The Role of Images in Astronomical Discovery Page 6
24 M. Hoskin, Discoverers of the Universe: William and Caroline Herschel, Princeton: Princeton University Press, 2011.
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Part I – Images and the Cosmos
Fig. 1.2 William Herschel. Engraving by James Godby showing Herschel against background of
stars in Gemini where he discovered Uranus in 1781. Credit: University of Cambridge, Institute of
Astronomy.
fabrication, building and selling hundreds of telescopes over his career. He rapidly turned
into a dedicated and proficient astronomical observer. In a serendipitous finding with a
superb 7-ft telescope, William recognized Uranus as a true planet of the solar system in
March 1781 (Fig. 1.2). King George III acknowledged the famous discovery by giving
Herschel a yearly stipend and requesting him to move to Windsor as astronomer of the
English court, a position created to entertain the Royal family and their visitors.
In a first telescopic program, Herschel had started observing and cataloguing stars, dou-
ble stars in particular. His interest was to build a database of pairs of stars all across the sky.
To assemble a large sample, William Herschel systematically scanned the sky. The goal
was to measure the proper motions of these stars, i.e. stars moving with respect to each
other in the celestial traffic, and more importantly to measure their parallaxes; the latter
was a key for deriving the absolute distance of nearby stars by trigonometry. The parallax
is a slight shift of the apparent positions of nearby stars with respect to the background
of more distant ones, as the Earth swings on its orbit around the Sun during the yearly
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revolution. Herschel’s assumption was that many of the stellar pairs were not physically
bound systems, but chance alignments of a relatively nearby star along the line of sight of
a more distant one, thus forming a proper set-up for determining the parallactic angle.25
Herschel published three catalogues of double stars (1782, 1784 and 1821).
The Herschels also embarked on another observing program. With the 4-inch “sweeper”
William had built for her, Caroline scanned the sky in search of new comets in the dawn
sky. In her systematic sweeps, she found something odd. She noted that there were many
more nebulous patches than those identified and listed by Messier and Méchain, even with
additions to their earlier catalogue. Caroline was intrigued. She suggested to her brother
that it might be useful to initiate a survey of “nebulae” and establish the extent of the popu-
lation of these many faint diffuse objects. From 1782 to 1802, well organized and strongly
motivated William, assisted by Caroline, systematically scanned the sky with a new pur-
pose: to identify and catalogue all non-stellar objects in order to better map our great stellar
system and establish the Sun’s position in it. The number and distribution of “nebulae”
were one aspect of Herschel’s investigation. Another characteristic of “nebulae” that drew
his attention was morphology or shape.
“United Luster of Millions of Stars”
Herschel spent thousands of hours observing and noting the appearances, features and struc-
tures of “nebulae,” emphasizing differences as well as similarities between the various cat-
egories. He found time to write extensively about his observations. In 1785, he detailed
the observed distribution of stars in the Milky Way and dealt with his preliminary findings
on “nebulae.”26 Several types of “nebulae” were identified as Herschel kept finding more
objects, confirming Caroline’s suspicion. A few hundred non-stellar objects turned out to
be star clusters or nebulosities congregating in a plane across the sky that corresponded to
the band of the Milky Way. However, the great majority of “nebulae” appeared well above
or below this band, numerous even in fields where stars were scarce. As we now know,
these were the far more numerous and distant extragalactic “nebulae,” or galaxies as they
were to be called a century and a half later.
Anxious to provide details, Herschel wrote descriptions of the shapes and colours of
many of the “nebulae” he found. He even speculated that the “great nebula” in Andromeda,
now known as the Andromeda Galaxy, was the closest to us. Herschel also described the
“nebula” in Messier 51 as a bright round nebula, surrounded by a halo or glory, and accom-
panied by a companion.27 This was a surprisingly good description of the distant galaxy,
taking into account the size of the telescope and the type of tarnishing metallic mirror he
25 For more on the parallax program, see A. W. Hirshfeld, Parallax: The Race to Measure the Cosmos, New York: W. H. Freeman and Company, 2001.
26 W. Herschel, On the Construction of the Heavens, Philosophical Transactions of the Royal Society of London, 1785, Vol. 75, pp. 213–266.
27 The Scientific Papers of Sir William Herschel, ed. J. L. E. Dreyer, London, 1912, Vol II, p. 657. Also cited in M. Hoskin, The First Drawing of a Spiral Nebula, Journal for the History of Astronomy, 1982, Vol. 13, p. 97.
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Part I – Images and the Cosmos
was using at the time. For the intrigued Herschel, “nebulae” remained a mysterious set of
cosmic objects.
A fundamental barrier obstructed Herschel and all his contemporaries: they had no idea
of the distances to “nebulae.” By assuming that stars were other suns, eighteenth-century
astronomers could get a rough estimate of stellar distances by comparing the apparent
brightness of the stars to that of the Sun; they inferred correctly that stars were light-years
away. However, there was no direct (or indirect) way of getting even a rough estimate of
distances for “nebulae.” There was no ‘nebula’ nearby to compare with and to serve as a
gauge.
Still, this did not deter the audacious Herschel from speculation. In his early article
Construction of the Heavens of 1785, he mused: “ . . . the naked eye, which, as we have
before estimated, can only see the stars of the seventh magnitude so as to distinguish them;
but it is nevertheless very evident that the united luster of millions of stars, such as I suppose
the nebulae in Andromeda to be, will reach our sight in the shape of a very small, faint
nebulosity.” Theorizing further, he imagined what an observer living in a distant star cluster
would see, adding: “ . . . If the united brightness of a neighboring cluster of stars should, in
a remarkable clear night, reach his sight, it will put on the appearance of a small faint,
whitish, nebulous cloud, not to be perceived without the greatest attention.”28 Although
correct, these ruminations were then entirely conjectural.
In the same 1785 article, William Herschel noted “that remarkable collection of many
hundreds of nebulae which are to be seen in what I have called the nebulae stratum of Coma
Berenices.” This was the first description of a cluster of galaxies (Fig. 1.3 and 1.4).29 The
stratum is now known as the Coma cluster and Virgo cluster, two large concentrations of
galaxies in the northern sky.30 Coma is one of the largest clusters of galaxies in
the universe,
containing 1,000 individual galaxies, located 333 million light-years away. Herschel’s com-
ment marked the discovery of some of the largest assemblies of cosmic matter. Using the
word “stratum,” Herschel implied layering in the structure of the universe, as for the geolog-
ical strata of the Earth, perhaps inspired by the innovative work of contemporary Scottish
geologist James Hutton (1726–1797). Any of us who has looked through a telescope eye-
piece can only admire how observant and clear-sighted were the Herschels when viewing
the sky more than 200 years ago.
Musing over the nature of the nebulous patches as assemblies of a multitude of unre-
solved stars, we see Herschel at first clearly favoring the “island-universe,” the magic
expression later put forward by Alexander von Humboldt: these numerous and distant
objects, he thought, are similar but detached from the Milky Way. “As we are used to call the
appearance of the heavens, where it is surrounded with a bright zone, the Milky-Way, it may
not be amiss to point out some other very remarkable Nebulae which cannot well be less, but
28 W. Herschel, On the Construction of the Heavens, Philosophical Transactions of the Royal Society of London, 1785, Vol. 75, p. 218.
29 Charles Messier had already noticed the exceptional concentration of “nebulae” in the Virgo constellation in his catalog of 1784.
30 M. Hoskin, The Construction of the Heavens: William Herschel’s Cosmology, Cambridge: Cambridge University Press, 2012, p. 52.
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Fig. 1.3 Transformational Image: A Remarkable Collection of Galaxies in Coma Berenices.
This image was viewed and sketched by amateur Michael Vlasov using a 25-cm f/5 Newtonian tele-
scope. The view is probably similar to that observed by the Herschels. Credit: Michael Vlasov.
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Part I – Images and the Cosmos
Fig. 1.4 Coma cluster of galaxies imaged with the 0.8-m Schulman telescope of the Mount Lemmon
Sky Center. Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona.
are probably much larger than our own system . . . for which reason they may also be called
milky-ways by way of distinction.”31 William Herschel held this daring opinion until 1791.
Herschel’s Paradigm Shift
That year Herschel went through a paradigm conversion and became an adept of the Nebular
Hypothesis. Having found a few indisputable examples of stars associated with nebulosity,
Herschel flipped opinion and fixed his mind on demonstrating that all “nebulae” had to be
close by. However, someone else likely stimulated his flip. In 1755, German philosopher
Immanuel Kant (1724–1804) had also presented a coherent, if qualitative, proposition of
“why will the middle point of every system consist of a burning body [ . . . ] with the sun
as the central body, and the fixed stars visible to us, all things considered, mid-points of
similar systems.”32
Kant’s concept was not entirely new. Inspired by the giant whirlpools of “universal
fluid” imagined by René Descartes (1596–1650), Swedish mystic Emanuel Swedenborg
31 M. Hoskin, The Construction of the Heavens: William Herschel’s Cosmology, Cambridge: Cambridge University Press, 2012, p. 255–256.
32 I. Kant, Universal Natural History and Theory of the Heavens, Translated by Ian Johnston, Arlington: Richer Resources Publications, 2008, pp. 105–113.
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(1688–1772) had proposed elements of the nebular condensation hypothesis in 1734.33 In
1809, French mathematician Pierre Simon Laplace (1749–1827) took Kant’s idea and trans-
formed it into a rigorous mathematical proposition. Laplace proposed a cosmogonical the-
ory that became known as the Nebular Hypothesis.34 As described in The System of the
World, the Sun and the solar system formed from a large cloud of gaseous material.35 The
primordial cloud had a slight rotation. It collapsed into a disk under the action of Newtonian
gravitation. With the fiery Sun at the center, the planets and their satellites kept the imprint
of the original cloud motion, forcing all rotational and orbital movements to be in the same
direction. Years before Laplace, Herschel already felt he was on solid ground and pushed
forward with Kant’s appealing idea.
Having pocketed many nebulae with a central star (e.g. the planetary nebula NGC 1514),
Herschel convinced himself that nebulae of such “singular appearance” were new stars and
planetary systems in formation (Fig. 1.5). “Impressed with an idea that nebulae properly
speaking were clusters of stars, I used to call the nebulosity of which some were composed,
when it was of a certain appearance, resolvable; but when I perceived that additional light,
so far from resolving these nebulae into stars, seemed to prove that their nebulosity was
not different from what I had called milky, this conception was set aside as erroneous.”36
Herschel invoked shining fluid not necessarily associated with a star. It was indeed a rather
judicious and correct statement about galactic nebulae, as we now know them to be fluo-
rescent clouds of interstellar gas.
Consequently “nebulae” of such “singular appearance” had to be relatively small sys-
tems and located nearby. Herschel then drew an assertive but incorrect conclusion: all “neb-
ulae” were part of the great Milky Way system, he declared, even if he allowed some sys-
tems to be at the periphery of the Milky Way. Our great system of stars had no bound, as
he wrote. In Herschel’s transformed view, “nebulae” all belong to the “Heavens” defined as
the Milky Way, and this same Milky Way system encompasses everything in the universe,
he concluded solemnly. From today’s perspective, Herschel had switched to the wrong
hypothesis. His meticulous observations had provided him a reasonable justification, as
more observational cases were to strengthen his “local” view. Aware of Kant’s speculative
work, Herschel was also attracted by its strong visual inferences and its natural link to New-
ton’s gravitational theory. Converted into a valorous champion of the Nebular Hypothesis,
Herschel was to be followed by several others in the following decades.
A note on planetary nebulae is in order. Today we now know that planetary nebulae are
not associated with the birth of stellar systems but are a phenomenon that happens at the
opposite end of star life. They are envelopes of gas and dust that have been spat out by stars
with masses between one and seven times that of the Sun. As the central nuclear fuel runs
33 E. Swedenborg, Prodromus Philosophiz Ratiocinantis de Infinito, et Causa Finali Creationis: de que Mechanismo Operationis Animae et Corporis, 1734.
34 S. G. Brush, Nebulous Earth, The Origin of the Solar System and the Core of the Earth from Laplace to Jeffreys, Cambridge: Cambridge University Press, 1996, pp. 14–36.
35 P. S. Laplace, The System of the World, Vols. 1 and 2, London: Printed for Richard Phillips, 1809.
36 W. Herschel, Astronomical Observations Relating to the Construction of the Heavens, Philosophical Transactions of the Royal Society of London, 1811, Vol. 101, p. 270.
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Part I – Images and the Cosmos
Fig. 1.5 Planetary nebula NGC 1514. Located 800 light-years away, the nebula was discovered
by William Herschel in 1790. The modern image on the left was taken in the visible. The
image on the right was taken in the infrared with the space observatory WISE. Credit: NASA/JPL
Caltech/UCLA/Digitized Sky Survey/STScI.
out, the star goes through a complex rearrangement of its internal structure. The star interior
resettles by contracting and adjusting to a new equilibrium, as its outer envelope inflates to
an enormous volume, the red giant stage. At some point, the outer layers of the evolved
star become detached from the star and expand further in a relatively non-violent process
lasting thousands of years; the star leaves behind a small central core, which becomes a
white dwarf. As the dying star evolves, its bright envelope becomes visible as a planetary
nebula excited by the hot central stellar remnant. Herschel called them “planetary” because,
as viewed through his telescopes, they resembled the disk of the planet Uranus.
In closing my discussion of Herschel’s work, a few remarks are in order. First, Her-
schel had been wise to put on hold his double-star program and to moderate his ambition
to measure the parallax of nearby stars. Several of the pairs he had identified turned out
to be physically bound systems, thus improper for parallax determinations. Furthermore,
his telescopes suffered from poor precision and accuracy; measuring a parallax with them
was impossible. He could not win the race. Indeed, it was some time later, in 1838, and
with much improved instrumentation, that German astronomer Friedrich Wilhelm Bessel
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Fig. 1.6 Herschel’s 20-ft telescope (18.7-inch mirror). Credit: University of Cambridge, Institute of
Astronomy.
(1784–1846) became one of three individuals who first measured a stellar parallax. Bessel
won the race by measuring a parallax of 0.314 arcsecond for the star 61 Cygni, putting it at