The Role of Images in Astronomical Discovery Read online

  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