The Role of Images in Astronomical Discovery Read online

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  The One-Thousand-Year Journey

  What are galaxies? No one knew before 1900. Very few people knew in

  1920. All astronomers knew after 1924.

  Allan Sandage 1

  The history of scientific discovery affords many instances where men

  with some strange gift of intuition have looked ahead from meager data,

  and have glimpsed or guessed truths which have been fully verified only

  after the lapse of decades or centuries.

  Heber D. Curtis 2

  Few discoverers, if any, tried more carefully to avoid the most fundamen-

  tal aspect of their findings than did Hubble.

  Stanley Jaki 3

  Why Did It Take One Thousand Years to Figure Out What “Nebulae” Were?

  “The best astronomer hardly known in those days,” is how Allan Sandage described his

  fellow countryman, American astronomer Vesto Slipher (1875–1969).4 Slipher was an out-

  standing spectroscopist. He measured the radial velocities of dozens of spiral “nebulae”

  and found that most were speeding away from us at speeds much higher than the escape

  velocity of the Milky Way. His finding was instrumental to the discovery of the expanding

  universe. Slipher had joined the Lowell Observatory in 1909, where the eccentric Perci-

  val Lowell, searching for life on Mars, had built a private observatory equipped with a

  24-inch refractor (Fig. 5.1). Lowell, strong adherent of the Nebular Hypothesis, believed

  spiral “nebulae” were solar systems in the process of formation. He had hired Slipher to

  study planets. Starting around 1912, and adding to these planet studies, Slipher developed a

  1 A. R. Sandage, The Hubble Atlas of Galaxies, Washington: Carnegie Institution of Washington, 1961, p. 1.

  2 H. D. Curtis, Modern Theories of the Spiral Nebulae, Journal of the Royal Astronomical Society of Canada, 1920, Vol. XIV, pp. 317–327.

  3 S. L. Jaki, The Milky Way, An Elusive Road for Science, New York: Science History Publications, 1972, p. 302.

  4 A. R. Sandage, Centennial History of the Carnegie Institution of Washington, Volume 1: The Mount Wilson Observatory, Cambridge: Cambridge University Press, 2004, p. 450.

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  Part II – Images as Galaxy Discovery Engines

  Fig. 5.1 Vesto Slipher in 1932. Credit: University of Chicago Photographic Archive, [apf6–04298],

  Special Collections Research Center, University of Chicago Library.

  spectroscopic research program on “nebulae.” Of independent mind, he slowly came to

  regard these objects as genuine galaxies.5

  Slipher was totally oblivious to fame. As Sandage writes further, “Slipher’s desire for

  astronomical recognition approached zero.” Slipher later became mayor of the growing

  nearby city of Flagstaff, Arizona, and was in control of the main utility departments. He

  and his brother Earl C. Slipher were successful businessmen and rangers in the developing

  wild southwest. As the booming town sprawled and city lighting grew, Slipher was able to

  turn off the city lights when needed for the observation of faint objects, simply by activating

  a main switch he had installed in his observatory office.

  The Gradual Dawning of Galaxies and the Law of Eponymy

  The previous chapters have explored the various ways of making images of “nebulae.” By

  examining the various means of “seeing things,” e.g. observing with the unassisted eye,

  5 M. J. Way and D. Hunter (editors), Origins of the Expanding Universe: 1912–1932, Astronomical Society of the Pacific Conference Series, 2013, Vol. 471. A goal of the conference was to re-establish Slipher’s recognition.

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  Fig. 5.2 Harlow Shapley on a radio broadcast at CBS in the 1940s. Photograph of Harlow Shapley,

  July 23, 1946. HUG 4773.80P (Box 2), Harvard University Archives.

  hand sketching and photographing re-enacted the slow recognition that our gigantic stellar

  system is only one island in a colossal cosmic archipelago of billions of others. Establishing

  the Sun’s position as a tiny member of the Milky Way itself has also followed a long and

  twisted path.

  In contrast to some other scientific controversies, the realization that spirals are other

  stellar systems like our own Milky Way, located millions and even billions of light-years

  away, had limited public impact. There was little response from broad society, certainly

  nothing like the uproar that greeted Darwin’s theory of evolution, Freudianism or the ficti-

  tious canals on Mars. There were reasons for the tame reaction. First, the “island-universe”

  view was not socially revolutionary. It did not disturb the order of things, even less the per-

  ceived course of events on Earth. Galaxies were not part of the scriptures or any other holy

  text. Secondly, the sequence of breakthroughs in physics, biology and psychology around

  the turn of the century may have acted as an intellectual desensitizer. Finally, compared with

  the practitioners of the nineteenth century, scientists of the twentieth century had become

  more specialized, more focused on one discipline; they often ignored or failed to participate

  in the major debates and controversies in fields other than their own. Naturalists or gener-

  alists of the eighteenth and nineteenth centuries who engaged in every intellectual debate

  were becoming an extinct group.

  Even among astronomers, the subject appeared to raise more curiosity than deep inter-

  est. The so called “Great Debate” between Harlow Shapley (Fig. 5.2) and Heber Curtis at

  the meeting of the National Academy of Sciences in Washington, DC, on 26 April 1920

  was a subdued affair, where a young, ambitious but insecure Shapley seemed to avoid any

  hard confrontation with the experienced Curtis. Reading the slides shown by Curtis, one

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  realizes how structured and assertive Curtis was at conveying his perspective of spirals

  being external systems, hundreds of thousands of analogues of the Milky Way.

  Shapley was skirting with the issue. Instead of tackling the nature and location of spirals,

  he tried to promote his arguments in favor of a “super Milky Way,” 300,000 light-years in

  size, which embraced everything. Sarcastically, Shapley wrote privately: “The Andromeda

  Nebula, a giant among the external galaxies, is apparently not larger than the star cloud in

  Sagittarius and Scorpio which appears to form the nucleus of our Milky Way.”6 Referring

  to spirals, he preferred “to believe that they are not composed of stars at all, but are truly

  nebulous objects.” Shapley was putting undue weight on nebular spectra that showed the

  emission lines typical of diffuse gaseous substances.

  Curtis (see Fig. 3.4) took this spectroscopic argument and skillfully turned it on its head,

  stating that the spectra of spirals were “as would be expected from a vast congeries of stars.”

  Five years before Hubble determined direct distances to spirals, Curtis surmised that the

  “great nebula” in Andromeda was at 500,000 light-years from us, and the more remote

  spirals at 10 million or more light-years. Mi
chael Hoskin has provided a wonderful and

  detailed overview of the debate and related events, by drawing on archival material. He

  warned that some historians have erred in dramatizing a debate that really never took place,

  creating “an historical romance.”7

  Many excellent books have told the story of the discovery of the world of galaxies,

  emphasizing several fascinating aspects; the reader should refer to these works for a more

  complete analysis and other perspectives.8 The same story seems to be retold each time

  with its variable and particular set of details. It is an intricate but wonderful story, and like

  kids at bedtime, we like to hear it over and over again. Here I will be brief. This chapter

  re-assembles in a more linear way some parts of previous chapters. There is a review of the

  battle to resolve “nebulae” into stars, a long and unsuccessful crusade, which at the same

  time helped in the mission to locate better telescopes at more favorable observing sites.

  The roles of key players in the discovery of galaxies are highlighted, and how this led to

  the dramatic follow-up finding, that the universe is expanding; and I consider how images

  drove the development of new ideas.

  If a particular lesson is to be remembered, it is the intricacy of the scientific discov-

  ery process. Stigler’s law of eponymy encapsulates this: no scientific discovery is named

  after its discoverer.9 Stephen Stigler, a professor of statistics at the University of Chicago,

  demonstrated his paradoxical point in a straightforward manner: it was science historian

  6 Memo from H. Shapley to J. C. Duncan, 29 January, 1930, cited by O. Gingerich, Through Rugged Ways to the Galaxies, Journal for the History of Astronomy, 1990, Vol. 21, p. 79.

  7 M. Hoskin, The ‘Great Debate’: What Really Happened, Journal for the History of Astronomy, 1976, Vol. 7, pp. 169–182.

  8 Among several books, I note many excellent ones: S. L. Jaki, The Milky Way – An Elusive Road for Science, New York: Science History Publications, 1972; R. Berendzen, R. Hart and D. Seeley, Man Discovers the Galaxies, New York: Science History Publications, 1976; R. W. Smith, The Expanding Universe – Astronomy’s ‘Great Debate’ 1900–1931, Cambridge: Cambridge University Press, 1982; H. S. Kragh, Conceptions of Cosmos – From Myth to the Accelerating Universe, A History of Cosmology, Oxford: Oxford University Press, 2007; M. Bartusiak, The Day We Found the Universe, New York: Vintage Books, 2010; A. Hirshfeld, Starlight Detectives: How Astronomers, Inventors, and Eccentrics Discovered the Modern Universe, New York: Bellevue Literary Press, 2014. For a concise overview, see J. D. Fernie, The Historical Quest for the Nature of the Spiral Nebulae, Publications of the Astronomical Society of the Pacific, 1970, Vol. 82, pp. 1189–1230.

  9 S. M. Stigler, Stigler’s Law of Eponymy, Transactions of the New York Academy of Sciences, 1980, Vol. 38, pp. 147–158.

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  Robert K. Merton who had initially proposed the “law,” not him. The law of eponymy is

  not a law in the scientific or logical sense, but a sociological statement about discovery.

  Imagining “Nebulae”

  As has been shown in the previous chapters, the stars of the celestial sphere have been

  observed, catalogued and mapped for several thousand years. Nebulous objects were found

  from time to time among the fixed stars as they moved rapidly across the sky; they were

  comets. However, a few diffuse objects appeared fixed against the sidereal vault; they

  became known as “nebulae.” Chapter 1 gives an account of how the Persian scholar Abd

  al-Rahman ibn Umar al-Sufi first identified, in the year 964, what we now know to be a

  galaxy: the “little cloud” in the constellation of Andromeda. There then followed a silence

  of almost seven centuries. In 1614, Marius observed the “little cloud” with a telescope and

  gave a vivid description of the “nebula” in Andromeda as “the light of a candle, seen at

  some distance, shining through horn.”10

  We had to wait more than a century and a half for the exploration of the nebular world

  to take on a new life, with a more rigorous approach. The study of the nebular phenomenon

  followed two parallel tracks: first, an empirical one, solidly observational, mainly by the

  German-born musician and dilettante astronomer William Herschel and his astute sister

  Caroline; and secondly, a highly speculative avenue, roamed by three theological and philo-

  sophical writers, Thomas Wright, Johann Heinrich Lambert and Immanuel Kant.

  Although being of cautious mind, William Herschel tried to make sense of the numer-

  ous objects scattered across most of the sky. Like others, he battled with the challenge

  of some “nebulae” being, or appearing to be, resolved into stars and others not, caught

  in the midst of many contradicting reports. Herschel made recurrent hypotheses and went

  through paradigm shifts: as we saw, he first surmised that “nebulae” could simply be clus-

  ters of unresolved stars, due to their large distances, other “milky ways”; later, he appeared

  to favor nebular matter of an unknown state, some shining fluid that existed between the

  stars; later still, he saw “nebulae” as solar systems in formation. Although he changed his

  mind regarding their nature, “William did more than any other astronomer to trigger the

  transformation from the eternal, clockwork universe of Isaac Newton and Gottfried Leib-

  niz to that of modern astronomy in which everything from individual stars to the cosmos

  itself changes over time.”11

  Herschel had been neither alone nor the first on the speculative path to interpret “nebu-

  lae.” Their mystery had already captured the attention of eighteenth-century philosophers.

  With an increased vigor, the most eminent thinkers, Wright, Lambert and Kant, undertook

  to describe the large-scale structure of the heavens. They employed a mix of theosophical

  and physical considerations.

  10 Cited in G. P. Bond, An Account of the Nebula in Andromeda, Memoirs of the American Academy of Arts & Sciences, 1848, Vol. 3, p. 76.

  11 M. Hoskin, Caroline Herschel as Observer, Journal for the History of Astronomy, 2005, Vol. XXXVI, p. 396.

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  The first, Thomas Wright of Durham (1711–1786), remains known for his highly spec-

  ulative and moral work, A Theory of the Universe, published in 1734. He dealt with the

  various astronomical phenomena and sought a unified explanation. He tried to explain

  novae, variable stars, nebulae, comets and the Milky Way in terms of volcanoes in the sky.12

  Wright had little impact on the thinking of contemporary astronomers. William Herschel

  had a copy of Wright’s book and annotated it. As commented by Michael Hoskin, Herschel

  never expected to learn anything from the work.

  Johann Heinrich Lambert (1728–1777) was the second moral “cosmologist” to be cap-

  tivated by “nebulae.” He elaborated on a hierarchical system of clusters of stars spreading

  across millions of light-years of sidereal space. William Herschel had no sympathy either

  for the “fantastic imaginations” of Lambert. It was an issue of character: an opposition

  between the hard, reluctant empiricist that Herschel was and dreamers of subjective moral

  order.

  Wright’s id
eas on trying to integrate the moral and scientific picture of the universe did

  influence fertile minds, including that of Immanuel Kant; Kant was less restrained by the-

  ological references and, fortunately, he wrote more clearly than Wright. Using a consistent

  argument and borrowing on Newton’s theory of gravitation, Kant’s speculative approach

  in explaining cosmic shapes showed good insight. He refuted Pierre Louis Maupertuis’

  idea that nebulae were huge flattened stars induced by their rapid rotation. Instead, Kant

  proposed a drastically different concept, “that there are no such individual huge stars but

  systems of many stars, whose distance makes them appear in such a narrow space, that the

  light, which cannot be seen for each individual star, because of the countless crowd of them,

  comes out in a uniform pale glow.”13

  It is remarkable that shape or morphology played a significant role in Kant’s inter-

  pretation and his model for “nebulae.” Before it became a contentious debate among

  astronomers, Kant became known as the champion of the “island-universe” concept: most

  “nebulae” are distant Milky Way-like systems. However, he could neither prove this, nor

  was he equipped to do it.

  The Nebular Hypothesis

  In proposing, in the same little book, a precursor view of the nebular hypothesis, Kant

  threw a snag onto the elusive path of unraveling the nature of “nebulae.” It was a con-

  cept that captivated, often distracted, many astronomers of later generations and acted as

  a double-edged sword. Why? Because the nebular shapes that were revealed by telescopic

  drawings and early photographs showed patterns consistent with the mechanism Kant had

  proposed to describe the formation of the Sun, its planets and their satellites. Much of the

  12 M. Hoskin, The Cosmology of Thomas Wright of Durham, in Stellar Astronomy: Historical Studies, Chalfont St. Giles: Science History Publications, 1982, pp. 113–114.

  13 I. Kant, Universal Natural History and Theory of the Heavens (translated by Ian Johnston), Arlington: Richer Resources Publications, 2008, p. 37.

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  controversy revolved around images of “nebulae” and their physical meaning.14 Many