, a bright, distended cloud of cometary fluff. It lies immediately northeast of a bowl of 7th-magnitude stars in a 2°-long Dipper asterism. The dimensions of the cluster swell dramatically when changing from the naked eye to the binocular view. Walter Scott Houston once said, “The apparent size of Omega Centauri depends not only on the size of your telescope but also on your reaction to seeing scattered light. It’s rather like the better-understood ‘cocktail-party effect’ – the

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more sensitive you are to hearing, the more conversations you will overhear.” With averted vision, Omega Centauri grows faint radial arms that project against a halo of teasingly faint stars that show hints of resolution. At 23×, in the 4-inch the cluster looks like a snowball illuminated from the inside. The east–west trending core seethes with glowing globs of melded starlight. Two “beams” of hazy starlight stretch to the northeast and southwest (the northeast beam is more prominent), while “claws” of stars jut out from the southern rim of the core. A 9th-magnitude star burns about 4′ north of the loose and visually serene center. Omega’s core lacks the intense central concentration of light that its rival 47 Tucanae displays, its texture more soft, like salt piled onto a wad of cotton. When the cluster is high in the sky, use low power to look for color. I see a slight yellow tinge to the core, while the outer halo has a pale blue luster.

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Increasing the magnification to 72× transforms the cluster into a web of gossamer threads surrounding a sizzling skillet of starlight. Look for a pair of 12th-magnitude stars directly at the center of the core, which is now framed by an arc of stars to the northeast and southeast. But it is at high power that the globular dissolves into innumerable beads of quicksilver. The northernmost section of the inner core has a dark and prominent keyhole or “footprint” stamped on it (oriented north to south). The footprint lies on the eastern fringe of a ribbed wedge of stars that extend like thin comet tails away to the northeast from the center. This bright, asymmetrical core lies on a bed of fainter stars that create a scintillating background of stellar static. A long, curving lane of darkness sweeps through the southern half of the cluster (like a big smile); its southern levee is lined with filigreed starlight. Undulating waves of stars flow away from this black wall to the south. All manner of clumps, stellar strings, and dark patches scar the globular’s face. The arms on the western side are more curled or flamboyant, while those on the eastern side are long wisps of decreasing intensity, like fading smoke trails from skyrockets. Visions of evaporating ice chips, crystal flames, and light-snapping glitter all come to mind as the cluster assaults the eye– brain system. It’s like peering into the working mind of the creator.

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63 63 Spiral Planetary NGC 5189 = IC 4274 Type: Planetary Nebula Con: Musca RA: 13h33.5m Dec: −65°59′ Mag: 9.9 Diam: 153″ x 10.3′ Dist: ~1,800 light-years Disc: James Dunlop, 1826 j ames dunl o p [July 1, 1826]: A very faint nebula, about 25″ diameter. It is very near a star of the 8th magnitude, and near the [northeast] extremity of a crescent of very small stars. (D 252) j o hn hers che l : A very strange object. A nebula of oval figure, but having a central and brighter axis somewhat curved, and terminating in two masses brighter than the rest; diameter about 90 to 100″. It involves 3 stars, one of which with 320 power is double. The principal star is 10th magnitude, the others extremely small; a multitude of other stars in field. (h 3514) ngc: Remarkable, bright, pretty large, considerably extended, bright in the middle with a curved axis, 4 stars involved.

N G C 5189 is a l ar ge and st u nni ng planetary nebula 1 3/4° southeast of 4.5-magnitude m Centauri, which is about 6 1/4° east-southeast of 1st-magnitude Alpha (α) Crucis (Acrux), the brightest and southernmost star in the Southern Cross. But don’t be deceived by the nebula’s brightness (magnitude 9.9); its light stretches across ~2 1/2°

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of sky, so it appears dimmer than expected as seen through a small telescope. Nevertheless, under a dark sky, the nebula’s irregular structure can be detected in telescopes as small as 3 inches. James Dunlop discovered NGC 5189 in 1826, but John Herschel first noticed the nebula’s curious figure. After observing it

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63 through his 18-inch speculum-mirror reflector, Herschel wrote that the object’s bright, central axis appeared “somewhat curved,” and it terminated “in two masses brighter than the rest.” Williamina Paton Fleming (1857–1911)  – one of Harvard College Observatory’s best-known female astronomers  – rediscovered the object in 1901 while visually scanning a photographic plate from the Observatory’s Draper Memorial collection. This collection consisted of plates covering the entire northern and southern skies, using telescopes equipped with an objective prism to record spectra. So, not only did Fleming and her associates measure the positions of more than 400,000 stars (which they compared against those in known catalogues), but they also searched eagerly for new objects by using their spectra. Thus, Fleming became the first to see the spectrum of NGC 5189, which she identified as a “gaseous nebula.” The object was added to the second Index Catalogue as IC 4274 because its north polar distance of 155° was erroneously typed as 115°, which, until recently, masked the object’s real identity. So IC 4274 equals NGC 5189. Interestingly, by the middle of the twentieth century, astronomers still had their doubts as to the object’s nature. It had been referred to as a double planetary nebula, a quasi-planetary (a transitional object between a real planetary and bright

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diffuse nebula), and, more simply, a peculiar object. Despite the historical puzzlements, we now know with certainty that NGC 5189 is a planetary nebula, having a spectrum that displays emission lines of ionized helium, hydrogen, sulfur, and oxygen  – all indications of elements being burned inside the star as it ages and dies. As the star contracts under gravity, its core ultimately condenses into a white dwarf, while its outer atmosphere is expelled into space, forming the shells of gases that we see as the planetary nebula. But why are NGC 5189’s shells so chaotic looking (like a parallelogram)? As the accompanying 2006 Gemini South 8-meter-telescope image release shows, the nebula has long streamers of gas, glowing dust clouds, and cometary knots that point away from the central star, suggesting some extraordinary action at the nebula’s

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63 core, where we find the hot, hydrogen-deficient star HD 117622. This dense central star appears to be blowing off its thin remnant atmosphere into interstellar space at a speed of about 2,700 kilometers (about 1,700 miles) per second, and we see this remnant as an expanding ring of gas nearly edge-on. “As the material leaves the star,” the image release states, “it immediately begins to collide with previously ejected gas and dust surrounding the dying star. This collision of the fast-moving material with slower motion gas shapes the clouds, which are illuminated by the star. These so-called ‘low ionization structures’ (or LIS) show up as the knots, tails, streamers, and jet-like structures we see in the Gemini image. The structures are small and not terribly bright, lending planetary nebulae their often-ghostly appearance.” Kevin Volk (Space Telescope Science Institute) further explains that the likely mechanism for the formation of this planetary nebula is the existence of a binary companion to the dying star. “Over time,” he says, “the orbits drift due to precession and this could result in the complex curves on the opposite sides of the star visible in this image.” To find this beautiful object, use wide-field chart 3 to find m Centauri, which should appear as a fine pair of roughly 5th-magnitude stars through binoculars. Center it in your telescope at low power and then use the accompanying chart to move about 35′

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east-southeast to magnitude 6.5 Star a. Now drop 40′ south-southeast to 7th-magnitude Star b and then drop another 20′ to similarly bright Star c. NGC 5189 is another 20′ due south of Star c, just about 6′ north-northwest of 8th-magnitude Star d. Through the 5-inch at 33×, NGC 5189 is a large (~2′), diffuse glow that looks irregularly round. With averted vision, the core appears brighter. Increasing the magnification to 60×, the nebula appears in two sections: a rectangular core, elongated east to west, with a fainter butterfly wing at each ansa. So it looks like a fat butterfly or a bow tie. At 94×, which is the best power to use with the 5-inch, the bow tie appears irregularly bright, beaded, and blotchy. Averted vision reveals the box to have a central bar, oriented northeast to southwest, though I could not see

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any S-shaped twists to it. An 11th-magnitude star can be seen northwest of the object’s center. The spiral can be inferred, however, from the “butterfly” shape.

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Through a 12-inch reflector, NGC 5189 is a stunning sight. Its long axis appears riddled with irregularities, while its squashed, inverted S-shaped extensions emerge from the central bar like butterfly wings open to the wind. Through her 12-inch telescope, Magda Streicher says her “first impression of the nebula, was that it showed a lot of detail with a relatively well-defined eastern part. It has a bright, curved bar … east to west, which is well edged to the north with a hazy inner southeastern part. With higher power, splinter faint stars can be seen embedded in this part of the nebula. The northeastern part sort of made a water drop impression bending down towards the southwest.”

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64 64 M83, Thousand Rubies Galaxy NGC 5236 Type: Mixed Spiral Galaxy (SAB(s)c) Con: Hydra RA: 13h37.0 m Dec: −29°52′ Mag: 7.5 Dim: 12.9′ × 11.5′ SB: 13.2 Dist: ~15 million light-years Disc: Abbé Nicolas Louis de Lacaille, 1752 abbé ni co l as l ou is de l acail l e : A small, shapeless nebula. (I-6) charles mes s ie r : [See the text.] j ames dunl o p: 185 Centauri (Bode) is a very beautiful round nebula, with an exceedingly bright well-defined planetary disk or nucleus, about 7″ or 8″ diameter, surrounded by a luminous atmosphere or chevelure, about 6′ diameter. The nebulous matter is rather a little brighter towards the edge of the planetary disk, but very slightly so. I can see several extremely minute points or stars in the chevelure, but I do not consider them as indications of its being resolvable, although I have no doubt it is composed of stars. (D 628) ngc: A very remarkable object. William and John Herschel found it very bright, very large, elongated in position angle 55°, very suddenly much brighter toward a central nucleus. Seen as a threebranched spiral by Leavenworth [with the 26-inch refractor of Leander McCormick Observatory].

Wi t h ou t qu e st i on, M 83 is my ­favorite galaxy in the Messier list. This gorgeous, nearly face-on spiral is powerfully condensed at low power in my 4-inch

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refractor, visible in binoculars, and just discernible to the unaided eye. It is a true showpiece for small telescopes and, with time and effort, displays much structure at high power,

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64 including bright knots, dark lanes, and oddly shaped arms. Abbé Nicolas Louis de Lacaille discovered the object during his exploration of the southern skies from the Cape of Good Hope from 1751 to 1753. French comet hunter Charles Messier included Lacaille’s find and listed it as the 83rd object in his now famous catalogue of nebulae and clusters (published in the Connaissance des Temps for 1784; published in 1781). Of it he wrote: “Nebula without a star, close to the head of Centaurus. It appears as a faint, even light, but is so difficult to see with the telescope that the slightest illumination of the micrometer’s crosshairs causes it to disappear. It requires considerable concentration to be seen at all. It forms a triangle with two stars estimated to be of sixth and seventh magnitude. Its position has been determined from the stars i, k, and h, in the head of Centaurus. M. de la Caille had already detected this nebula.” In 1781, William Herschel surveyed M83 and found that it “more than fills the field.” He became the first to notice the object’s “faint branches,” which we now know are the galaxy’s spiral arms. William Lassell (1799–1880) later refined Herschel’s description, adding that, through his 48-inch reflector on the island of Malta, M83 is “a 3-branched spiral.” Today we know that M83 is the brightest spiral in the nearby Centaurus A group of galaxies – a loose gaggle of island universes whose brightest member is NGC 5128 (Centaurus A [Southern Gem 61]). This grand-design spiral lies about 15 million light-years distant and spans some 70,000 light-years of space. Its massive spiral arms are of high surface brightness and cover most of the disk. The dust lanes are intricate and, in general, are closely associated with the luminous areas on the inside of these arms, as usual. The spiral’s

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nucleus is nestled in a prominent bar and appears “amorphous” at optical and infrared wavelengths, which is almost certainly associated with a nuclear starburst. In fact, the nucleus and spiral arms are experiencing starbursts. It was once believed that the starbursts were possibly triggered by an encounter with NGC 5253, an elliptical galaxy 10 times less massive than M83 and that now lies about 2° to the southwest. But as R. C. W. Houghton and N. Thatte (University of Oxford) reported in a 2006 paper in Monthly Notices of the Royal Astronomical Society (vol. 385, p. 1110), the closest passage of NGC 5253 (which also contains recent star formation) to M83 occurred some 1–2 billion years ago. “So the activity in these galaxies,” they say, “is likely driven by internal inflow of gas via the bar rather than from direct interaction as the clusters are mostly 1–10 million years in age.” The starburst phenomenon indicates an evolved population of supergiants. In fact, a large number of supernovae have been observed in this galaxy (six in the last 100 years). Among them was a Type II supernova in the nucleus; the association of the Type II supernova with H ii regions in the spiral arms and in the nucleus is in agreement with the general scenario of a starburst. Aside from its starbursts and high supernova rate, M83’s spiral arms are also awash with large numbers of high-mass star clusters and billions of blue stars because they have a relatively large fraction of young blue stars populating them. The core of M83 itself is bright at x-ray energies, showing a high concentration of neutron stars and black holes left from an intense burst of star formation. M83 is also known to have a remarkably extensive neutral-hydrogen disk, containing 80 percent of the galaxy’s mass outside its optical length.

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64 In a 2011 Astronomical Society of the Pacific Con­ ference (vol. 440, p. 149) early science release, Hwihyun Kim (Arizona State University) and colleagues announced a multiwavelength photometric study of individual stars in M83 based on observations with the Hubble Space Telescope, which they used to determine the recent (< 1 billion year) star-formation history of the galaxy. Among their findings was that stars younger than 10 million years are located preferentially along the active star-forming regions on the spiral arm. To find M83, use wide-field chart 3 to find the 3°-wide Lambda-shaped ­asterism  – with 4.5-magnitude 1 Centauri as its southwesternmost star – about 5° northwest of 2ndmagnitude Theta (θ) Centauri (Menkent). Center 1 Centauri in your telescope at low power and then switch to the accompanying chart. From 1 Centauri, move about 40′ to 8th-magnitude Star a. Now make a careful 1 1/4° sweep north-northwest to the 10th-magnitude galaxy NGC 5253 (Southern Gem 65), also identified as “b” on the chart. Now move slowly about 2° north-northwestward to Pair c, comprised of a magnitude 5.5 star and a 7th-magnitude star almost 20′ apart. M83 lies just about 20′ southwest of Pair c. (If you live under dark skies, try using binoculars first to scout out the galaxy before employing your telescope.)

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Through the 4-inch at 23×, the field is a rich vista of starlight, including a beautiful string of 10th-magnitude stars bordering the galaxy to the southeast and two fainter stars bracketing its farthest spiral arms. The galaxy immediately looks elongated southeast to northwest, with obvious patches on either side of the nucleus. One patch in the outer, southeastern arm is glorious. And with time several knotty, star-forming regions pop in and out of view all over the galaxy. (The wealthy assortment of these red regions in deep color photographs led to its nickname of the Thousand Rubies Galaxy); through larger telescopes, one can spend many enjoyable hours just hunting them down. To see the most detail in my 4-inch, I alternate between medium and high power. I use

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64 72× to see the faintest details in the arms and 130× to pick out structure in the nuclear region. And I’m amazed at how incredibly detailed the nucleus and its surroundings appear  – how well defined even the tiniest bits of dark dust show up. The nucleus itself is tiny, looking like a star trapped in a maelstrom of shimmering light. While concentrating on the nucleus, I notice that a bright knot in the southeastern arm becomes clearly apparent, as does another knot equidistant from the nucleus in the northwestern arm. The knots are oriented in a row that is skewed about 20° from the galaxy’s main “bar,” which appears to extend across the major axis. The bar is composed of numerous nebulous patches along the galaxy’s major axis and ultimately branches off the axis to form the stunning spiral arms. The sight reminds me of the great controversy over Percival Lowell’s canals on Mars: in imperfect seeing, the bar looks like a straight line of light, but under excellent conditions the line breaks up into individual spots. If you own a small telescope, use high power and try to follow some of the spiral patterns (plan to spend some time in the search because the irregular structure can confuse the eye). I suggest focusing not on the bright arms themselves but on the dark lanes between them. The biggest challenge beyond that will be trying to make sense out of the innermost region – that nuclear maelstrom of light and dark. I spent hours confirming and

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reconfirming what I was seeing. But don’t struggle. Take it piecemeal. First look southwest of the nucleus, where the arms appear brighter than those to the northeast. There is one beautiful dark lane just northeast of the nucleus, which I find the most striking feature in the galaxy. It looks like a black canal whose sandy banks are being illuminated by starlight. On the south side, use averted vision to search for a broad arcing band of patchy nebulosity between the nuclear pool and the closest spiral arm. When you concentrate on that arm, do you see it separate into two parts – with one arm closer to the nucleus and more tightly wound than the other? Be sure to use your acute averted vision. By the way, the numerous supernovae that have occurred in M83 indicate one appears every 10 to 15 years, compared to perhaps one every 30 to 50 years in our own galaxy. This makes M83 one of the most popular hunting grounds for these stellar explosions, so be on the lookout.

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65 65 NGC 5253 Type: Blue Dwarf Galaxy Con: Centaurus RA: 13h39.9m Dec: −31°39′ Mag: 10.2 Dim: 5.1′ × 2.3′ SB: 12.7 Dist: ~11 million light-years Disc: William Herschel, 1787 willi am hers c he l [March 15, 1787]: Pretty bright, small, little extended [southwest to northeast]. (H II-638) j ames dunl o p: A very small and very bright nebula, very much resembling a small star, surrounded by a very strong burr; this is a singular body. (D 623) j o hn hers che l : Very bright, much elongated, pretty suddenly brighter in the middle, 2.5′ long, 1′ broad. (h 3526) ngc: Bright, pretty large, extended toward position angle ~45°, pretty suddenly much brighter in the middle.

N G C 5253 i s a smal l bu t bri ght dwarf galaxy in northern Centaurus about 1 3/4° south-southeast of brilliant M83, with which it may have interacted in the distant past. Although William Herschel discovered the object from the Northern Hemisphere, James Dunlop included it in his 1828 catalogue, listing it as the 623rd object. Users of small telescopes will find it just as Dunlop described it, being very tiny, like a star with a “very strong burr.” 256

Early twentieth-century observers recognized its curious spindle shape with a bright center. But classifying it was difficult. In time it was considered perhaps an irregular lenticular, an irregular dwarf, or an elliptical with a very bright core and traces of complex dark lanes surrounded by a smooth outer halo. Adding to the visual perplexity, later observations also identified faint knots on one side of the “amorphous” galaxy, with plumes similar to those in M82.

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65 Today astronomers categorize NGC 5253 as a nearby dwarf galaxy (11 million light-years distant) in the NGC 5128 group of galaxies – the brightest members of which include M83 (Southern Gem 64), NGC 5128 (Southern Gem 61), and NGC 4945 (Southern Gem 60). NGC 5253 is clearly the runt of the litter. This tiny, nearly edge-on system spans only 16,000 light-years in true physical extent. It shines with a total luminosity some 700 million times that of the Sun and has a total mass of only about 650 million solar masses. It is receding from us at 407 kilometers per second. NGC 5253’s classification has since been further refined to a starburst, “blue dwarf” galaxy  – a particular class of relatively small galaxies with features that might reveal the secrets of star formation in the early universe because they contain some interstellar clouds that may resemble those from which the first stars formed. Although NGC 5253 is poor in dust and heavier elements, its nuclear region is presently undergoing violent star formation. As reported in a 2004 European Space Agency press release, NGC 5253, which is almost 100 times smaller than our own Milky Way Galaxy, can produce hundreds of compact stellar clusters. “The youngest of these clusters are still deeply embedded in their natal clouds, but when observed with infrared-sensitive instruments like ISAAC at the [Very Large Telescope], they stand out as very bright objects indeed.” In fact, NGC 5253 is the site of the youngest starburst known. As the Hubble Space Telescope (HST) recently revealed, the core contains a super star cluster and the very youngest globular cluster – only about a million years old – that has ever been observed. The starburst in this galaxy may have been started by the accretion of a small cloud of

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gas from intergalactic space. The peculiar morphology and enhanced star formation strongly suggest an accretion event, presumably as a result of a recent encounter about 1 billion years ago with the nearby gas-rich spiral M83. As reported in a 2007 paper in Astronomical Journal (vol. 134, p. 1799), T. J. Davidge (Dominion Astrophysical Observatory) used Gemini South to take deep infrared images of NGC 5253 and traced out the red giant branch (RGB) stars along its major axis, finding the outer region of the galaxy metal poor. Also, roughly 1–10 percent of the stellar mass may have formed during the past few hundred million years, and it is suggested that the progenitors of the two recent Type Ia supernovae in this galaxy (discussed later) may have formed at this time. “That NGC 5253 has experienced either episodic or continuing elevated levels of star formation during the past few hundred million years is reminiscent of what is seen in other dwarf starburst galaxies, such as NGC 3077,” Davidge says. He agrees that the current episodes of star formation in NGC 5253 were triggered up to ~1 billion years in the past. Similarly, in a 2011 paper in the Bulletin of the American Astronomical Society (vol. 43), Daniel R. Harbeck (WIYN Observatory) and colleagues note that they studied star clusters in the field of NGC 5253 based on three-color photometry with the HST. They found that the ages and masses of the cluster candidates are consistent with an episode of star formation about 1 billion years ago that was able to produce three surviving star clusters with masses around 500,000 solar masses. To find this little wonder, use wide-field chart 3 to locate 3rd-magnitude Iota (ι) Centauri and then 4.5-magnitude 1 Centauri in a pretty, 1°-long Y-shaped asterism of

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similarly bright stars. Center 1 Centauri in your telescope at low power and then switch to the accompanying chart. From 1 Centauri, move 40′ west-northwest to the roughly 20′-long Y-shaped asterism of 8th- to 10th-magnitude stars (a). From the western side of the Y, now move about 50′ west-northwest to 8.5-magnitude Star b. NGC

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5253 is about 40′ northeast of Star b. Through the 5-inch at 33×, the galaxy is a tiny, condensed starlike glow with a tiny halo of light. Its spindle shape and crisp starlike core become evident at 60× and are easily seen as a nice bright glow at 94×. I could not detect any further details. Through her 12-inch Schmidt-Cassegrain, Magda Streicher spied NGC 5253 immediately, noticing a “very dense, compact nucleus with a large envelope around it. The outer edge of the galaxy, which is very broad and more prominent than those of most other galaxies, is something quite special to see  – soft and hazy, almost as if covered with a nearly see-through veil. It is elongated in a northeast to southwest direction, with three field stars towards the north quite neatly completing the picture.” The galaxy has had two bright supernovae in the last century and more. A magnitude 8.4 supernova (1895B; originally designated Z Centauri) was discovered by Harvard astronomer Williamina P. Fleming on December 12, 1895 (5 months after maximum) by examining a spectrum plate taken on July 18, 1895. Charles T. Kowal (Caltech) photographically discovered another 8.5-magnitude supernova (1972e) in NGC 5253 in 1972 at Mount Palomar, so be on the lookout for more (perhaps equally bright) eruptions.

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66 66 NGC 5286 Type: Globular Cluster Con: Centaurus RA: 13h46.4m Dec: −51°22′ Mag: 7.3; 7.2 (O’Meara) Diam: 11′ SB: 12.6 Dist: ~36,000 light-years Disc: James Dunlop, 1826 j a mes dunl o p [April 29, 1826]: A bright exceedingly well-defined rather elliptical nebula, about 1′ diameter, exceedingly condensed almost to the very edge, and gradually a little brighter to the centre. This is about 6′ north of M Centauri. – I have strong suspicion that this is resolvable into stars. (D 388) j o hn hers chel : Very bright; gradually much brighter to the middle; 2.5′ or 3′ diameter; resolved into 15th magnitude stars; has one star 12th magnitude [to the southeast]; the centre near the edge. It is in the field with Brisbane 4618 a star of 6th magnitude. (h 3533). ngc: Globular cluster, very bright, pretty large, round, well resolved, stars of magnitude 15.

NG C 5 2 8 6 i s o n e of t he sk y’s brightest but most neglected globular clusters. It is easily located 5° southeast of the great globular cluster Omega Centauri (Southern Gem 62), where it lies directly between magnitude 2.6 Zeta (ζ) and magnitude 2.3 Epsilon (ε) Centauri, though a little bit closer to the latter. The cluster literally hides in the topaz fire of magnitude 4.6 M Centauri just 4′ to

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the southeast. M Centauri is a rather complex star, being a double with a magnitude 4.6 (G8) primary and an 11th-magnitude secondary (a suspected variable star), and a spectroscopic binary. NGC 5286 is also part of a 2 1/2°-long binocular asterism that looks like the constellation Scorpius. Shining at magnitude 7.3, the cluster is as bright as, or brighter than, about half of the Messier

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66 catalogue’s 29 globulars. Is it any wonder, though, that Omega Centauri steals all the attention in this region of sky? Aside from some of its physical parameters, little is known about the nature of NGC 5286. It has a normal globular cluster metallicity, with each of its member stars having, on average, nearly 1/50 as much metal as the Sun. The cluster has an integrated spectral type of F5. The cluster is rich in variable stars, including 52 RR Lyrae variables, 4 long-period variables, and 1 type II Cepheid of the BL Herculis type (M. Zorotivic et al., Astronomical Journal, vol. 139, p. 357). A study of the cluster by Nikolai Samos (Institute of Astronomy of Russian Academy of Science) and his colleagues determined an age between 14 and 18 billion years, with the likeliest value being 17 billion years. This finding remains quite controversial, however, both because the cluster has been insufficiently studied and because recent estimates of the Hubble constant and other cosmological parameters suggest that our universe may be only 12 or 13 billion years old. NGC 5286 lies about 28,000 light-years from the Galactic center and 36,000 light-years from the Sun. In brightness, apparent size, and distance, NGC 5286 resembles M68 in Hydra. The cluster is moving toward us at 58 kilometers per second, a quarter of Omega Centauri’s radial velocity. Interestingly, if you were to reel in NGC 5286 to Omega Centauri’s distance, NGC 5286 would still appear about 2.5 times smaller, and 10 times fainter, than that great cluster. In fact, NGC 5286 may once have belonged to the globular cluster system of the Canis Major dwarf spheroidal galaxy (discovered in 2003), which our Milky Way is currently comsuming. If so, it has been ripped free and lies in the dwarf spheroidal’s tidal stream.

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To find this little wonder, use wide-field chart 3 to first locate brilliant Alpha (α) and Beta (β) Centauri. Then look about 7° northwest for 3rd-magnitude Epsilon Centauri. M Centauri lies just 2° north-northeast of Epsilon Centauri, and NGC 5286 is only 4′ northwest of that star. Use the accompanying chart to pinpoint its location near M Centauri. From Hawaii, I could see M Centauri with the unaided eye but could not resolve the cluster next to it. I do not think it impossible to do so, however, especially for someone who observes from the Southern Hemisphere with young eyes. The globular reveals itself with the slightest of aperture. In 7 × 35 ­binoculars, it looks like a tiny ghost image of M Centauri, a dim puff of smoke wafting away from that alluring flame-colored star. At 23× in the 4-inch, the color contrast between the two objects is remarkable. M Centauri looks strongly yellow with a tinge of orange – a dim Arcturus – while the globular cluster glimmers with a very pale ashen blue. Still, at this low magnification little else can be seen; the cluster still looks like a circular wafer of uniform light, like a breath spot on a mirror. The late, great comet discoverer Jack Bennett of South Africa included it as the 64th

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66 object in his list of comet-like objects as seen with a 5-inch rich-field refractor. Indeed, I have spotted NGC 5286’s cometary form several times in my own comet sweeps in the Centaurus area. At 72×, the cluster appears as a moderately condensed glow with stumps of starlight poking out at the cardinal directions in a cruciform shape. These are probably the “faint star points” that the late Ernst Hartung glimpsed through a four-inch telescope. The cross lies on an elliptical bed of fainter stars. With a dedicated gaze, star patterns seem to spiral out from the condensed core, which appears very mottled or patchy, as it is interspersed with dark veins. The latter view is rather reminiscent of Dunlop’s discovery impression of “a bright exceedingly well-defined rather elliptical nebula, about 1′ diameter, exceedingly condensed almost to the very edge, and gradually a little brighter to the centre.” Dunlop also had “a strong suspicion that this is resolvable into stars,” which is most certainly true. Although NGC 5286’s horizontal-branch magnitude (its fully resolvable limit) is magnitude 16.5, the cluster’s brightest stars shine at a respectable magnitude 13.5, so partial resolution does occur in telescopes of optical quality as small as 3 inches. At high magnifications, the cluster sports some seven arms. Two of them  – one to the south and one to the east  – form a counterclockwise spiral. The other arms look more straight or rigid. Each is dappled by a few magnitude 13.5 to 14 stars. A dense kite-shaped clustering of roughly 14th-magnitude stars can be seen in the southern arm just beyond the 1.5′’-diameter core (which has a very stellar center). The “kite” is cleanly separated from the core by a dark lane running roughly east to west. A few bright 10th-magnitude and

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fainter foreground stars occupy the outskirts of the 11′-diameter cluster. Through her 12-inch Schmidt-Cassegrain, South African observer Magda Streicher saw NGC 5286 as a “bright, round, and large globular cluster with nice faint pinpoint stars randomly visible. It displays a very dense and large core, which is slightly hazier towards the edges with stars forming random outliers (95×). It shows an elongated impression in a northeast to southwest direction.” If you move 3/4° to the east-southeast of NGC 5286, you’ll find the magnitude 11.2 planetary nebula 5307 (not plotted here). It spans a mere 13″ in diameter and appears starlike, so high magnification and averted vision (or an O iii filter) are needed to see its uniformly bright elliptical disk. Owners of big telescopes will find that the central star shines at magnitude 14.6. Just one more ancillary note: While observing this cluster and dodging clouds one night, I noticed a peculiar phenomenon. When the clouds rolled over me at the 4,200-foot-high summit of Kilauea, I could feel the temperature and humidity rise. I also heard the crickets and cicadas, who were loudly chirping

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66 beforehand, cease their gnawing cacophony of song. When the wind blew, the temperature dropped and the insects resumed their singing. Actually this phenomenon has been known for ages; in fact, a correlation table between insect song and temperature has

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appeared in the Old Farmers Almanac. Well, I suppose the point is that though we have our eyes glued to our telescopes, we should keep our other senses open for equally fascinating occurrences in nature happening right here on Earth. Observing is a full sensory experience.

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67 67 Little Scorpion Cluster NGC 5281 Type: Open Cluster Con: Centaurus RA: 13h46.6m Dec: −62°55′ Mag: 5.9; 6.1 (O’Meara) Diam: 8′ SB: 10.4 Dist: ~4,200 light-years Disc: Abbé Nicolas Louis de Lacaille, listed in his 1755 catalogue a bbé ni co l as l ou is de l acail l e : Heap of 4 or 5 stars, slightly compressed. (I-7) j a mes dunl o p : (201 Centauri, Bode) This is a curved line of faint stars, about 1 1/2′ long, with a star of the 7th magnitude in the north extremity; a group of extremely faint stars on the [western] side of the crescent, and a multitude of very faint stars extended [west] and [east]. (D 273) j o hn hers chel : Small, compact irregularly round, one 8th-magnitude star, and 15 or 20 smaller in a knot. No. 1 in Sweep 578 [NGC 5269] is an outlier of it. (h 3531) ngc: Cluster, bright, small, pretty compressed, irregularly round, stars magnitude 10 to 12.

N G C 5281 is a b ri ght and ­b e au t i fu l open cluster in the dense southern Milky Way, just 3 1/4° southwest of Beta (β) Centauri (Hadar), the 10th brightest star in the sky. Abbé Nicolas Louis de Lacaille (1713–1762) discovered this cluster while surveying the night sky over the Cape of Good Hope in South Africa from April 19, 1751 to March 8, 1753. Through his 1/2-inch f/50 telescope, Southern Gems

the object appeared as a “Heap of 4 or 5 stars, slightly compressed.” Interestingly, he lists it as a Class I object (“nebulae without stars”). I say that’s interesting because when I look at it through my antique telescope I see it at first as an uncharacteristically large 6th-magnitude “star” that with even the slightest bit of concentration appears as three very prominent stars in a 263

67 slight curve  – like tiny jewels  – surrounded by an irregular glow. On the other hand, NGC 5316, an equally bright but larger cluster, lies just 1 1/4° to the northeast. Unlike NGC 5281, NGC 5316 does not resolve into starlight with the antique telescope but remains a diffuse glow. Is it possible that Lacaille got confused in this rich Milky Way region? Probably not. In his paper “On the Nebulous Stars of the Southern Sky,” which appeared in the 1755 Memoires de l’Academie Royale des Sciences, Lacaille describes his Class I objects as being “no more than a whitish, ill-defined area, more or less luminous and of a very irregular shape: these patches are quite similar to the nuclei of faint, tail-less comets.” And that is just how NGC 5281 appears in my antique telescope. In true physical extent, the cluster is relatively small, measuring only 10 light-years across. In a 2001 paper in Astronomy and Astrophysics (vol. 369, p. 511), German astronomer J. Sanner (Sternwarte der Universität Bonn) and his colleagues presented CCD photometry and proper-motion studies of three open star clusters, including NGC 5281. The researchers found that NGC 5281 has an age of about 45 million years, making it about half as young as M45, the Pleiades. As A. Marco (Universidad de Alicante, Spain) and colleagues reported in a 2009 paper in Advances in Space Research (vol. 44, p. 348), the orbiting Chandra and XMM-Newton telescopes have identified the x-ray source 1WGA J1346.5–6255 with the Be star HD 119682 – a member of NGC 5281  – which displays all the characteristics of the new class of x-ray sources known as “γ-Cas analogues.” Their spectroscopic study of the cluster revealed a sequence of evolved stars with an age of 40 million years, which agrees well with Sanner et al.’s research just mentioned. “These results

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imply that HD 119682 is a blue straggler in NGC 5281,” the researchers say, “as it is much more massive than any other cluster member, and its membership is strongly suggested by proper motion analysis. This is the second γ-Cas analogue found to be a blue straggler in an open cluster, suggesting that evolution plays a role in the formation of these systems and that a supernova explosion has not occurred in them.” To find NGC 5281, use wide-field chart 3 to find Beta Centauri. The cluster lies just 3 1/4° to the southwest. Under dark skies, NGC 5281 is visible to the naked eye as a 6th-magnitude star, even when it is very low in the sky. The object appears swollen and ill defined in 7 × 50 binoculars  – it does indeed look like the nuclear region of a tailless comet. But if you want to starhop to it with your telescope, center Beta Centauri in your telescope at low power and then switch to the accompanying chart. From Beta Centauri, move about 35′ west-southwest to 8th-magnitude Star a. Now drop nearly 1° south to magnitude 6.5 Star b. Next, sweep about 55′ southwest to open star cluster NGC 5316, which is about 45′ northwest of 7th-magnitude Star c. From NGC 5316, swoop about 35′ west-southwest to 7th-magnitude Star d and then another 30′ southwest to a pair of 7th-magnitude stars (e). NGC 5281 is a little less than 20′ south of the southernmost star in Pair e. Through the 4-inch at 23×, the cluster is a tiny package of light in the choppy seas of the Milky Way that appears to be frothing haphazardly here and there with “windblown” clumps of dim starlight. What’s interesting is that, because of its compactness, once I lock my eye onto NGC 5281, the cluster has the power to draw my attention away from everything else around it. It is as if I have found a

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67 N

a

E

b 5316

c

1˚ S

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floating chest and want to do nothing else but use my telescope as a key, lift the lid of the chest, and see what lies within. When I concentrate on Centaurus NGC 5281 with low power, the first thing I notice is a bejeweled crescent, oriented northeast to southwest, consisting of stars 8th magnitude W and fainter. Two arms jut out from the northeast end, one to the north and one to the d east. A fainter string of stars extends from the southwest e end of the curve and gradually curls away to the west. NGC 5281 With 72× and 101×, the cluster looks like a little scorpion with claws and a raised tail. And like the Milky Way around it, the cluster has all manner of geometrical patterns: long lines of stars, patches, tiny agglomerations of dim stars, arcs, diamonds, triangles. Despite the richness of the field, only 40 stars belong to the cluster. The remainder are background stars. In larger telescopes, the main form of the cluster remains the same. The only difference appears to be the sudden magnificence of the background Milky Way and the intensity of the stars’ colors. As the late Ernst J. Hartung observed through his 12-inch reflector, NGC 5281 is a “beautiful scattered cluster of fairly bright stars merging into a fine field and concentrated at the centre in a pattern of two crossing curved lines of brighter stars, yellow, bluish, white and orange.… [O]n a clear dark night this is a most lovely field.”

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68 68 NGC 5460 Type: Open Cluster Con: Centaurus RA: 14h07.6m Dec: −48°18′ Mag: 5.6 Diam: 35′ SB: 13.3 Dist: ~2,300 light-years Disc: James Dunlop, 1826 j ames dunl o p [May 7, 1826]: A curiously curved line of small stars, of nearly equal magnitudes; two stars of 7th magnitude to the [east]. (D 431) j o hn hers che l : A region of large, bright stars, 8, 9 … etc. magnitude; a very coarse cluster. Place that of a brilliant group, one of which is a double star class III. (h 3555) ngc: Cluster, very large, very little concentrated, stars 8th magnitude and fainter.

NG C 5 4 6 0 i s a bri ght and l oose open star cluster about 2° east-southeast of magnitude 2.5 Zeta (ζ) Centauri and about 7° east-southeast of the magnificent globular star cluster Omega Centauri. It’s visible in binoculars and is a wondrous sight in telescopes of all sizes, being well resolved with a “curiously curved line of small stars, of nearly equal magnitudes,” as its discoverer James Dunlop noted in 1826. And, as John Herschel later added, the coarse cluster is also the place of a pretty double star. Curiously, some modern catalogues classify it as an I3m system, meaning it is a .

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moderately rich cluster detached from the Milky Way, with a strong central concentration of stars of nearly equal magnitude. Yet this does not match the large and sparse assortment of stars seen through telescopes. In fact, Robert J. Trumpler originally listed it as a Class II3m system, meaning it has little or no central concentration, which is more befitting of the visual situation. The cluster lies about 2,300 light-years distant and spans about 23 light-years in true physical extent. It’s an intermediate-age cluster, having an age of about 160 million years. This makes it close in age to NGC

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68 6025 in Triangulum Australe and NGC 6281 (Southern Gem 87) in Scorpius. In a 1995 paper in Astronomy and Astro­ physics Supplement Series (vol. 111, p. 275), D. Barrado (Universidad Complutense) and P. B. Byrne (Armagh Observatory) tell us that studies of young, nearby open clusters are of particular interest to astronomers because some harbor very rapidly rotating late-type, main-sequence stars. The rapid rotation appears to be the result of the conservation of angular momentum (the principle that the angular momentum of an object remains constant as long as no external moment of force acts on that object) during the later phases of contraction toward the main sequence. It’s been found that in progressively older clusters rapid rotators are found only in progressively later spectral types, so rotational braking appears to be mass dependent. In addition, the timescale for braking appears to be shorter in younger clusters, suggesting that at least two braking mechanisms might be at work, the researchers say. To place constraints on possible braking mechanisms the researchers began a study of NGC 5460 to identify cluster members for future research. Their photometric observations of 353 stars at NGC 5460’s core with the 1-meter telescope at the South African Astronomical Observatory at Sutherland identified 25 cluster members, and possibly 27 more. In a 2011 paper in Monthly Notices of the Royal Astronomical Society (vol. 413, pp. 1132–1144), L. Fossati (Open University, Milton Keynes) and colleagues reported their spectroscopic

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study of 21 members of NGC 5460 with the European Southern Observatory’s Very Large Telescope. They found that these members have a nearly solar metallicity. They also provided new estimates for the cluster distance (~2,350 light-years), age (158 million years), and mean radial velocity (–17.9 kilometers per second). To find NGC 5460, use wide-field chart 3 to find Zeta Centauri. You could then use binoculars to locate the cluster roughly 2° to the east-southeast. Or you could center Zeta in your telescope at low power and then switch to the accompanying chart. From Zeta Centauri, move about 45′ southeast to 7th-magnitude Star a and then drop 40′ south to similarly bright Star b. NGC 5460 marks the eastern vertex of an acute triangle made with those two stars and is about 25′ north of 7th-magnitude Star c. Through the 5-inch at 33×, the cluster is a loose and fractured assortment of stars with about two dozen 9th- to 10th-magnitude stars in sinuous shapes across an area of sky equal to the apparent diameter of the full Moon. An inchworm-like row of stars snakes across the field from the northwest to

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the southeast, with the worm’s raised back pointing to the northeast. One of the stars central to that prominent grouping is a very pretty double star. The cluster can be broken down into four main groupings of similarly bright stars: (1) a central arc of six stars (one of which is a double); (2) a triangle of three stars a few arcminutes northwest of the central arc;

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(3) a gentle curve of four stars (in two wide pairs) a few arcminutes south-southeast of the central curve; and (4) a pretty trapezoid of four stars a few arcminutes south of that gentle curve. Being so coarse and large, it’s hard to know where the cluster truly ends, so also look for several wide pairs and arcs of stars around these groups. Overall, the low-power wide-field view shows the cluster to have a loose spiral form emanating from the central arc. Magda Streicher says that the cluster as seen through her 12-inch is “beautifully composed with stars that gleam in separate small groups, resembling a half-moon, a type of square and another separate triangle combination. It stands out well from the background star field in a north–south line and is an exceptional sight to behold with mix[ed]magnitude stars in small concentrated groups. This star field is one of the most interesting and to me just beautiful.”

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69 69 Dracula Cluster NGC 5617 Type: Open Cluster Con: Centaurus RA: 14h29.7m Dec: −60°42′ Mag: 6.3 Diam: 10′ SB: 11.3 Dist: ~6,500 light-years Disc: James Dunlop, 1826 j a mes dunl o p [May 8, 1826]: A cluster of small stars of mixed magnitudes, considerably congregated towards the centre, 4′ or 5′ diameter. (D 302) j o hn hers chel : Class VI object, very rich; irregularly round; pretty much compressed in the middle, but scattered at borders; 15′; there are three stars of 10th magnitude, 5 or 6 of 11th magnitude; the rest below 11th magnitude. (h 3570) ngc: Cluster, large, pretty rich, pretty concentrated in the middle, stars 8th magnitude and fainter.

NG C 5617 i s a smal l bu t pre t t y little open star cluster only about 1° west-northwest of brilliant Alpha (α) Centauri (Rigil Kent)  – the star system closest to the Sun. The cluster is one of six such objects in the area – the others being NGC 5606 (about 1° to the north-northwest), Lynga 2 (about 1° to the southwest), Pismis 19 (about 20′ to the south-southeast), Trumpler 22 (about 30′ to the south-southeast), and Hogg 17 (about 50′ to the south-southeast)  – so it’s quite a Southern Gems

complex of stellar gatherings, though many of them are small and sparse. When James Dunlop discovered NGC 5617 in 1826, he found it to be about 5′ in diameter, but this clearly refers to the tight knot of stars at the cluster’s center. John Herschel later extended the very compressed and rich cluster of stars to three times that size, citing in a second observation the cluster’s “irregular figure” with a “good sprinkling” of 12th- and 13th-magnitude stars. In 1928, 269

69 Robert J. Trumpler classified it as I2r  – a rich and detached cluster of stars of mixed magnitude with a strong central concentration. Some modern references, however, list it as I3r, which better reflects what John Herschel implied by his observations: that the cluster is comprised of bright and faint stars. In a 2004 paper in Monthly Notices of the Royal Astronomical Society, Giovanni Carraro (University of Padova, Italy) and Ulisse Munari (Astronomical Observatory of Padova, Asiago, Italy) reported a reddening of 0.48 magnitude, a distance of 6,500 light-years, and an age of 80 million years, which would make NGC 5617 as young as the Pleiades. If we accept their distance, NGC

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5617 would span about 19 light-years of space in true physical extent. In a 2010 paper in Astronomy and Astrophysics (vol. 513, p. 75), A. M. Orsatti (Observatorio Astronómico, La Plata, Argentina) and colleagues presented polarimetric observations of 72 stars in NGC 5617 to determine the cluster’s general characteristics and to study the characteristics of the dust lying between the Sun and the cluster. They added 32 stars to the list of members of NGC 5617 and found five blue straggler stars in the region. They also confirmed membership of two red giant stars and say that in the direction of the cluster, the interstellar medium is apparently free of dust, “from the Sun’s position up to the Carina-Sagittarius arm, where NGC 5617 seems to be located at its farthest border.” To find this little gem, use wide-field chart 3 to locate Alpha Centauri. Center that bright star in your telescope at low power and then switch to the accompanying chart. From Alpha Centauri, move 30′ to Asterism a, which is a 20′-wide grouping of 7th- to 8th-magnitude stars, shaped like a pentagram. NGC 5617 is just about 40′ west and a tad north of Asterism a. Through the 5-inch at 33×, NGC 5617 is a small, l ow –su r f a c e - b r ig ht n e ss glow with a tight ellipse of stars 5′ in extent, oriented north-northeast to southsouthwest. This central knot

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is bracketed between some 8th-magnitude stars: a triangle to the southeast and a wide pair of similarly bright stars. With averted vision, the ellipse resolves into a mottled glow, with sparkling stars that flit in and out of view. At 60×, the knot resolves into about a half dozen stars surrounded by an irregular web of fainter starlight, the geometry of which changes depending on how I position my eye at the eyepiece. Increasing the power to 94× brings a fractured ellipse of about a dozen stars crisply into view. The ellipse breaks down into little stellar aggregations, all populated with dim stars. With averted vision, I see

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the main axis of the cluster (oriented northwest to southeast) bordered to the northeast and southwest by what appears to be an open cape of starlight. With imagination, I envision Dracula standing with arms spreading his cape wide. Through her 12-inch telescope, Magda Streicher says NGC 5617 appears as one of the most elongated open clusters she’s seen. “It is very dainty in composition,” she adds, “with a sprinkling of faint stars grouped well together. The inner core is quite interesting, with a few stars in a stringy, curled formation. The field is filled with faint pinpoint stars, but the cluster makes a strong impression with a lovely display overall. On the northeastern tip of the cluster a few stars appear in a half-Moon shape.” By the way, at the tip of the southeastern triangle is the tiny, pale glow of Pismis 19. (I’ve shown just the tiny tip of it in my drawing.) This cluster, which actually lies about 1,100 light-years in the foreground, is estimated to be around 800 million years old (100 times that of NGC 5617). Indeed, in color images, Pismis 19 glows with a warm hue  – its stars being mostly yellow and orange compared to those blue crystals of NGC 5617. Quite the dramatic photographic pairing!

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70 70 NGC 5643 Type: Mixed Spiral Galaxy (SAB(rs)c) Con: Lupus RA: 14h32.7m Dec: −44°10′ Mag: 10.4 Dim: 5.12′ × 4.2′ SB: 13.6 Dist: ~55 million light-years Disc: James Dunlop, 1826 j ames dunl o p [May 10, 1826]: An exceedingly faint, extended nebula, about 10′ long; rather ill-defined. (D 469) j o hn hers che l : Pretty faint, large, round, very gradually a little brighter in the middle; has many stars involved. (h 3572) ngc: Pretty bright, large, round, very gradually a little brighter in the middle, stars involved.

N G C 5643 i s one of t hose rare ­extragalactic wonders that lies near the dusty plane of our galaxy yet shines brightly enough to be appreciated in moderate to large backyard telescopes. When I first spied it through my 5-inch, I was amazed that James Dunlop had discovered it with his 9-inch speculum-metal reflector, which was probably only about as good as a modern 6-inch telescope. That he did discover it only made me appreciate his keen eyesight. Even John Herschel, who viewed it years later with an 18-inch speculum-metal-mirror telescope, logged it as “pretty faint.” NGC 5643 is truly a wonder. This beautiful, grand-design nuclear dust spiral is

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a prototype of its class  – sporting not only tightly wound dust spirals in the circumnuclear region but also two more widely sweeping dust arms of greater contrast. A member of the Centaurus spur of galaxies, it spans about 80,000 light-years of space, has a total luminosity 33 billion times that of the Sun, and boasts a total mass of 100 billion solar masses. It is receding from us at 1,199 kilometers per second. Because we see NGC 5643 only 15° from the Galactic plane, its face is littered with foreground stars. Shining among them is the galaxy’s very small and very bright nucleus. This nuclear “spark” is embedded in a slightly elliptical bulge populated with moderately

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70 old stars. A narrow but prominent bar threads through the bulge, but it’s skewed by about 30° from the bulge’s major axis. The bar also has a strong twist, which is interesting because late-type (Sc) galaxies like NGC 5643 are not expected to show a twist and, at the same time, have a Seyfert nucleus. Bright, lumpy arms emerge from the ends of the bar and wrap tightly around the bulge, forming a feathery spiral structure that can be traced out for several windings. The outer, wider arms spread outward in branches from this region. Robust star formation is occurring currently in the nearly circular arms. In a 1997 paper in Astro­ physical Journal (vol. 404, p.121), Chris Simpson (now of John Moores University, Liverpool) and colleagues presented high-resolution observations of NGC 5643 taken with the WFPC2 on board the Hubble Space Telescope. The images reveal a halo of emission centered on the nucleus and high-excitation extranuclear emission that extends eastward for at least 6,000 light-years. The high-excitation gas is distributed within a well-defined V-shaped structure, presumably the projection of a three-dimensional ionization cone. The researchers expect there to be another cone on the other side of the nucleus, which is presumably behind the galaxy’s disk, a bit of which may be seen through the galaxy’s dust lanes.

The fine-scale structure of the ionized gas (which is related to a radio jet oriented at position angle 87°) shared the same axis as the cone. Simpson et al. also detected a dust lane straddling the nucleus and oriented perpendicular to the radio axis, and an unusually blue region of nearly 300 light-years in extent to the east of the nucleus. “Our data strongly support the unified model* for Seyfert galaxies, in which an optically thick structure perpendicular to the radio axis blocks the nucleus from direct view and allows the optical and ultraviolet radiation to escape preferentially along and around this axis.” To find this faint but fascinating galaxy, use wide-field chart 3 to find magnitude 2.5

* Unified models of active galactic nuclei in Seyferts unite two or more classes of objects, based on the traditional observational classifications, by proposing that they are really a single type of physical object observed under different conditions.

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Eta (η) Centauri and then look about 3 1/2° south-southeast for the pretty 4th-magnitude binocular pair Tau1 (τ1) and Tau2 (τ2) Lupi. Once you confirm these stars, center them in your telescope at low power and then switch to the chart on page 273. From the Tau pair, move about 40′ east to 6th-magnitude Star a. Now jump about 40′ northeast to 7th-magnitude Star b. NGC 5643 lies about 40′ north-northeast of Star b. Through the 5-inch at 33×, NGC 5643 is a fairly dim and pasty oval of light some 3′ across. Be patient if you don’t see it at first. The overall amorphous glow has a feeble sheen

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and, depending on the size of your telescope, field stars may interfere with the view  – the glow “hidden” among them. But, once I knew exactly where to look, I had no trouble seeing the galaxy with averted vision. At 60× and 94×, the dim boundaries of NGC 5643 swell to a very faint ellipse that gradually becomes almost imperceptibly brighter toward the middle. With time and averted vision, I could suspect traces (hints) of an outer glow of material; this no doubt is the feeble contribution of the wider outer arms. I could not detect the nucleus, but two dim stars are involved in the inner ellipse. When South African observer Magda Streicher observed it in April 2009 through her 16-inch Schmidt-Cassegrain, she found it “very round in figure, quite outstanding with a very gradual brightening towards its nucleus. It appears to me like a round hazy bubble blowing away in the wind. Higher power brings out a stronger small nucleus which is somewhat uneven in shape. A few pinpoint stars can be seen on the surface. A nice string of stars can be seen a few arc minutes away from the galaxy towards the northeast star field.”

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71 71 NGC 5824 = NGC 5834 Type: Globular Cluster Con: Lupus RA: 15h04.0m Dec: −33°04′ Mag: 9.1 Diam: 7.4′ SB: 13.4 Dist: ~105,000 light-years Disc: James Dunlop, 1826; rediscovered by Edward Emerson Barnard in 1884 j a mes dunl o p [May 10, 1826]: A very singular body resembling a star with a burr. The light is equal to that of a star of the 7th [or] 8th magnitude, and the diameter is not sensibly larger, with various magnifying powers. This has the appearance of a bright nucleus, surrounded by a strong brush of light; and the nebulosity surrounding the bright point has not that softness which nebulae in general possess. I consider this different from nebulae in general. (D 611) j o hn hers chel : A very strongly suspected nebula; but I cannot be quite sure (from the low situation) it is not a star. (h 4036) N G C: Pretty bright, small, stellar, pointlike nucleus.

In the far northwestern corner of the imaginary boundaries of Lupus the Wolf, clinging to the western folds of the Galactic plane about 2 1/2° southwest of 5th-magnitude 1 Lupi, lies a distant denizen of our Milky Way, globular star cluster NGC 5824, which has an interesting history related both to its discovery and to its association with our galaxy.

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Most modern sources credit eagle-eyed Edward Emerson Barnard with the discovery of NGC 5824 while sweeping for comets in June 1884. He described it as a nebula with a stellar nucleus. But, John Herschel likely saw it before him in 1831, and as will be discussed, James Dunlop found it first with his 9-inch telescope in 1826.

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71 Barnard’s rediscovery of NGC 5824 was timely enough to make it into the New General Catalogue (NGC), where it received the number designation 5824. In the 1990s, however, Glen Cozens of Australia identified NGC 5824 with the 611th object in Dunlop’s 1828 catalogue. As you might agree, Dunlop’s extensive description fits the modern visual view through a small telescope perfectly. When R. T. A. Innes observed NGC 5824 in 1925 through the 26.5-inch refractor at Union Observatory in Johannesburg, he noticed that although the Cordoba Durchmusterung marked it as a “nebula,” he found it “has almost exactly the appearance of NGC 104 (globular cluster 47 Tucanae [Southern Gem 2]), as seen in a small telescope. It is a very condensed globular cluster, its outliers resolved into stars,” he said. Indeed, the cluster was found to be a rich system with a strong central concentration, of Shapley-Sawyer Class I, meaning it has the highest known concentration of stars at the core, while the surrounding halo of stars decreases in brightness the farther away from the core one looks. In 1961, L. Rosino of the Asiago Astrophysical Observatory of the University of Padua noted in a paper in Publications of the Astronomical Society of the Pacific (vol. 73, p. 309) that, “Like many other clusters in the southern hemisphere, NGC 5824 has never been examined for variable stars.” Thus, during a two-month stay at the Radcliffe Observatory in Pretoria, South Africa, he searched the cluster using the 74-inch telescope and found 27 RR Lyrae variable stars near or inside the cluster. “The cluster NGC 5824 appears to be rich in variable stars,” he concluded, “and has such interesting characteristics that a further study from southern stations would be advisable.” R. D. Cannon (Anglo Australian Observa­ tory) and colleagues did just that. As reported

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in a 1990 paper in Monthly Notices of the Royal Astronomical Society (vol. 243, pp. 151–158), they sampled 285 stars to magnitude 20.4 in NGC 5824 and observed for the first time a well-defined blue horizontal branch in its color–magnitude diagram – an important tool for estimating the cluster’s distance from the stars’ apparent luminosities. We now know that NGC 5824 is a distant spectral type F4 globular cluster 225 light-years in true linear extent. It seems to be similar to typical moderately metal-poor globular clusters such as NGC 6752 in Pavo (Southern Gem 111) and M13 in Hercules. It is extremely distant, lying about 105,000 light-years from the Sun and some 85,000 light-years from the Galactic center, in the Milky Way’s outer halo. In 2009, Heidi Jo Newberg (Rensselaer Polytechnic Institute, Troy, New York) and her colleagues reported that NGC 5824 may be associated with a new, polar-orbiting debris stream in the Milky Way’s stellar halo. While studying spectra of Milky Way stars from the Sloan Digital Sky Survey, Newberg et al. noticed a co-moving population of low-metallicity blue horizontal-branch stars with positions and velocities near, but not coincident with, the Sagittarius dwarf spheroidal galaxy’s trailing tidal stream in the South Galactic Cap, about 110,000 light-years from the Sun. Because the most concentrated parts of this new stream are located in Cetus and the orbital path is nearly polar, Newberg et al. named it the Cetus Polar Stream (CPS). Although the CPS is spatially coincident with the Sagittarius dwarf trailing tidal tail, the metallicities, ratio of blue stragglers to blue horizontal-branch stars, and velocities of the two are significantly different. “The globular cluster NGC 5824 is located on the CPS orbit with the correct radial velocity, distance, and plausible metallicity,” they say.

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“It is additionally elongated along the orbit. NGC 5824 could be the progenitor, the core of a dwarf galaxy progenitor, or associated with a dwarf galaxy progenitor.” To find this deep gem, use wide-field chart 3 to first locate 3.5-magnitude Psi1 (ψ1) Lupi in the Wolf’s head and similarly bright Sigma (σ) Librae a little more than 10° (a fist held at arm’s length) to the north-northwest. Now look midway between them with unaided eyes or binoculars for 5th-magnitude 1 Lupi, which has similarly bright 2 Lupi about 1 1/2° to the northeast. Then sweep your gaze just 2 1/2° west-southwest to 5.5-magnitude Star a, which marks the eastern side of a 2°-wide trapezoid of similarly bright stars, best confirmed in binoculars. Center Star a in your telescope at low power and then switch to the accompanying chart. NGC 5824 is in the same low-power field as Star a, being just 30′ south-southeast of it. At 33× in the 5-inch, the globular matches Dunlop’s and Barnard’s descriptions of a star with nebulosity. Before I even knew about their descriptions, I had penned my first impression as “A star with a halo.” And that’s the way the cluster remains at all magnifications from 33× to 94×. With averted vision at 94×, I can make out a tiny and slightly swollen core.

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At powers of 165× to 330×, the cluster’s core appears fractured or mottled, while the halo remains uniformly tight and dim, extending to perhaps about 5′ at best. It might have been my imagination, but with time, relaxed breathing, and a wandering averted gaze, I suspected some hyperfine “veins” of unresolved starlight, making it appear like a moonlit dandelion. Also there are some stars superimposed that do not belong to the cluster, but these are very faint. Through her 12-inch Schmidt-Cassegrain at 218×, Magda Streicher said the cluster reminded her of a “streetlight on a wet and misty night. With higher magnitude the tight core roughly covers one third of the globular and displays a slightly soft envelope around it, changing into a soft outward haze. No stars are revealed, although the edges become faintly granular with just a few faint stars visible.” That is why I have no problem understanding why William Herschel might have missed this stellar object in his sweeps. The object, being so low in the sky from England, would have been slightly swollen because of bad seeing, and extinction might have removed its dim, nebulous appearance.

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72 72 Ghost of Uranus NGC 5882 = IC 1108 Type: Planetary Nebula Con: Lupus RA: 15h16.8m Dec: −45°39′ Mag: 9.4 Diam: 7″ Dist: ~1,800 light-years Disc: John Herschel, 1834; possibly James Dunlop, 1826 j o hn hers che l [September 27, 1834]: A most elegant and delicate planetary nebula. Diam[eter] in right ascension equals 1.35 seconds by many observations. Long contemplated with 180×, 240×, and 320×. The disc is magnified by the power in due proportion. It is equal to a star of 8.9 magnitude; perfectly sharp, not the slightest haziness. A very fine object. It has no “satellites.” My attendant, to whom I showed it, said it was like the moon, only smaller, and not in the least like a star. (h 3594) j ames dunl o p: A very small nebula, with a very minute star involved in the north side; the nebula is about 1′ north of a star of the 9th–10th magnitude. (Possibly D 447) NG C: Planetary, very small, round, quite sharp.

NGC 5882 is a bright but small ­planetary nebula about 1 1/4° east-southeast of 4.5-magnitude Lambda (λ) Lupi or about 1 1/4° southwest of 4th-magnitude Epsilon (ε) Lupi. It’s bright enough to be seen in supported binoculars, but it is a challenge to observe details in it through most backyard telescopes.

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John Herschel’s discovery record of the nebula is of particular interest for two reasons. First, note how he “long contemplated” it with various magnifications, with each increase making the perfectly sharp disk proportionally larger so that it looked more like the Moon (as seen with the unaided eye I presume)

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72 than a nebulous object. Second, note how the apparent view seemed so planet-like that he obviously took the time to search for orbiting satellites. After all, it is similar in size to Uranus, which his father, William, had discovered. As the younger Herschel noted in a second observation, the object has “a clear round planetary white disc, at most 4 arcseconds diameter.” Nevertheless, by 1835, John Herschel had clearly identified the object as a nebula. In his extract of a letter to Francis Baily from the Cape of Good Hope and dated October 22, 1834 (Monthly Notices of the Royal Astronomical Society, vol. 33, p. 75, 1835), the younger Herschel relays his discovery of NGC 5882: “On the 2[nd] of July I was fortunate enough to light on another very delicate and beautiful planetary nebula … having a perfectly sharp disc.… My assistant J. Stone, to whom I [showed] it, said it was like the moon, round and clean, only smaller.” In 1894, Williamina Paton Fleming (1857–1911) anno­ unced in Astronomische Nach­ richten (vol. 137, p. 71) that a superposition of a chart and spectrum plate taken at Harvard College Observatory’s Peruvian Station of a star “shows that this object is in reality a gaseous nebula.” This discovery, along with four others, brought to 60 the number of planetary nebulae known at the time. For some reason, though, perhaps owing to a typo, the object was not recognized as NGC 5882 and was added to the second Index Catalogue as IC 1108. So IC 1108 equals NGC 5882.

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The planetary is reasonably near but also small. As W. Martin (Planetarium Wolfsburg, Germany) explains in a 1994 paper in Astronomy and Astrophysics (vol. 281, p. 526), he sought to derive a distance to NGC 4882 using what’s known as the “extinction method,” which requires knowledge of the interstellar reddening of the object (usually a well-known quantity). By using a diagram of color excess versus distance derived from neighboring field stars, he says, “the object distance is easily deduced.” His study led to an extinction distance of about 1,800 light-years. If we accept that distance, the planetary has a true linear extent of only 0.1 light-year. Nevertheless, the planetary is rich in detail. In a 2000 paper in Astrophysical Journal (vol. 542, pp. 861–869), Romano L. M. Corradi (Instituto de Astrofísica de Canarias) and colleagues present images and high-resolution spectra of NGC 5882. Their modeling of its

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structure, based on observations obtained at the European Southern Observatory’s 3.5-meter New Technology Telescope and with the NASA/ESA Hubble Space Telescope (HST), shows that it is composed of a markedly elongated inner shell and a less aspherical outer shell that’s expanding at a considerably higher velocity than the inner one. Several low-ionization knots are embedded in NGC 5882’s outer shell. “The lack of symmetry in the distribution of the observed low-ionization structures,” they

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say, “makes it possible that they are the result of in situ instabilities.” Indeed, look at the accompanying Hubble Space Telescope image (see page 279) released in April 2011. It shows in striking detail both the elongated inner shell of gas and fainter aspherical shell – each of which displays an intricate array of knots, filaments, and bubbles. At the center is the nebula’s dying star, blazing forth with an incredible surface temperature of about 70,000 Celsius. (By comparison, the Sun’s surface temperature is only about 5,500 Celsius.) The high surface temperature of NGC 5882’s central star results from its struggle to survive by finding new ways to prevent itself from collapsing under its own gravity. To find this little wonder, first use widefield chart 3 to find Epsilon Lupi. Center that star in your telescope at low power and then switch to the accompanying chart. From Epsilon Lupi, move 20′ southwest to 7th-magnitude Star a. Then drop 1° south-southwest to 7.5-magnitude Star b. NGC 5882 is a little more than 10′ northwest of Star b. Through the 5-inch at 33×, the nebula shines simply as a 9th-magnitude star without any hint of a disk. This holds true up to powers of 94×, with no obvious change at all. Increasing the power to 165× finally reveals a sharply defined disk, a pale “planet,” like Uranus without a hue. Pushing the magnification in increments up to 330× reveals an everincreasing disk of light. Studying the nebula

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72 long and hard brings to view a tiny inner disk that appears like a swollen star. This is not the central star but more likely the unresolved inner shell. Through her 12-inch Schimidt-Cassegrain at 218×, Magda Streicher says it’s still “an

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out-of-focus, very blue, frosted star-like object amongst a field of various magnitude stars.” Don’t be afraid to push the magnification to the limit on this one, especially if you are using a larger telescope.

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73 & 74 73 NGC 5927 Type: Globular Cluster Con: Lupus RA: 15h28.0m Dec: −50°40′ Mag: 8.0 Diam: 6′ SB: 11.9 Dist: ~25,100 light-years Disc: James Dunlop, 1826 j ames dunl o p [May 8, 1826]: A very fine round pretty bright nebula, about 3′ diameter, gradually brighter towards the centre, and well defined at the margin: this is resolvable. With a power of 260[×] it has a beautiful globular appearance. The stars are considerably scattered on the south side. (D 389) j o hn hers che l : Globular, bright, light, round, gradually brighter in the middle, diameter in right ascension equals 16 seconds. Comes up to a bright blaze in middle. Resolved by left eye. Stars 17th magnitude. (h 3604) NG C: Globular cluster, considerably bright, large, round, very gradually brighter in the middle, very well resolved, clearly consisting of stars, stars 15th magnitude.

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73 & 74 74 NGC 5946 Type: Globular Cluster Con: Norma RA: 15h35.5m Dec: −50°40′ Mag: 9.5 Diam: 3′ SB: 10.8 Dist: ~ 34,600 light-years Disc: James Dunlop, from his handwritten notes not included in his 1828 catalogue j a mes dunl o p : (From Dunlop’s handwritten notes, not included in his 1828 catalogue) j o hn hers chel : Not very bright, small, gradually a little brighter in the middle, 90 arcseconds, resolved into stars of 16th magnitude, with one of 12th magnitude, at or a little beyond the [southwest] edge. (h 3607) N G C: Globular cluster, considerably bright, pretty large, round, very gradually a little brighter in the middle, well resolved, stars of 16th magnitude.

A lt h ou g h we se e ce l e st i al pairings projected on what appears to be a two-dimensional sky, knowing the estimated distances of the objects helps us to break through the crystalline sphere and see them in our mind’s eye in 3D. Such is the dramatic case with our next two targets, globular star clusters NGC 5927 and NGC 5946  – a visual yin-yang pairing about 3 1/2° east-northeast of 3.5-magnitude Zeta (ζ) Lupi. The yin-yang aspect becomes evident in a low-power field of view. While NGC 5927 is Southern Gems

a pretty globe of light 12′ across, NGC 5946 is a much smaller and dimmer patch of light about 1′ to the east. This is understandable, because NGC 5927 lies 25,100 light-years from the Sun and 15,000 light-years from the Galactic center, while NGC 5946 is nearly 10,000 light-years farther from us and 19,000 light-years from the Galactic center. Dunlop discovered both globulars on May 8, 1826. Australian astronomer Glen Cozens, however, found NGC 5946 described in Dunlop’s handwritten notes, which were not 283

73 & 74 included in his 1828 catalogue. Even John Herschel’s impression of NGC 5946 through his 18-inch speculum-metal-mirror telescope was “not very bright.” Let’s look now at the two clusters separately, as clearly they are not physically related. Globular clusters that lie at low galactic latitudes are difficult to study owing to the interference by intervening dust and the contamination of foreground stars. Both NGC 5927 and NGC 5946 suffer from these effects. Nevertheless, in a 1979 paper in Acta Astronomica (vol. 29, p. 281), G. Alcaino (Ministerio de Educacion de Chile, Chile) made color–magnitude diagrams for NGC 5927 and found a flat giant branch and a stubby red horizontal branch with metal-rich features typified by 47 Tucanae (Southern Gem 2). His analysis of the giant branch indicates that the reddening is different in two distinct sectors of the cluster field. A study by Eileen D. Friel (Dominion Astrophysical Observatory, Victoria, Canada) and Doug Geisler (Cerro Tololo InterAmerican Observatory, La Serena, Chile) in a 1991 paper in Astronomical Journal (vol. 101, pp. 1338–1351) also revealed that the cluster has well-defined giant and subgiant branches. Their photometry for more than 100 giants in the cluster led to the determination that NGC 5927 is metal-rich, with each star in the cluster having about as much metal as the Sun. More recently, Laura K. Fullton (Space Telescope Science Institute) and her colleagues presented Hubble Space Telescope photometry of NGC 5927 that indicates that the cluster is somewhat younger than other disk globular clusters with known ages. Observations of the relative numbers of stars on the red giant branch and red horizontal branch indicate a large helium abundance. “Comparison of the seven known thick disk globular clusters that

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have estimated ages with ages of globulars that belong to the halo,” they say, “reveals a significant overlap in age between the two cluster systems. As a group, the disk clusters appear somewhat younger than their halo counterparts, indicating that the Galactic halo began to form some 1–2 [billion years] before the thick disk.” If we accept the cluster’s distance as 25,100 light-years, we see its form stretching across about 90 light-years of space. Dimmer and fainter NGC 5946, on the other hand, avoided photometric study until the mid-1980s. In a 1991 paper in Astronomical Journal (vol. 102, p. 1371), Gonzalo Alcaino (Instituto Issac Newton, Santiago, Chile) and his colleagues report how they used the 2.2-meter Max-Planck-Institut telescope at the European Southern Observatory in La Silla, Chile, in 1985 to perform the first known photometric investigation of the cluster of 514 stars ranging from about magnitudes 14 to 21.5. This study probes about four magnitudes below the brighter edge of the horizontal branch  – the part of the cluster’s Hertzsprung-Russell diagram that contains stars with masses of 0.6 to 0.8 solar masses. These stars, which are fainter and hotter than those on the diagram’s giant branch, are burning helium steadily in their cores and hydrogen in a surrounding envelope. Alcaino et al. found NGC 5946 to be a medium-metallicity object with about 1/25 as much metal in each of its stars as in the Sun. The cluster’s giant branch extends in magnitude from about 14 to 17, while the horizontal branch has a more heavily populated blue part. Four years later, however, T. J. Davidge (University of British Columbia) published in a 1995 paper in Astronomical Journal (vol. 110, p. 1177) new near-infrared photometry of the cluster that found the upper giant branch of its color diagram to

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73 & 74

be relatively steep, indicating that the cluster is actually metal-poor, having only about one-hundredth as much metal in each star as the Sun. To find this odd globular pairing, use widefield chart 3 to locate Zeta Lupi. Then use binoculars to find 6th-magnitude Star a, about 2 1/2° east-northeast and refer to the accompanying chart. Confirm Star a with the chart. Note how it is the easternmost star in a triangle of stars about 1° in length. Center Star a in your telescope at low power. NGC 5927 is a little less than 1° north of Star a, about 12′ west-northwest of 7.5-magnitude Star b. Through the 5-inch at 33×, NGC 5927 looks like a comet with a moderate degree of central concentration in a rich and very attractive field of view, littered with stars of mixed magnitudes and colors. With averted vision, the cluster’s outer halo appears to twinkle with clumps of starlight at the limit of vision. Increasing the power to 60×, the view is more pronounced and requires attention. Several stars interact with the cluster’s halo. Two conspicuous field stars also frame the globular beautifully, one to the northwest and the other

Southern Gems

to the south. The cluster’s inner halo appears patchy and asymmetric, especially to the northeast, where, with imagination, it looks like it is being pulled like taffy. The cluster’s inner region really comes alive at powers between 94× and 330×. An obvious wedge of starlight tapers to the northeast, and flares of star clumps branch off to the northwest and east. The entire northern side of the cluster is weighty with stars, while its southern half is an anemic and frayed haze. With keen averted vision, four “arms” extend symmetrically away from the center to the northwest, northeast, southwest, and southeast, so it looks like a decapitated starfish. The cluster resolves well in larger telescopes, with the brightest stars shining at magnitude 14.5. Through her 12-inch f/10 SchmidtCassegrain at 218×, for instance, Magda Streicher found NGC 5927 a “large oval which grows increasingly brighter to a small round compact core. It displays fringy edges with

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73 & 74 well-defined outliers to the south. It is outstanding in a beautiful busy star field.” NGC 5946 lies a little more than 1° east and slightly north of Star b. It is a more troublesome object to observe. Even Harvard Observatory’s director, Harlow Shapley, referred to its image on photographic plates as “a small poor loose cluster in a rich region” and included it on a list of “doubtful objects.” Indeed, at 33× in my 5-inch under very dark skies, I needed to know the cluster’s exact location before I could glimpse its weak glow, appearing only a few arcminutes across, like comet fluff feebly shining far from the Sun. Imagine just a little puff of light. But this is understandable. If we accept the cluster’s distance of 34,600 light-years, we see its form stretching across only about 50 light-years of space, so the cluster is not only farther away than NGC 5927 but almost twice as small in true linear extent. Add extinction effects and you can see why NGC 5946 is such a poor visual performer. At 60×, I found the low–surface-brightness object more noticeable just 5′ northwest of a pretty 12′-long rowboat-shaped asterism of 10th- and 11th-magnitude stars. The boat reminds me of the ancient Egyptian depiction of a solar boat – the kind that carried Ra, the Sun god, through the Underworld on his way from setting to rising. With averted vision at this power, I could detect a core with a slight enhancement to the northwest. At powers between 94× and 247×, it becomes apparent that the bright “core” I saw was actually a combination of the inner core and a field star to the southwest. Once that became apparent, the already tiny cluster diminished further in size. I could make out an apparent asymmetry to the northeast, though I believe this is an artifact of the proximity of the star to the southwest. Still, I could see what appears to be three stubby

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“arms” creating a wedge of dense starlight to the northeast, beyond which appears a hazy bubble of unresolved stars, so it looks like a fuzzy bubble about to burst. Through her 16-inch Schmidt-Cassegrain, Streicher says that NGC 5946 displays itself only as a “small cloud of mist, but surprisingly obvious. Higher magnification reveals a sandpaper impression, and averted vision brings out the faint flickering of starlight on its surface. The edges also become slightly rough and uneven.” By the way, New York astronomer Lewis Swift perhaps rediscovered this object on May 24, 1898, and he listed it as the 4,550th entry in the Index Catalogue, so IC 4550 equals NGC 5946. As Harold Corwin, director of the NGC/ IC Project (www.ngcic.org), explains, “Swift’s RA for this globular is 40 seconds of time too small, but his declination and description are good. He apparently did not resolve the cluster – not surprising as it would have been, at most, only five or six degrees above his horizon. I actually am surprised that he called this ‘[pretty bright]’ (Dreyer changed this to ‘[bright]’ for the IC entry) – this is one of the fainter Galactic globular clusters.”

Deep-Sky Companions

75 75 NGC 5986 Type: Globular Cluster Con: Lupus RA: 15h46.1m Dec: −37°47′ Mag: 7.6; 7.8 (O’Meara) Diam: 9.6′ SB: 12.5 Dist: ~34,000 light-years Disc: James Dunlop, 1826 j a mes dunl o p [May 10, 1826]: A beautiful round pretty bright nebula, about 2′ diameter, pretty well defined. (D 552) j o hn hers chel : Globular, very bright, round, very gradually bright in the middle, diameter in right ascension 10″; all stars; a star of 10th magnitude [east of ] the centre by four seconds, and is involved; three stars 13th magnitude in the middle. (h 3611) N G C: Remarkable, globular cluster, very bright, large, round, very gradually brighter in the middle, stars from magnitude 13 to 15.

NG C 5986 is a lit tle-studied, compact globular cluster lying in the Milky Way’s inner halo 15,600 light-years from the Galactic center. It is partly immersed in a rich swath of Milky Way that sweeps through northern Lupus the Wolf. An ancient constellation, Lupus probably dates to the Akkadians of Mesopotamia (~2350 BC), who saw the stars in this region not as a wolf but as Urbat, the Beast of Death. Who, or what, that beast was remains unknown. The Greeks and Romans later identified it as a wild beast, and

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in the second century BC Aratos referred to it in his Phaenomena as the sacrificial beast clutched in the Centaur’s hand. The constellation did not become recognized as a wolf until Renaissance times, apparently after an erroneous translation of Al Fahd, the Arabian name for either a lioness, leopard, or panther. And while the constellation has no clear mythology, some Greek scholars believe that the wolf is the wicked King Lycaon of Arcadia. According to one version of the story, Zeus, disguised as a day laborer, descended to

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Earth to investigate some rumors of Lycaon’s evil ways. When Zeus arrived at Lycaon’s palace, the “laborer” was greeted with mock hospitality and treated to a feast that included one of his own sons as dessert. Outraged and disgusted, Zeus disposed of his disguise and blasted Lycaon’s palace with thunderbolts. The terrified king tried to escape Zeus’s wrath by fleeing into some neighboring fields, but Zeus was quick to transform this savage beast into, well, a savage beast. Lycaon tried to cry out for help, but all he could do was howl, as his royal robes transformed into shaggy hair and his mouth filled with hideous fangs, which he used to ravage his own sheep with Lycaon barbarity. James Dunlop discovered this sheepish cluster on May 10, 1826, while surveying the southern skies from Parramatta, New South Wales. He listed it as the 552nd object in his 1828 catalogue of nebulae and star clusters. Through his 9-inch f/12 reflector, he described it as a “beautiful round pretty bright nebula … pretty well defined.” An inner halo object, NGC 5986 has an intermediate metallicity with 1/30 to 1/49 as much iron (relative to hydrogen) as the Sun. Two very bright and highly evolved Type A–F stars have also been identified in the cluster; these pulsating red giants shed matter in the form of a strong stellar wind and are probably

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nearing the planetary nebula stage of their evolution. NGC 5986’s color–magnitude diagram reveals a blue horizontal branch of stars  – those of spectral type B3 to A0 that have evolved past the red giant stage and are now burning helium instead of hydrogen at their cores. The cluster’s color-magnitude diagram is very similar to those of M2 or M13, which are similar in age, being around 13 or 14 billion years old. But, as William Harris says, “An age difference of ± 1 billion years is at the limits of distinguishability for present-day precision photometry. Any difference quoted smaller than that, I simply would not believe unless it is based on really exceptional data and really careful model fitting.” The cluster also contains about a half dozen RR Lyrae variables, which yield a distance of about 36,500 light-years from the Sun, in good agreement with William Harris’s distance to the cluster. NGC 5986’s true physical diameter, then, is about 100 light-years – almost one-third the size of Omega Centauri, the biggest globular in the Milky Way Galaxy. To find NGC 5986, use wide-field chart 3 to find 3rd-magnitude Eta (η) Lupi. Center that star in your telescope at low power and then switch to the accompanying chart. From Eta Lupi, move about 55′ north-northwest to 6.5-magnitude Star a. Now make a slow sweep 1 1/4° west and slightly north to a pair of 7.5-magnitude stars (b). NGC 5986 lies another 1 1/4° to the southwest, just west of another pair of stars (c) of 7.5 and 6.5 magnitude. Under a dark sky, the cluster is easily spied in 7 × 50 binoculars. It’s an absolutely

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75 beautiful object in the antique telescope, having a clearly defined comet-like glow. Had this object been just a little farther north, Messier and his contemporaries would have certainly nabbed it for masquerading as a comet. At 23×, in the 4-inch NGC 5986 displays a bright, moderately condensed core surrounded by an aura of dim stars. These stars, the brightest of which shine at 13th magnitude, flit about like milkweed seeds on a hot summer’s day. At low power, the cluster has a soft aquamarine hue, and at higher powers it looks somewhat yellowed, like an eye with a cataract. In fact, depending on how you look at it, the cluster’s total color can alternate between the two hues. This confusing view made sense after I saw a CCD of NGC 5986 made with the 35-inch reflector atop Cerro Tololo in the Chilean Andes. It shows the cluster to be comprised of a fine mix of yellow and blue stars distributed equally throughout the cluster. At 72× in the 4-inch, I can see a number of stubby arms, like those of a starfish. The cluster is punctuated on the eastern flank by a 12th-magnitude foreground star. Look also for two 13th-magnitude stars in the cluster’s northeast quadrant; these are the two Type A–F stars just mentioned. NGC 5986 takes magnification well, so go as high as you want. I found 216× best for seeing the finer details in a 4-inch. The inner core measures about 2.5′ across and is very complex. Several stars or groups of stars punctuate a circular central patch, which has rays of stars

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extending to the south, southwest, and west. The larger outer halo is fractured and frayed, with long tufts of unresolved starlight that look like weeds growing out from the base of a circular rock; some are populated with bits of starlight, looking like ants in the grass. The most prominent tufts extend to the southwest and northwest. Can you see a dark, L-shaped rift in the cluster’s halo on the southwest side? All these features make the cluster look disheveled, aged, and worn, like a weather-beaten flag flapping above a pirate’s rig. It’s quite a wonderful globular.

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76 76 NGC 6025 Type: Open Cluster Con: Triangulum Australe RA: 16h03.3m Dec: −60°25′ Mag: 5.1; 5.5 (O’Meara) Diam: 15′ SB: 11.0 Dist: ~2,500 light-years Disc: Abbé Nicolas Louis de Lacaille, included in his 1755 catalogue abbé ni co l as l ou is de l acail l e : Three faint stars in a straight line, surrounded by nebulosity. (III-10) j ames dunl o p: (λ Circini, Bode) Lacaille describes this as three small stars in a line with nebula. No particular nebula exists in this place. A group of about twenty stars of mixed magnitudes, forming an irregular figure, about 5′ or 6′ long, answer to the place of the λ. This is in the milky way; and there is no nebula in the group of stars except what is common in the neighbourhood. (D 304) j o hn hers che l : Large, brilliant cluster VII class; fills field, not rich, stars 8, 9, 10, 11th magnitude [and fainter]. (h 3616) NGC: Cluster, bright, very large, pretty rich, little compressed, stars of 7th magnitude and fainter.

Alpha (α) and Beta (β) Centauri point almost directly to the 5th-magnitude open cluster NGC 6025 in the south circumpolar constellation Triangulum Australe. The cluster literally kisses the border of Norma

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and is only 3° southwest of NGC 6087 (Southern Gem 78). The Southern Triangle is a fine constellation. It first appeared in Johann Bayer’s 1603 Uranometria, but Richard Hinckley

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76 Allen notes that Pieter Theodor had formed it nearly a century earlier. Unlike the sky’s dim Northern Triangle, Triangulum Australe is obvious, being comprised of three stars of nearly equal brightness  – the “three patriarchs” (probably Abraham, Isaac, and Jacob) of Jansenius Caesius, a seventeenth-century Dutch globe maker. Beta marks the Triangle’s northern tip. Astronomers used to believe that Beta was 10 parsecs (32.6 light-years) distant, so it was the standard for determining absolute magnitudes. Hipparchos data, however, have since revealed that Beta is 40 light-years away. In 2009, A. Feinstein reported photoelectric measures for 78 stars in NGC 6025, from which he derived a distance of about 2,500 light-years and an age of 100 million years, making the cluster a contemporary (loosely speaking) of the Pleiades (M45) star cluster in Taurus. Feinstein also notes that the brightest star, HD 143448, is a known emission star and is very probably a member of the cluster. Other studies have shown that more than 40–50 percent of the cluster’s members are probable binary systems, a high frequency for an open cluster. These binaries have components of similar spectral type and high rotational velocity. To find NGC 6025, use wide-field chart 3 to find Alpha Centauri. Then use your binoculars to look 10° (a fist held at arm’s length) due east for a bright but fuzzy string of starlight. It’s a lot easier, though, to hop 3° northeast of magnitude 2.8 Beta (β) Trianguli Australis (see the accompanying chart). The Milky Way runs through that constellation, so the region is rich with faint stars. From a dark-sky site, the cluster stands out to the naked eye as a puff of soft light. NGC 6025 lies 1° due east of a tight curve of three 6th-magnitude stars oriented north

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to south. The cluster’s brightest star shines at magnitude 7.1, so it makes a good naked-eye challenge. Note that my naked-eye estimate of the cluster’s apparent magnitude, given earlier, is 0.4 magnitude dimmer than Brent Archinal’s published value. But from Hawaii the cluster culminates only about 10° above the horizon, so I encourage observers in the Southern Hemisphere to try estimating its brightness with the naked eye and see if you don’t get a value closer to Archinal’s. Through 7 × 35 binoculars, NGC 6025 looks like a dagger of stars, with a row of three stars surrounded by an elongated halo of unresolved stars. And this is exactly what Abbé Nicolas Louis de Lacaille saw when he discovered it in his 1/2-inch 8× telescope: “three faint stars in line in nebulosity.” (Seen this way, the cluster resembles Carolyn Shoemaker’s famous “squashed comet,” Comet Shoemaker-Levy

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76 9, which smashed into Jupiter, for it looks like the fragmented nucleus of a comet shrouded in a coma of scintillating dust.) James Dunlop observed it on five occasions and saw no nebulosity. Lacaille’s nebulosity, of course, was just the haze of unresolved starlight. The nineteenth-century British observer Rev. Thomas W. Webb called it simply a “brilliant cluster.” With north up at 23× in the 4-inch, the cluster’s dozen or so brightest stars form the Greek letter Chi (χ). The brightest star resides in a very wide double of nearly equal intensity and punctuates the cluster’s southeastern arm. This very wide double also forms the eastern half of a trapezoid along with two slightly dimmer stars, the southwestern most of which is an even tighter double. The cluster contains no obvious central condensation, though the χ asterism lies on a blanket of fainter stars. Because NGC 6925 is large and scattered, the view is best at low power, but increasing the magnification to 72× and higher will bring out some nice pairings of stars and a Southern Cross asterism just north of the cluster’s “center.” It is hard to determine the full extent of the cluster because of the rich Milky Way background. Although most observers see 30 stars here out

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to about 5′, Archinal has tallied 139 members out to 15′ (11 light-years). If you return to the 1°-long arc of three 6thmagnitude stars and move 1° southwest of its southernmost star, you’ll encounter the small, pale glow of NGC 5759, an 11.5-magnitude planetary nebula less than 25″ across (not shown on the chart here). And if you return to Beta Trianguli Australis and look about 35′ to the southeast, you’ll find the copper-colored Cepheid S Trianguli Australis, whose light varies between magnitudes 6.1 and 6.7 about every 6.3 days.

Deep-Sky Companions

77 77 NGC 6067 Type: Open Cluster Con: Norma RA: 16h13.2m Dec: −54°13′ Mag: 5.6 SB: 11.5 Diam: 15′ Dist: ~5,700 light-years Disc: James Dunlop, 1826 j a mes dunl o p [May 8, 1826]: A pretty large cluster of small stars of mixed magnitudes, about 12′ diameter; the stars are considerably congregated towards the centre, extended [southwest] and [northeast]. (D 360) j o hn hers chel : The chief star in middle of a most superbly rich and large cluster, 20′ at least in diameter, as it much more than fills field; not much compressed in the middle, stars 10th to 12th magnitude. (h 3619) NGC: Cluster, very bright, very large, very rich, little compressed, stars of 10th magnitude and fainter.

Fo r s u ch a “su pe rflu ou s constellation” ­created by Nicolas de Lacaille in the 1750s (as author Ian Ridpath tells us in his book Stars & Planets), Norma contains one of the sky’s most stunning open star clusters: NGC 6067. Visible as a twinkling little gem in handheld binoculars, just 25′ north-northwest of 5th-magnitude Kappa (κ) Normae, NGC 6067 truly packs a visual punch through telescopes of all sizes and offers a myriad of fanciful shapes. Southern Gems

NGC 6067 is a small but compact star cluster in the Norma Star Cloud, located in the same inner spiral arm of the Milky Way that contains M24 (the small Sagittarius Star Cloud). The Norma spiral arm contains the most massive molecular clouds as well as the most far-infrared-luminous regions of massive star formation in the Galactic disk. Not unlike M11 in the Scutum Cloud, E. E. Barnard’s Gem of the Milky Way, the Norma Star Cloud has NGC 6067 at its visual heart. 293

77 NGC 6067 is superposed on, but some 3,000 light-years closer than, the Norma OB1 association, a gathering of O- and early B-type stars 1.5° southwest of the cluster. The cluster is some 15 to 20 times richer than the Pleiades (M45) in Taurus and shares a similar age (60,000 years). Robert J. Trumpler initially classified it as I2r, meaning it is a rich cluster detached from the Milky Way with a strong central concentration and stars of medium range in brightness. Many modern sources have reclassified it as a I3r system, the only difference being that the cluster is comprised of bright and faint stars. What’s important about NGC 6067 is that, as reported in a 1983 paper in Astronomical Journal (vol. 88, p. 379), Olin J. Eggen (Cerro Tololo Inter-American Observatory, La Serena, Chile) discovered two Cepheid variable stars (great distance indicators) possibly associated with it: one 18′ from the cluster’s center and one 2′ from the cluster’s center. The former one he found to lie at a common distance with the cluster at about 4,300 light-years. The latter one, however, Eggen suspected did not belong to the cluster but rather to the background Norma OB1 association. But a detailed study reported two years later concluded that both Cepheids are likely cluster members with a common distance of about 5,700 light-years. Eggen also noted the presence of a previously known Cepheid (GU Normae) in the cluster field but one that, being 1° away, is not likely a member. To find this superb cluster, use wide-field chart 3 to locate Kappa Normae, which is about midway between 3rd-magnitude Zeta (ξ) Arae and 3rd-magnitude Zeta Lupi. Center Kappa Normae in your telescope at low power and then switch to the accompanying chart. NGC 6067 is just 25′ north-northwest of it; you can’t miss it!

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At 33× through the 5-inch, the cluster is big and beautiful  – about the apparent size of a half Moon and rippling with starlight. The cluster is loosely concentrated, with a circular core that has an obvious “hole” in it. This core is boxed in by field stars, so it looks like a stellar gift. At a glance, the stars seem to glitter about a prominent double star near the cluster’s center. About a dozen stars form a U-shaped asterism to its southwest, and a rich oval of starlight extends to its northeast. Seen another way, the double star appears in the northeastern segment of an S-shaped asterism of stars. A wide halo of irregular starlight blazes forth around it, and the eye just can’t seem to catch up with the stellar multitude that shines forth. At 60×, the cluster resolves nicely into myriad stars (about 100 have been noted through amateur telescopes). I did not bother to count. I just wanted my eyes to soak up the view and set my imagination free. If I relax my gaze and just watch, the cluster takes on an elliptical form, northeast to southwest, surrounded by a large U-shaped outer halo of dim stars. Deep-Sky Companions

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At 84×, the cluster has a wide, sweeping spiral form more open to the northeast than to the southwest. The wide arms appear to be turning like a hurricane that’s breaking up.

Southern Gems

Through her 16-inch at 290×, Magda Streicher said the “very lively compact cluster [takes] on the shape of a flower, [whose] leaves fold into one another. It is a delicate and stringy cluster with a double star to show off the pollen towards the centre. . . . Even visible with the naked eye in dark skies. I guess it could hold more or less 150 various magnitude stars. Lovely 6.5 magnitude star on the southwest edge and even a planetary in the fringes slightly more south.” The planetary nebula she refers to is Menzel 2. It’s actually not near the cluster but about 20′ south-southeast of Kappa Normae. The nebula shines at magnitude 12.0, appears only 25″ × 21″ across, and has a ring structure surrounded by a fainter halo. Its catalogue position is 16h14.5m in right ascension and –54°57′ declination.

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78 78 S Normae Cluster NGC 6087 Type: Open Cluster Con: Norma RA: 16h18.9m Dec: −57°54′ Mag: 5.4; 5.2 (O’Meara) Diam: 15′ SB: 11.3 Dist: ~3,300 light-years Disc: James Dunlop, 1826 j ames dunl o p [May 8, 1826]: A group of very small stars of an irregular branched figure, 15′ or 20′ diameter. The central part is very thin of stars. (D 326) j o hn hers che l : Cluster VIII class, large, loose, brilliant, irregular figure, fills field. (h 3622) NG C: Cluster, bright, large, little compressed, stars of magnitude 7 to 10.

NGC 6087 is a bright, scattered, and rich open cluster in Norma, 3 3/4° south and slightly east of NGC 6067 (Southern Gem 77). The 5th-magnitude cluster lies just beyond a fabulously rich and dense region of Milky Way. Known as the S Normae Cluster, NGC 6087 is a dynamic assortment of 349 stars centered on the bright Cepheid variable star S Normae. James Dunlop discovered this “group of very small stars of an irregular branched figure” in 1826 and listed it as the 326th object in his Catalogue of Nebulae and Clusters of

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Stars in the Southern Hemisphere. Later, John Herschel would observe it three times. In his last observation, he called it a “superb cluster; very large and rich . . . a fine object. Much more than fills field.” Today we know that NGC 6087 is a relatively young open cluster 65 million years young. It has been of particular interest to astronomers because of its association with its Cepheid Lucida. Cepheids have extremely stable periods of variability and very reproducible light curves. In the early twentieth century, Harvard astronomer Henrietta S.

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78 Leavitt discovered that Cepheids have a very special relationship between their intrinsic brightness, or luminosity, and their period of variability. Basically the brighter a Cepheid is the longer it takes to complete one cycle of brightening and fading. By observationally determining the period of a Cepheid variable, we learn its intrinsic brightness. Then, by comparing this figure to how bright it really is, we can deduce its distance. Applying the period–luminosity rule to S Normae, we get a distance of 1 kiloparsec, or 3,300 light-years. To find it with the naked eye, use widefield chart 3 and locate Alpha (α) and Beta (β) Centauri. Just 5° east of Alpha is the 4thmagnitude, naked-eye pair Beta and Gamma (γ) Circini. And a little more than 5° farther east-northeast of Gamma Circini is Iota1 (ι1) Circini, the westernmost of four 5th- and 6thmagnitude stars that form a 2°-long, east– west trending chain. The chain’s easternmost “star” is the open cluster NGC 6087, about 25′

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east of 6th-magnitude Star b, as shown in the accompanying chart. Under dark skies, the 15′-wide cluster is just visible to the unaided eye, though I find it difficult to separate it from Star b. Averted vision is required to see the combined glow of the cluster’s brightest stars. Most of NGC 6087’s light comes from S Normae, a true cluster member. A Cepheid variable, S Normae fades from magnitude 6.12 to 6.77, then brightens again, every 9.75 days. The naked-eye view may take a lot of squinting; try brief, relaxed bouts of averted vision. In 10 × 50 binoculars, I estimated the cluster’s magnitude to be 5.2, which is 0.2 magnitude (20 percent) brighter than the published value. Actually, I was quite taken with the binocular view. Train your binoculars on the cluster, and you should see it as an east–west trending chain of stars surrounded by a magnificent halo of unresolved stars. S Normae immediately draws attention to itself with its orange color. A 9th-magnitude star pops into view at the extreme northern edge of the halo, and a pair of 8th- and 9th-magnitude stars mark the halo’s southern boundary. Overall, the binocular view reminds me of a granite Celtic cross, the kind you’d see in a cemetery, as if moonlit on a foggy night. I cannot see the background glow with the naked eye, only the bright bar of stars. The background swells into view when binoculars and averted vision are used. At 23× in the 4-inch Genesis refractor, it’s almost hard to believe you’re looking at a star cluster half the angular size of the full Moon, because NGC 6087’s brightest stars do not seem to blend together very well. Except for a 5′-wide gathering of stars around S Normae, the cluster’s remaining parts appear to be scattered like stellar marbles, forming all manner of geometrical patterns  – right angles, lines,

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78 arcs, diamonds  – all in the most haphazard way. Add the rich Milky Way background and it’s a visual mess. But if you relax your gaze and let your eye roam around the field, you should see the cluster’s edges fitting within the confines of a boxlike arrangement of field stars. It looks as if several trucks loaded with stars had approached S Normae from various cardinal points, dropped their wares on top of it, and then departed. In fact, if you look 7′ southwest of S Normae, you will see a wonderful 17′long trail of seven stars (ranging from 8th to 9th magnitude and oriented north to south) that look as if they had fallen out of one of the imaginary trucks as it sped away. Immediately south of S Normae is a tight acute triangle of 8th-magnitude stars, 5′ northwest is a near perfect right triangle of 10th-magnitude stars, and 5′ due east is a stellar rhomboid. The area immediately surrounding S Normae and the Southern Triangle glows with the nagging light of stars near the limit of visual resolution, and this background glow continues to the east, tapering along the way until it reaches the rhomboid. With averted vision, this patch has the shape of a decapitated fish. At 72×, the cluster is a stellar Rorschach test. The entire 15′ diameter of the cluster is dappled with at least a dozen obvious stellar groupings and several other seemingly nebulous patches that crumble into stars with any scrutiny. Pale orange S Normae looks like a nova at the tip of a tiny Sagitta-like asterism. (Note that S Normae is not centered in my drawing because, when I was at the eyepiece, the greatest concentration of stars seemed to lie south of that star.) The southern end of this arrow is a fine double star; it marks the southeastern corner of the acute triangle I saw at low power. Some 5′ southeast of S Normae lies

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a small circlet of about a dozen dim stars – one of those nebulous patches that crumbles into stars when examined with averted vision. Those of you with moderate- to large-sized telescopes should use high power on this circlet and see if you can’t “discover” a roughly 13th-magnitude double on the western edge. A really nice double lies 5′ east of S Normae in the rhomboid. And the northern star in that pair is an even closer double that should be scrutinized at high power. These descriptions of doubles could go on forever. The field is littered with them. And it’s really futile for me to say what geometrical patterns you’ll see beyond those I’ve mentioned because these are only the obvious ones (to me anyway). If you happen to have a visitor at your telescope when you’re looking at NGC 6087, share with him or her the fact that each arcminute of cluster spans about 1 light-year (9.5 trillion kilometers) in space. This should enable your companion to fathom the astounding vastness of our Milky Way. Detlev Koester (Louisiana State University) and his colleagues discovered a previously unknown planetary nebula in the field of NGC 6087 (its equinox 2000.0 coordinates are RA 16h19m16s, Dec –57°58′25″). In a 1989

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78 paper in Astronomy and Astrophysics, however, they concluded that the cluster and the nebula are not associated (see also Southern Gems 39 and 40). PN G327.7–05.5, the magnitude 19.3 planetary, is 28,000 light-years

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distant – nearly nine times farther away from us than the cluster. The planetary spans 14.2″, which translates into a shell of gas nearly 2 light-years in diameter. Now there’s a wonderful challenge for CCD users!

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79 79 NGC 6124 Type: Open Cluster Con: Scorpius RA: 16h25.3m Dec: −40°40′ Mag: 5.8; 5.3 (O’Meara) Diam: 40′ SB: 13.8 Dist: ~1,500 light-years Disc: Abbé Nicolas Louis de Lacaille, 1752 abbé ni co l as l ou is de l acail l e : It resembles a fairly bright small comet. (I-8) j ames dunl o p: A round cluster of small stars of nearly equal magnitudes, about 12′ diameter, considerably congregated to the centre, not rich in small stars. This answers to the place of 44 Normae (Bode), but there is no nebula. (D 514) j o hn hers che l : Cluster, bright, large, loosely scattered, not much compressed in the middle, fills nearly a field, consists of about 50 or 60 stars of 9th to 11th magnitude. (h 3626) NG C: Cluster, bright, large, pretty rich, little compressed, stars of magnitude 9 to 11.

Few s ig h ts i n nat u re can mat ch the splendor of the Milky Way stretching across the heavens like a garland of moonlit holly. Here is infinite complexity in structure, infinite mystery in design. It is limitless, untouchable, a vision to be grasped by our minds and hearts. And it is here that we ­continually turn our telescopes to admire its multitude of visual splendors. One aspect of the Milky Way that has continually captured the attention of skywatchers

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throughout time has been the small patches of comet-like nebulosity that splinter into magnificent clusters of stars with the slightest of aperture. The dazzling naked-eye duo M6 and M7 in Scorpius are two of the finest examples of these easily resolved “clouds.” On the other end of the spectrum are clusters too faint to be seen with the naked eye – the ones that manifest themselves with a sweep of the binoculars or a small telescope. If you follow the Milky Way southwest from the tail of

Deep-Sky Companions

79 Scorpius toward Norma, you’ll find a dozen or more of these tiny clusters in an area only 10° across. But there is one curious object on the western outskirts of the Scorpius Milky Way straddling this cluster corridor that seems to exist in a visual limbo. Open cluster NGC 6124 marks the western apex of an isosceles triangle with Mu (μ) and Zeta (ζ1,2) Scorpii in the Scorpion’s tail. It is also exactly halfway between Zeta1,2 Scorpii and Eta (η) Lupi (see wide-field charts 2 or 3). It occupies a dim patch of Milky Way surrounded by blobs of dark nebulosity. The cluster’s catalogued magnitude (5.8) seems bright, but it is also deceiving. The open cluster is 1.3 times larger than the full Moon. In other words, take a 6th-magnitude pinpoint of light and smear it out across 40′ of the sky – against the Milky Way, no less. Now consider that none of the cluster’s 100 known members shine brighter than 9th magnitude. NGC 6124 is one of those objects that can take you by surprise under a dark sky, for it is not immediately obvious to the naked eye, but if you happen to be lying back in a lounge chair and admiring that part of the Milky Way for aesthetic reasons, you might find yourself springing up, convinced that you’ve just discovered the large, diffuse head of a naked-eye comet. And so it was in the eighteenth century when Nicolas Louis de Lacaille came across this object from the Cape of Good Hope, not with his unaided eye but with his 1/2-inch 8× telescope, writing that, “It resembles a big comet without tail.” He then added it to his list of Class I objects (“nebulosities without stars”). Some catalogues credit James Dunlop with the discovery because he was the first to describe its true nature as “a round cluster of small stars of nearly equal magnitudes,” in his 1828 catalogue, but credit for its discovery should remain with Lacaille.

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When I first saw NGC 6124, I did jump up with a start. While observing the cluster with 7 × 35 binoculars on September 9, 1996, I noticed not one object but two: a large, round haze some 20′ in diameter and another round haze separated from the first by a narrow lane of darkness. I immediately looked at my star charts and found nothing there. “A comet!” flashed through my mind. Clouds began moving in, so I went to the 4-inch and looked at the field and confirmed the existence of this ill-defined patch before clouds covered the sky. That night, many thoughts went through my mind. Is this how Lacaille felt when he first encountered NGC 6124? Much like Lacaille, I was using the rough equivalent of a 1/2-inch instrument at 7×. And, like him, I chanced upon a large “nebula” that appeared to lack stars. Perhaps, I thought, this secondary haze was part of NGC 6124. But, if that were true, why did Dunlop and some modern-day observers call the cluster “round” instead of “irregular,” “dumbbell-shaped,” or a “double cluster forming a figure-eight pattern”? I checked various sources for the size of NGC 6124, and all agreed that it was between 20′ and 29′, which placed most of this fuzzy oval partly outside the cluster’s perimeter. The next night fell clear, and I checked for movement in the curious glow but found none. It wasn’t a comet. Figuring the entire sky had been covered well enough by now for a new cluster’s discovery to be unlikely, I presumed the glow to be nothing but a spurious patch of Milky Way. Through the 4-inch, NGC 6124 appeared about 1/3 times larger than this “patch,” which was contained by an obvious oval-shaped asterism of 9thto 12th-magnitude stars. Closer inspection revealed that the single oval was comprised

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79 of two fainter ovals side by side, each scintillating with the light of fainter stars. I checked the Astronomical Journal, the Astrophysical Journal, and the Monthly Notices of the Royal Astronomical Society but did not find a reference to this “cluster.” Apparently, no modern research has been done on it or the surrounding field. The situation remained at a standstill until the year 2000, when Brent Archinal revised the cluster’s size from 29′ to 40′. When I plotted his position for the center of the cluster (see the table on the opening page of this essay) and placed a 45′-diameter template around it, I was pleased to discover that the new dimensions included this curious patch of dim starlight and the bright ovals of stars. But why do I see two distinct “clusters”? The answer may be simple. On National Geographic Society-Palomar Observatory Sky Survey prints, I noticed an almost perfect semicircle of dust (open to the northeast) slicing through the northeast quadrant of the cluster – just where I see one cluster ending and the other cluster beginning. This semicircle of dust outlines the southwestern edge of the large oval of stars I detected in binoculars. Of course, the other possibility is that there really is just a spurious patch of Milky Way here. Check this object out and decide for yourself. The reason NGC 6124 looks so large and diffuse is because it is relatively nearby (1,500 light-years). If NGC 6124 indeed spans 40′, then its true physical diameter is 18 light-years. I find NGC 6124 looks best in binoculars. (Use wide-field chart 3 and the accompanying chart to pinpoint its location.) At a glance, it is a circular object with a dense 10′ core. With time, though, the core begins to break up. Soon it’s a mottled haze with prominent clumps of stars to the northeast, southwest, and east. Particularly absent is any notable

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mottling to the west, so it looks somewhat like an arrowhead pointing to the east. At 23× in the 4-inch, a pair of parallel and slightly curving bands of stars (oriented northwest to southeast) flow through the center of the cluster’s densest part. Look for a tiny gang of knitted stars at the northwestern end of the core, between these two parallel streams. The streams open out into a delta of darkness at the southeastern end of the core; look for a prominent pair of 10th-magnitude stars here. Christian Luginbuhl and Brian Skiff note that the densest portion of the cluster contains about 25 stars in an area 6′ across. All kinds of pairs and groupings dapple the cluster. Using my imagination, NGC 6124 looks like the remains of a globular cluster torn apart by tidal forces. Sweep your eyes across the field at low power and see if the entire region doesn’t look as if you’re viewing it through a black fog. That’s because the entire region is veiled in dark nebulosity. The extinction of starlight by dust, I thought, must be substantial along this sight line through our galaxy’s disk. Indeed, in a 2010 paper in

Deep-Sky Companions

79 Monthly Notices of the Royal Astronomical Society (vol. 403, p. 2041), M. Marcela Vergne (Observatorio Astronómico, Paseo del Bosque, Argentina) and colleagues report the results of an investigation into the properties of the interstellar medium along the line of sight toward the cluster. Their analysis yields a reddening of nearly 1 magnitude. “Our analysis also indicates that the observed visual extinction in NGC 6124 is caused by the presence of three different absorption sheets located between the Sun and NGC 6124,” they say, noting also the presence of dust within the cluster. At 72×, the cluster’s dual aspect is most apparent. Also look for two asterisms resembling the Southern Cross interspersed among several groupings, pairs, and strings of stars. The late Ernst Hartung called NGC 6124 a “scattered galactic cluster [with] several orange stars as well as numerous pairs.” Although I couldn’t see any colored stars through my 4-inch, Hartung used a telescope with three times my aperture. What do you see? If you throw the cluster out of focus, it looks like a barred spiral galaxy with a large dust lane slicing though the bar and core, or a coat of arms. The magnitude 10.9 planetary nebula NGC 6153 in Scorpius lies just a little over 1° east-northeast of NGC 6124. Once I zeroed in on the field, I found the 25″-diameter object

Southern Gems

easy to see. It’s the southernmost “star” in a diamond-shaped asterism of 9th- and 10th-magnitude stars. NGC 6153 is an annular planetary with a magnitude 15.4 central star. Through the 4-inch, it’s just a swollen star at moderate magnifications. Nearly 2° almost due north of NGC 6124 is the small but bright (magnitude 9.1) globular star cluster NGC 6139 (Southern Gem 81), which I found once on a comet sweep. Before leaving the area, do sweep that region I mentioned earlier, southeast of the Scorpion’s tail, toward Norma, and see how many tiny open star clusters and asterisms you encounter. You’ll be taking a fantastic voyage through some of the most spectacular Milky Way regions, one that is guaranteed to keep you occupied on many a moonless evening.

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80 80 NGC 6101 Type: Globular Cluster Con: Apus RA: 16h25.8m Dec: −72°12′ Mag: 9.2 Diam: 5′ SB: 12.7 Dist: ~50,000 light-years Disc: James Dunlop, 1826 j ames dunl o p [June 1, 1826]: A pretty large rather faint round nebula, about 3½′ or 4′ diameter, a little brighter in the middle. There is a very small nebula on the [northwest] side joining the margin of the large nebula. (D 68) j o hn hers che l : Globular cluster, large, faint, round, very gradually a little brighter in the middle, all resolved into stars 15th to 18th magnitude, 4′ diameter, with stragglers. A delicate and beautiful object. (h 3623). NG C: Globular cluster of stars, pretty faint, large, irregularly round, very gradually brighter in the middle, partially resolved, stars of 14th magnitude and fainter.

A p u s t h e b ird of parad i se has an intriguing history. Its name is derived from the Greek apous, meaning “without feet.” It refers to the fabled legless sparrows of Greek mythology, which Keats later glorified in his “Eve of St. Mark” as “legless birds of paradise.” But it is not the sparrow that Apus represents but the large-feathered curiosities brought aboard the European sailing vessels of the sixteenth century. 304

These birds the explorers called Apus (also Avis) Indica, the Indian Bird. The bird was first introduced to the people of Europe in 1522, when some skins were found on the sole surviving ship of Magellan’s fateful expedition. It is from these and subsequent remains that the Spaniards, and later other Europeans, created their own myths about the bird. So it is of little wonder then that Apus ultimately found its way into a section of southern sky

Deep-Sky Companions

80 dedicated to the tools and treasures of these great explorers. Ironically, when the great bird finally appeared in Johann Bayer’s original 1603 atlas (and its corresponding page of text), it did so under the name “Apis Indica” (the Indian Bee). But this, notes Richard Hinckley Allen, is obviously a typographical error. Remember, Bayer did have a constellation called Apis, which Nicolas Louis de Lacaille later renamed Musca the Fly. The typographers and engravers obviously misread Apus for Apis. (So book reviewers take note: to err has long been human.) The error was fixed in later editions of Bayer’s atlas. Apus is conveniently placed just south of Triangulum Australe, and its two brightest stars, both of 4th magnitude, lie about 10° from the South Celestial Pole. Covering 206 square degrees, the constellation ranks 67th in area. Two globular clusters lie within these boundaries: dim IC 4499, the closest globular star cluster to the South Celestial Pole, and NGC 6101, our target. James Dunlop discovered NGC 6101 in 1826 during his southern sky survey and catalogued it as his 68th object. In his description of the object, he adds this interesting note: “There is a very small nebula on the [northwest] side joining the margin of the large nebula.” This statement alarmed me at first; did he chance upon a comet and not know it? But I quickly recalled Herschel’s dismay with Dunlop’s catalogue after he set out for South Africa on his own southern sky survey. “A want of sufficient light or defining power in the instrument used by Mr. Dunlop,” Herschel said, “has been the cause of his setting down objects as nebulae where none really exist.” On photographs of NGC 6101, I have identified a distinct oval of roughly 14th-magnitude stars in the northwest halo, and I believe this

Southern Gems

could have manifested itself to Dunlop as a nebulous extension, especially if his optics were poor. NGC 6101’s brightest stars shine at magnitude 13.5, but the horizontal-branch magnitude, which determines the cluster’s resolvability, is a dim 16.6  – a challenge for a 12-inch telescope. NGC 6101 is intrinsically a rather large globular, measuring 160 light-years, which places it roughly halfway between NGC 362 (Southern Gem 5) and 47 Tucanae (Southern Gem 2). NGC 6101 appears small and faint to us because it is so distant (about 3 1/2 times farther away than 47 Tucanae). It is about 13,000 light-years closer to the Galactic center than to the Sun. The cluster’s iron-to-hydrogen ratio is slightly below normal for a globular; each cluster member has only 1/66 as much metal as the Sun. According to William Harris (McMaster University, Canada), its integrated spectral type is F5. A 1991 Astronomical Journal article by Ata Sarajendini (Yale University) described CCD photometry that revealed a significant blue straggler population in NGC 6101. It was found that these stars are more centrally concentrated than the cluster’s similarly bright subgiants. This indicates that NGC 6101’s blue stragglers have larger masses than their ostensible counterparts, supporting the theory that they are either a binary system or have been formed by the merging of stars. In a 2001 paper in Astronomy and Astrophysics (vol. 380, p. 478), G. Marconi (European Southern Observatory) and colleagues reported CCD photometry for stars within 3.4 arcminutes of NGC 6101’s core using the ESO telescope at La Silla, Chile and the Hubble Space Telescope. Their research confirmed the existence of a sizeable population of 73 blue stragglers, 27 of which had

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been already detected. They found the cluster’s horizontal branch narrow and the bulk of the stars blue, as would be expected for a typical metal-poor globular cluster. They derived an age of 13 billion years. Before 2011, 15 variable stars had been discovered in the region of NGC 6101. Eleven of these appeared to be RR Lyrae variables, one of the types of variables used to size up our galaxy. In a 2011 Astrophysical Journal paper (vol. 727, p. 9), Roger E. Cohen (University of Florida) and his colleagues announced the discovery of seven new RR Lyrae variable stars and confirmed 10 candidates.

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To find NGC 6101, use wide-field chart 3 to find 2nd-magnitude Alpha (α) Trianguli Australis (Atria), then 4th-magnitude Zeta (ζ) Trianguli Australis 2º to the southwest of Alpha. Then switch to the accompanying chart. From Zeta Trianguli Australis, move 1° southeast to 5.5-magnitude Star a and then slide about 25′ west-southwest to 7.5-magnitude Star b. Now move about 55′ south-southwest to 7th-magnitude Star c. NGC 6101 is a little more than 25′ south-southwest of Star c. The globular can be seen in binoculars. Look for a tight triangle of 7th- to 8th-magnitude stars whose westernmost member appears double. The cluster should be just south of the triangle and will appear as a dim glow with a slight central condensation. It’s best to have some power handy if you want to confirm your sighting. With an 8-inch from Wellington, New Zealand, I saw the cluster as a moderately bright sphere surrounded by a dim gray halo. The roughly 3′ core is fractured into three parts. The central most section is a wedgeshaped mass of uneven stars. A dark lane separates it from a tiny cap of stars to the northwest (Dunlop’s “very small” nebula?), while a chevron of starlight appears to have “calved off” the core. Through Auckland Observatory’s 20-inch Zeiss reflector, the core of NGC 6101 looks herniated. A splash of 13th- to 15th-magnitude stars crosses the cluster’s entire face, as if some disgruntled artist shook a wet paint brush at it. The drawing here is a composite of all these views. Through his 12-inch, the late Ernst Hartung described it as a “rather faint but very rich globular cluster . . . irregularly round, rising broadly to the centre, about 3′ across with rays of faint stars emerging.” And through her

Deep-Sky Companions

80 12-inch Schmidt-Cassegrain, South African observer Magda Streicher saw that the cluster had a “granular appearance . . . with very faint stars going out in flimsy flares to the edges. On the northwest edge of this globular cluster a group of four stars that strongly reminds me of the well-known Trapezium in the Orion Nebula (290×) can be seen. In the heart of the globular an outstanding pair of stars [is] seen with ease. Fainter stars to the north [run] out in curls.”

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81 81 NGC 6139 Type: Globular Cluster Con: Scorpius RA: 16h27.7m Dec: −38°51′ Mag: 9.1 Diam: 8.2′ SB: 13.7 Dist: ~31,000 light-years Disc: James Dunlop, 1826 j ames dunl o p [May 13, 1826]: A round nebula, about 1′ diameter, bright immediately at the centre, and very faint from the bright nucleus to the margin. Another observation makes the figure rather elliptical, with a bright nucleus. (D 536) j o hn hers che l : Pretty bright, round, pretty gradually brighter in the middle, resolvable, with left eye I can barely discern a few of the stars. (h 3628) NG C: Bright, pretty large, round, pretty suddenly brighter towards the middle, partially resolved, some stars seen.

N G C 6139 i s a smal l bu t ni ce ly compact globular star cluster in southwestern Scorpius almost midway along an imaginary line between the naked-eye double star Mu (μ) Scorpii in the Scorpion’s tail and 3rd-magnitude Eta (η) Lupi in the Wolf’s chin. It’s also in the same region of the Milky Way as the binocular open star cluster NGC 6124 (Southern Gem 79) and planetary nebula NGC 6153. I once found our target on a comet-hunting sweep, so I admire James Dunlop’s discovery

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description of it (noted earlier), which could be used to describe many 9th-magnitude comets, namely “a round nebula, about 1′ diameter, bright immediately at the centre, and very faint from the bright nucleus to the margin.” In a 1920 Mount Wilson Report, Edwin C. Hubble noted its globular nature. Two years later, Harvard College Observatory director Harlow Shapley verified this on Harvard plates; it was Shapley who, from 1920, plotted the distances of globular clusters and made

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81 an early determination of the shape and size of our Milky Way and the position of the Sun within it. As for NGC 6139, it lies about 33,000 light-years from the Sun and only about 12,000 light-years from the Galactic center. It spans nearly 80 light-years of space in true physical extent and is metal-poor, with each of its stars, on average, having only about 1/45 as much metal (per unit hydrogen) as the Sun. In a 1991 paper in Astronomical Journal (vol. 101, pp. 170–174), Martha L. Hazen (Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts) presented photometry for 10 new variables within the tidal radius of NGC 6139, five of which have periods indicating that they are probable RR Lyrae stars. These variables, often used as standard candles to measure galactic distances, seem likely to be members of the cluster. “The RR Lyrae stars in the cluster show period shifts appropriate to a cluster of considerably lower metallicity than has been observed for NGC 6139,” Hazen says. Indeed, a study in 1998 (Astronomical Journal, vol. 116, p. 1736) by Robert Zinn and Sydney Barnes (Yale University, New Haven, Connecticut) revealed that the cluster’s color– magnitude diagram showed not only variable interstellar reddening across the cluster’s face but a metallicity of the giant branch of about 1/50 as much iron in each of its members as in the Sun, which agrees with some but not all previous studies. “NGC 6139 has a very blue horizontal branch,” they say, “which appears to be unusually short in color extent compared with other clusters of its high central density and concentration, although additional observations are needed to confirm this. Its metal and horizontal brach morphology and kinematics are typical of other inner halo globular clusters.”

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In a similar study presented in 1999 (Astronomy and Astrophysics Supplement Series, vol. 138, pp. 267–273), S. Ortolani and colleagues derived a reddening of 0.70 and a distance from the Sun of about 31,000 light-years. While some earlier studies had placed the cluster in the inner regions of the Galaxy but outside the Galactic bulge, Ortolani et al. concluded that NGC 6139 is “within the bulge volume.” This is important because studies of these metal-poor stars in the inner spheroid (where the first episodes of star formation may have occurred) may tell us something about the earliest epochs in the history of our galaxy’s formation. More recently, in 2011, P. H. T. Tam and colleagues (Astrophysical Journal, vol. 729, p. 90) used data obtained from the Fermi Gamma-ray Space Telescope to discover that globular clusters are an emerging new class of gamma-ray emitters – and NGC 6139 is one of eight globulars known to emit gamma-rays at energies greater than 100 million electron volts. The gamma-ray emission from NGC 6139 was detected within the globular cluster’s tidal radius. “The increasing number of known [gamma-ray globular clusters] at distances out to ~10 kiloparsecs is important for us to understand the [gamma]-ray emitting mechanism and provides an alternative probe to the underlying millisecond pulsar populations of the [globular clusters],” they say. To find this interesting globular specimen, use wide-field chart 3 and your unaided eyes or binoculars and take the time to look for magnitude 5.5 Star a, which is the brightest star in the region of the globular. Look about 4° southeast of Theta (θ) Lupi. Center Star a in your telescope at low power and then confirm the field with the chart on page 310. NGC 6139 is only about 50′ northeast of Star a.

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Through the 5-inch at 33×, NGC 6139 is a compact and easy target that gradually then

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suddenly gets brighter in the middle to a starlike nucleus, especially when viewed with averted vision. Even at this low power, I suspect a mottling in the inner halo, while the outer halo looks irregular with rays. At 60×, the cluster remains small, about 5′ across, with a tight and mottled core, though the outer halo now seems to twinkle with myriad dim stars. But the magic occurs between 94× and 247×, when the cluster’s starlike core burns forth most noticeably, the inner halo burns brightly with clumpy starlight, and the outer halo resolves into a rosette of shimmering starlight that seems to fracture as I move my eye around the field. A bright ray of starlight seems to pinch off to the northeast with about four other stubby arms oriented symmetrically about it in a weak starfish pattern. A dark band cuts across the face just below the intense core running from the eastnortheast to the west-southwest. Through her 12-inch scope at 218×, Magda Streicher says the globular “looks very hazy, almost comet-like. With averted vision the core is not exactly round with the focus the large outer envelope around the core. No stars reveal[ed] but slightly granular.”

Deep-Sky Companions

82 82 Little Pincushion NGC 6134 Type: Open Cluster Con: Norma RA: 16h27.8m Dec: −49°10′ Mag: 7.2 Diam: 8′ SB: 11.1 Dist: ~4,600 light-years Disc: James Dunlop, 1826 j a mes dunl o p [May 10, 1826]: A pretty large round nebula, about 4′ diameter, gradually a little brighter towards the centre. There is a [faint] star on the north, and another on the south side, both involved. This is easily resolved into stars, with slight compression to the centre. (D 412) j o hn hers chel : A pretty rich, loose, large, roundish cluster of stars 12th to 14th magnitude, 7′ diameter, not much compressed in the middle. (h 3627) N G C: Cluster, considerably large, pretty rich, little compressed in the middle, stars of magnitude 13–15.

N G C 6139 is a l ar ge and l oose bu t aesthetically pleasing open star cluster just 1 1/2° south of 4.5-magnitude Epsilon (ε) Normae. It lies in the rich Scorpius-Norma Milky Way almost on the Galactic plane. But because of its low surface brightness, light pollution or atmospheric haze will affect the cluster’s visual impact, especially when seen through small apertures. But, under a dark sky, the cluster can be seen in the smallest of Southern Gems

telescopes or with mounted or well-braced binoculars with effort, appearing as a little puff of light. Robert J. Trumpler initially classified NGC 6134 as II2r, meaning it’s a rich cluster detached from the Milky Way with a medium brightness range of stars. Modern catalogues list it as II3m – a detached cluster, moderately rich, with bright and faint stars. Early investigations of the cluster led to the detection 311

82 of 17 red giant members, six spectroscopic binaries, six Delta Scuti stars, and a variable blue straggler. NGC 6134, in fact, is the record holder in terms of the number of Delta Scuti stars it contains (though a fair number have been detected in other clusters). Delta Scuti stars are important in that they display small but regular light variations, making them powerful tools for determining cluster distances and other properties. In a 1999 paper in Astronomy and Astrophysics (vol. 140, p. 135), H. Bruntt (Aarhus University, Denmark) and colleagues report how they used the Danish 1.5-meter telescope at European Southern Observatory, La Silla, Chile, to make photometric observations of NGC 6134. Their data revealed a cluster distance of about 4,500 light-years and an age of about 700 million years, making it almost as old as the Hyades in Taurus. Because the cluster lies so close to the Galactic plane, intervening dust along our line of sight reddens its light by about 0.3 magnitude. If we accept the distance just given, the cluster spans only about 10 light-years of space.

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The cluster is easy to find. First use chart 2 to locate 3rd-magnitude Zeta (ζ) Scorpii (actually the pairing of stars Zeta1 and Zeta2 Scorpii) in the Scorpion’s tail; it forms the head of the “False Comet” asterism (see Southern Gem 85). Now look about 7° southwest for Epsilon Normae. You can center Epsilon Normae in your telescope at low power and then drop 1 1/2° to the south to find it. Alternatively, you can refine the search by looking about 3° farther south for 4th-magnitude Gamma (γ2) Normae, which is the middle star of three 4th- to 5th-magnitude stars in a 1° area of sky. You’ll want to center the northernmost star of the three (Star a) in your telescope at low power and then switch to the accompanying chart. NGC 6134 is only 1° to the east-northeast of Star a and a little less than 30′ east of 7.5-magnitude Star b. At 33×, in the 5-inch NGC 6139 appears as a hazy globe of faint starlight. The vast bounty of stars in an area almost 10′ across makes it appear as a fully resolved globular star cluster punctuated by a roughly 9th-magnitude star immediately to the south. With averted vision, the round cluster becomes gradually brighter toward the middle, though it remains loosely compact. Rows of stars slice through it from the east-northeast to the west-southwest, so there appears to be some sort of divine order to the stars. At 60×, the cluster loses a bit of luster, owing to the object’s low surface brightness, but keep looking, because the longer you look the more you should see. The view seems to slowly ingrain itself in you.

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With imagination, I think the cluster looks like a little pincushion (see Southern Gem 49), though more anemic. Stars of mixed magnitudes at the core form a distinct ellipse

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oriented east-northeast to west-southwest. Sweeping my vision outward from the core, strings of stars radiate away in all directions. At 90× and higher, I could trace a wide, sweeping spiral structure that, overall, makes the cluster appear as a clockwise-turning system of stars. The numerous stars and irregular distribution of magnitudes will keep the eye busy looking for fanciful patterns  – if you’re the imaginative type. Through her 12-inch Schmidt-Cassegrain at 218×, Magda Streicher found the stars “evenly separated, forming strings that merge with the field stars. The cluster is reasonably bright, round in shape and dark lanes can be seen in between this loose cluster. . . . Here and there stars appear as double embedded into the cluster.”

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83 83 NGC 6192 Type: Open Cluster Con: Scorpius RA: 16h40.3m Dec: −43°22′ Mag: 8.5 Diam: 9′ SB: 13.3 Dist: ~5,500 light-years Disc: James Dunlop, 1826 j ames dunl o p [May 13, 1826]: A cluster of very minute stars, of a round figure, about 4′ diameter, following v Normae. (D 483) j o hn hers che l : A coarse but rich cluster of stars 11th- to 12th-magnitude, which leaves dark lines unoccupied, forming sections. (h 3641) NG C: Cluster, pretty large, pretty rich, irregularly round, consisting of 11th- to 14th-magnitude stars.

One of the most alluring qualities of open star clusters is their myriad forms and shapes. Some are large and sprawling, others appear small and compact, and some are a mix. Most lie close to the Galactic plane, where their members mix and mingle with stellar backgrounds and intervening dust, adding awe and confusion to the view. NGC 6192 is one of those mixed varieties and a pleasing sight despite its listed photographic magnitude of 8.5. As a general rule, an object’s photographic magnitude is on the order of 1 magnitude fainter than its visual magnitude, but I’ve also seen the cluster’s magnitude listed as visual magnitude 8.5.

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Regardless, the cluster is rich in stars and nicely compact, so it’s a pleasing sight even in small telescopes. It’s a rich cluster detached from the Milky Way with a strong central concentration and a medium range in the brightness of its stars (Trumpler class of I2r). Adding to the cluster’s luster, it not only lies near the Galactic plane but also appears superimposed, if not centered, on a concentration of B stars some 13,000 light-years distant. In a 1991 paper in Astronomy and Astrophysics Supplement Series (vol. 87, p. 119), K. Kjeldsen and S. Frandsen (University of Aarhus, Aarhus, Denmark) determined the ages and distances of 13 open clusters

Deep-Sky Companions

83 (including NGC 6192) using photometry performed at the Danish 1.5-meter telescope at the European Southern Observatory in La Silla, Chile. The data revealed that NGC 6192 lies at a distance of about 5,500 light-years from the Sun and has an age of about 100 million years. The new findings differ greatly from a study in 1987 by D. J. King (The Observatory, vol. 107, p. 107) that found the cluster’s age to be about 1 billion years. “The reason we find a completely different age,” they report, “is that we get a much larger reddening [0.68 magnitude]. . . . The larger reddening moves the main sequence upwards, so that the turn-off is at a brighter magnitude and the age decreases.” Kjeldsen and Frandsen became suspicious of King’s result when they saw that he did not detect any Delta Scuti stars in the cluster, which should have been present if the cluster’s age were about 1 billion years. More recently, in a 2010 paper in Astronomy and Astrophysics (vol. 523, p. 11), Laura Magrini (Osservatorio Astrofisico di Arcetri, Firenze, Italy) and colleagues used the European Southern Observatory’s Very Large Telescope at Cerro Paranal to make high-resolution spectroscopic observations

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of three open clusters located within ~23,000 light-years from the Galactic center, one of which was NGC 6192. They derived memberships of a group of evolved stars for each cluster, including a sample of four member stars in NGC 6192. The purpose of the investigation was to determine the slope of the inner disk metallicity gradient as traced by open clusters; the gradient suggests that metallicity is higher in the Galactic center and decreases as one moves outward. Because the density of stars is greater toward the Galactic center, more metals have been returned to the interstellar medium and thus incorporated into new stars. By studying these inner galaxy clusters, the researchers wanted to learn more about the chemical evolution of our Galaxy. They found that the cluster has an oversolar abundance of metals (1.3 times as much for each star in the cluster as in the Sun), consistent with the belief that all internal open clusters are richer in metals than those in the solar neighborhood. This finding, they say, confirms, together with other open cluster and Cepheid data, that a steep gradient exists in the Galaxy’s inner disk, which is a signature of an evolutionary rate different from that in the Milky Way’s outer disk. To find this little wonder, use wide-field chart 2 to find 3rd-magnitude Zeta (ζ) Scorpii (actually the pairing of stars Zeta1 and Zeta2 Scorpii) in the Scorpion’s tail; it forms the head of the “False Comet” asterism (see Southern Gem 85). Then look for 5th-­magnitude Mu (μ) Normae, which is about 4° to the west-southwest. Center

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83 Mu Normae in your telescope at low power and then switch to the chart on page 315. From Mu Normae, move about 1° northeast to 6th-magnitude Star a in Scorpius. NGC 6192 is just about 20′ east of Star a. Through the 5-inch at 33×, NGC 6192 is a small and fractured cluster near a bright “Y” of stars to the southwest and a wide pair of stars to the southeast. The cluster is detached and loosely concentrated, with a broken core about 5′ across surrounded by an irregular sprinkling of stars, some in rays, extending to about 10′. The stars are mixed in magnitude, and the field appears marginally rich in starlight. At 60×, the core looks boxy and highly segmented, and dark lanes run throughout it. One obvious lane slices across the core from west to east before branching southeastward along a pretty string of stars. Actually, that string continues in the opposite direction as well, to form the northern segment of the core. The southern part of the core is a warped rectangle of about a half dozen stars of nearly equal magnitude congregated in an area about 2′ across. Weak rays of starlight also extend to the northeast and southwest, so the brightest members form an X pattern. The cluster is nicely resolved at 90× and higher, which also shows numerous stellar pairings and groupings. Now return to low power and slightly defocus the view. Relax your gaze and try to concentrate on the dark veins crossing the

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cluster, not the stars. Do you see how they form a beautiful web? It’s as if the cluster is being hauled up from the depths of a dark sea like fish in a net. Through her 12-inch Schmidt-Cassegrain telescope, Magda Streicher describes NGC 6192 as a “pretty, loose, rich open cluster with a selection of about 60 mix[ed]-magnitude stars, slightly elongated north to south in shape. This is an appealing cluster that displays in a unique way a wealth of detail. The inner section is rich in stars, with short strings that mingle well with the busy star field. The northern side of the cluster’s field of view is rather poor in starlight. To me the stars resemble a tick-like figure or even a small baby scorpion. The upright tail projects quite prominently southeast, increasing its overall size to about 10.0′.”

Deep-Sky Companions

84 84 NGC 6193 Type: Open Cluster Con: Ara RA: 16h41.3m Dec: −48°46′ Mag: 5.2; 4.8 (O’Meara) Diam: 14′ SB: 10.9 Dist: ~4,300 light-years Disc: James Dunlop, 1826 j a mes dunl o p [May 14, 1826]: A cluster of small stars, with a bright star in the [western] side. A very considerable branch or tail proceeds from the north side, which joins a very large cluster. (D 413) j o hn hers chel : Cluster VIII; consists of about a dozen stars 10th- to 11th-magnitude, and perhaps as many less, with stragglers, which fill field. In its preceding part is a fine double star . . . and yet more preceding is a very large, faint nebula, in which the [western] part of the cluster is involved. (h 3642). N G C: Cluster, very large, little rich, little compressed, well resolved, faint nebula involved.

Ge n t ly s t ir cre am i nt o coffe e and the cream will form a spiral pattern before it blends. Can the spiral structure of a galaxy be viewed in much the same way? Only up to a point. As compression waves course through a disk galaxy’s plane, matter piles up at the crests of the waves. These density enhancements become the spiral arms as they are curved backward by radial differences in the galaxy’s angular rotation speed. But the waves Southern Gems

don’t blend themselves out of existence as the coffee-cup analogy would suggest. In the Milky Way and spirals like it, the disk’s stars, gas, and dust catch up with these so-called density waves from behind. When a dense interstellar cloud encounters a density wave, it compresses, triggering star formation. Things get really interesting when the compressed cloud’s self-gravity becomes a dominant force. Once this happens, the cloud 317

84 splinters into swirling ribbons, fingers, and globs of cold, opaque gas that can manifest themselves into dark animate shapes; the Horsehead Nebula in Orion and the Eagle Nebula (M16) in Serpens are excellent examples. As the cloud continues to collapse, sections squeeze together so tightly that their interiors start to heat up. Then, suddenly, nuclear reactions fire up within the densest, hottest concentrations. Radiant energy pours out from each newborn star. Some of the radiant energy scatters off the surrounding dust particles, which we can see as icy blue reflection nebulae. The energy also can boil away other sections of the cloud, causing it to glow with the fiery reddish light of hydrogen-alpha emission. Few places in the night sky reveal this drama of stellar evolution in details large enough and bright enough that they can be spied in amateur telescopes. But one region places all the performers on stage, ready for you to critique: it’s the Ara OB1 Association, a 1°-wide swath of cosmic splendor centered on our target  – the naked-eye open cluster NGC 6193. NGC 6193 is still shedding its swaddling clothes, which can be seen in photographs as the part reflection–part emission nebula NGC 6188. Cold, opaque clouds of dusty gas make NGC 6193’s neighborhood one of the most visually intriguing regions in the Milky Way. In color photographs, the area becomes an elaborate mosaic of cosmic clouds and starlight, replete with baby blue stars, turbulent black eddies, red walls of glowing gas, and veils of reflected starlight. Recent studies have shown that the 18-light-year-wide cluster is also on the border of a vast (310 light-year-wide) expanding cloud of atomic hydrogen gas. The dark clouds appear to be part of two molecular complexes, one of which might be associated with the cluster.

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As S. L. Skinner (University of Colorado, Boulder) and colleagues report in a 2005 paper in Monthly Notices of the Royal Astronomical Society (vol. 361, p. 191), the Chandra X-ray Observatory detected 43 x-ray sources in a 2′ × 2′ core region centered on the stars HD 150135 and HD 150136, whose bright x-ray emission dominates the cluster; the stars are separated by only 10″. The x-ray luminosity of HD 150136 makes it one of the most luminous O-star x-ray sources known. All the fainter x-ray sources in the core region have near-infrared (near-IR) counterparts. “It is likely that some are young low-mass stars in the cluster, but cluster membership remains to be determined,” they say. The researchers found the x-ray spectral properties of HD 150135 and HD 150136 similar, but not identical. Both have emission dominated by cool plasma, pointing to a wind-shock origin. “However,” they continue, “the emission of HD 150136 is slightly hotter and four times more luminous than its optical twin HD 150135. . . . A surprising result is that the X-ray emission of HD 150136 is slowly variable on a time-scale of