The Role of Images in Astronomical Discovery Page 17
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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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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