The Role of Images in Astronomical Discovery
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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?