The Role of Images in Astronomical Discovery Read online

  disentangled the scientiic discovery process: historians, sociologists and epistemologists.

  Their perspicuous investigations have explored the minds of scientists and dissected their

  products. These scholars have helped us understand why it took 1,000 years from the irst

  viewing of nebulae to the inal unveiling of the world of galaxies in the initial part of the

  twentieth century. Thus, I encapsulate the works of many. Ambitiously, I am striving to

  build a bridge between the genre of science writing typical of scientists and the history of

  science literature.

  On Nebulae

  “Nebulae” are far away, and most are astonishingly distant. The concept of “nebulae” has

  been at times very confusing, even chaotic, as successive attempts were made to distinguish

  the categories of these elusive cosmic objects. It took a long time to igure out the diversity

  of “nebulae” and to understand them. For a long time, astronomers were unable to determine

  their distances, which many thought implausible. Moreover, the puzzling objects could not

  be related to anything familiar. The nature of “nebulae” was considered to be out of the

  ordinary: were they made of a mysterious cosmic luid, a bunch of unresolved stars or just

  illusions in the mind of imaginative observers?

  To assist you in navigating through this long and foggy history, I give here the gist of the

  different classes of “nebulae” as we now know them. This will help you to stay the course

  through the maze of the long-lasting unraveling of their nature.

  There are two main classes of “nebulae”: (i) diffuse nebulae (clouds of gas and dust)

  and (ii) extragalactic “nebulae” or galaxies (huge assemblies of stars). Diffuse nebulae

  are members of our Milky Way; they also exist in other galaxies as components of the

  interstellar medium – the space between stars. Diffuse nebulae can be divided further into

  two broad categories, (a) emission nebulae where the atoms of the cosmic gas are stripped

  of their electrons and made luorescent by the ultraviolet light of hot massive stars, and

  (b) relection nebulae whose dust relects the light of stars in their vicinity.

  By far the largest category of “nebulae” are the non-galactic or extragalactic “nebulae,”

  now called galaxies. Much larger physical entities than emission and relection nebulae,

  they are totally different from diffuse nebulae. Galaxies are made of billions of stars and

  contain huge quantities of interstellar matter often seen as diffuse nebulae and dust clouds.

  They form two main categories, ellipticals and spirals.

  A major epistemological stumbling block was the following: for centuries, most

  researchers tried to bring all nebulae under one umbrella, making them a single class of

  physical objects. This is not an uncommon approach in the development of science. The

  long quest is not without parallel to Plato’s allegory of the cave, where people try to under-

  stand the world by watching shadows on the walls from the things passing in front of the

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  Preface

  ire behind them. For centuries, astronomers, like the prisoners of Plato’s cave, puzzled

  over “nebulae.” The challenge and key to a successful epistemic exit were to distinguish

  the different categories of “nebulae,” and, as a critical step, to establish their distances. Key

  breakthroughs came with reliable distance determinations and from spectroscopy. The lat-

  ter technique revealed the physical nature of sidereal matter in its various states, providing

  the tool to distinguish stars from true nebular material.

  Throughout the initial chapters of the book, I will use the word “nebula” in quotes since

  historically the object discussed could be either a cloud of gas and dust, an unresolved

  cluster of stars or a distant galaxy, the observers not knowing or being unable to make

  the distinction. When unquoted, nebula refers to diffuse or relection nebulae. More

  confusing for the unfamiliar reader, galaxies were initially called “non-galactic nebulae,”

  “extragalactic nebulae,” or “anagalactic nebulae.” After the death of Edwin Hubble in

  1953, they became simply galaxies. Just watch for the shifting of names, especially when

  I cite original material.

  Plan of the Book

  The book is divided in three parts. In the introduction, I deal with the challenges of images

  and their role in scientiic discovery. I discuss the issue of images not being self-evident.

  Part I deals with the speciicity of astronomical imaging and its challenges at inding and

  revealing galaxies: the long path from the visual discovery of fuzzy celestial clouds to the

  photography of multitudes of spirals, a long quest that lighted the path to our inding of

  the universe. I show how images provided the exacting and essential steps for unveiling

  the world of galaxies: irst from written descriptions of what was seen through the tele-

  scope (Chapter 1), then sketched in the drawings of nineteenth-century visual observers

  (Chapter 2), later photographed by the pioneers of the end of the nineteenth century and

  early twentieth century (Chapter 3), then abstracted as images for the mind (Chapter 4).

  Chapter 5 acts as a gateway. It is a transition chapter: I recap the whole story in a more

  traditional way, bringing together the names of the key actors, their places and dates, as

  well as the ideas that contributed to the unveiling of the world of galaxies. I describe how

  galaxies became stepping stones for measuring the size and the age of the universe, and

  not least can be used to determine the exact position of the Sun and solar system in cosmic

  space and time. I chronicle the crucial decade of 1915–1925, where reliable distances to

  galaxies were established. I refer extensively to the works of astronomers, both profession-

  als and amateurs, as the latter often contributed in most innovative ways. For example, the

  early recognition that imaging techniques (photography and spectroscopy) could be valu-

  able to study nebulae and galaxies came from amateur astronomers. It took decades for the

  professionals, who initially mistrusted photography, to be convinced.7

  7 See A. Hirshfeld, Starlight Detectives: How Astronomers, Inventors and Eccentrics Discovered the Modern Universe,New York: Bellevue Literary Press, 2014.

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  In Part II, I summarize the current knowledge about galaxies, emphasizing the role of

  several imaging techniques that helped to unravel the complexities and extraordinary prop-

  erties of galaxies: galaxies as viewed in the optical/infrared domain (Chapter 6), and at

  radio and X-ray wavelengths (Chapter 7). A particularly gripping story is how dark matter

  was discovered and inferred by what I call “imaging the invisible” (Chapter 8).

  Part III of the book is my most original contribution. It is about at
lases of galaxies.

  The scientiic atlas is a standard tool to share and disseminate knowledge using carefully

  selected sets of images. “Scientiic atlas images are images at work, and they have been at

  work for centuries in all the sciences of the eye, from anatomy to physics, from meteorol-

  ogy to embryology.”8 Astronomy is no exception. Atlases of galaxies have been trailblazers

  in the development and sharing of new knowledge about these great assemblies of stars.

  Classiication of objects is the foundation of any scientiic atlas. Chapter 9 tells the fasci-

  nating story of the building up of a classiication system of galaxies and what role images

  played in the controversial process, and how morphology became a fundamental criterion

  to classify galaxies. In Chapter 10, I discuss and review all the major galaxy atlases that are

  published and used by astronomers and their students. In Chapter 11, using speciic exam-

  ples, I illustrate the impact of these atlases on the way research programs were proposed

  and conducted, and their role in the design and building of new astronomical cameras and

  telescopes. I conclude the book with some personal relections on how images are helping

  us to understand the universe better, and what great tools they are for sharing that knowl-

  edge more broadly. Finally, I relect on the changing role and future of galaxy atlases in the

  digital age.

  From the very beginning, I wish to highlight the cumulative approach I will take you

  through as we move along. The main thesis (why it took so long) and theme (images as

  discovery tools) will gradually bulk up over the course of the book. Concepts, ideas, theo-

  ries, observations, objects and historical actors will occur in different ways as we progress

  from one chapter to another. There will be repetitions. As we come back to these notions

  over and over again, we will build a fuller picture. My goal is that by the end of the book,

  you will appreciate these things in a more rounded fashion and be able to embrace a deeper

  perspective than provided by the standard astronomy textbook.

  The research for this book is based on a mix of primary and secondary sources. Primary

  sources include research works published in observatory reports, in journals of professional

  societies, conference proceedings and galaxy atlases. Secondary sources are other scientiic

  atlases, books and articles by researchers in history, sociology and epistemology of astron-

  omy and natural sciences. The Biographical Encyclopedia of Astronomers was a rich source

  of information and provided many hints for further search. The Smithsonian Astrophysical

  Observatory/NASA Astrophysics Data System Bibliographic Services has been an ines-

  timable resource. Several Wikipedia articles provided useful content and were indicators

  for other material.

  8 L. Daston and P. Galison, Objectivity, New York: Zone Books, 2007, p. 19.

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  This work was inspired and completed with the help of many individuals with whom I

  interacted during my career. I owe enormously to colleagues who read the evolving versions

  of the manuscript and provided most helpful comments, criticisms and suggestions.

  I am deeply indebted to Omar W. Nasim of Regensberg University, who generously

  shared his extraordinary perspective, unique knowledge and insights on the role of image

  making in astronomy of the nineteenth and twentieth centuries. Pierre Martin (University

  of Hawai’i/Hilo), Ronald Buta (University of Alabama), and Anton Koekemoer (Space

  Telescope Science Institute) read initial versions of the manuscript and provided extremely

  helpful feedback. Pekka Terrikorpi of the University of Turku, Finland, provided clariica-

  tion on the Hubble vs. Lundmark altercation regarding priority on the design of a proper

  morphological classiication of galaxies. David L. Block of the University of Witwatersrand

  and Kenneth Freeman of the Australian National University shared key information on the

  early work of John Reynolds on galaxies, and on his forgotten contribution to what later

  became the “Hubble sequence.”

  Zoltan Levy (STScI) instructed me on the subtle art and science of making “portraits”

  from the images obtained with the Hubble Space Telescope. Lars Lindberg Christensen of

  European Southern Observatory provided provocative insights on the future of imaging in

  the evolving world of interactive archives and the challenge of creating “ethically correct

  colour imagery” with raw data from telescopes.

  In my exploration of the impact of atlases of galaxies, I interacted with several people

  by e-mail and telephone. I am most grateful to Alar Toomre (MIT) and François Schweizer

  (Carnegie Observatories) for their wonderful recollections on the development of the con-

  cept of interacting galaxies and of the impact of Halton Arp’s Atlas of Peculiar Galaxies

  on their own thinking. I also extend my thanks to Wendy Freedman of the University of

  Chicago, Kenneth Freeman of the Australian National University, Marshall McCall of York

  University, Preethi Nair of the University of Alabama, Robert J. Hanisch of the National

  Institute of Standards and Technology, Eduardo Hardy of Associated Universities, Inc. and

  Harold G. Corwin, Jr. for sharing on how galaxy atlases did inluence their work.

  Communicating with librarians around the world has been a most rewarding experience.

  I thank Jill Langstrom, Head Librarian, and her team at the Space Telescope Science Insti-

  tute, and Xiaoyu Zhang, the Gemini Observatory Librarian. I am particularly grateful to

  Paul Espinoza, Curator of the magniicent George Peabody Library, Johns Hopkins Uni-

  versity, for hosting me several times and providing me with the publications of the Earl

  of Rosse. Professor Earle Havens, the William Kurrelmeyer, Curator of Rare Books and

  Manuscripts of the Sheridan Libraries of the Johns Hopkins University, shared his passion

  for ancient scientiic books and introduced me to the glory of JHU collections of remark-

  able books. Cynthia Hunt, Social Media Strategist at Carnegie Observatories, and Chair

  of the History Committee for the Carnegie Observatories, provided important images, and

  permissions to reproduce them. I owe much to Daina Bouquin (Acting Head Librarian)

  and Maria McEachern at the John G. Wolbach Library of the Harvard-Smithsonian Cen-

  ter for Astrophysics, Harvard University. They gave me access to several key nineteenth-

  and early twentieth-century works and references, while shoring up generous and most

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  professional assistance in identifying images for digitization. Loma Karklins of Caltech,

  Lianne Smith of King’s College London and Janet Laidla of University of Tartu Museum

  came up with notable illustrations. Several librarians helped me to identify sources of

  ima
ges and to clarify ownership of material. Some individuals provided me with hints for

  other images or different versions, and help in identifying the exact sources. I am most

  grateful to Mark Bailey of Armagh Observatory and Planetarium, Jaan Pelt of Tartu Obser-

  vatory, and Harold G. Corwin Jr.

  I am in unbounded debt to Matt Mountain for inviting me to spend a year working in

  the Science Mission Ofice of the Space Telescope Science Institute, Baltimore, MD. This

  opportunity and multiple exchanges with STScI colleagues led me to relect about scientiic

  images and their function in astronomical discovery. My stay at the institute launched me

  into a small project on the role and impact of atlases of galaxies and that ultimately evolved

  into this book.

  I acknowledge the unrelenting and most professional support of Vince Higgs and Lucy

  Edwards of Cambridge University Press throughout all phases inalizing the manuscript.

  Zoë Lewin, my copyeditor, deserves special recognition for her superb work.

  Hélène Allard, my companion of decades, remained by my side all the time and patiently

  reviewed and criticized the earliest versions of the manuscript.

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  Introduction

  As the dome shutters begin to close, they [coyotes] emit a high-pitched

  squeal that every coyote within a three-mile radius answers with a howl.

  Their primeval lamentations play a fitting coda to a night of solitude with

  the stars, the dome, and the slowly turning telescope.

  Allan Sandage 1

  To learn to observe and to depict in a science is to acquire at once an

  ethos and a way of seeing.

  Lorraine Daston and Peter Galison 2

  In one sense, that theory of the spiral nebulae to which many lines of

  recently obtained evidence are pointing, cannot be said to be a modern

  theory. There are few modern concepts which have not been explicitly

  or implicitly put forward as hypotheses or suggestions long before they

  were actually substantiated by evidence.

  Heber D. Curtis 3

  How Are Images Discovery Engines?