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The Role of Images in Astronomical Discovery

Page 5

by Rene Roy


  “nebulae.”

  Observing by Viewing

  Recalling an impromptu nightly excursion in the Amazonian jungle, American naturalist

  Edward O. Wilson writes: “The best science doesn’t consist of mathematical models and

  experiments, as textbooks make it seem. Those come later. It springs fresh from a more

  primitive mode of thought, wherein the hunter’s mind weaves ideas from old facts and

  fresh metaphors and the scrambled crazy images of things recently seen. To move forward

  is to concoct new patterns of thought, which in turn dictate the design of the models and

  experiments. Easy to say, difficult to achieve.”9 With this vivid expounding, Wilson gives

  us a fine summary of scientific methodology and a close description of the successive steps

  that pace most research ventures.

  Wilson’s text also illustrates in a few words the long process of humankind finally real-

  izing that most “nebulae” in the sky were distant independent stellar systems, other giant

  Milky Way-like conglomerates of stars. Notable is that the process of sensing and concoct-

  ing patterns depends on handling images and on the interpretation of these images by the

  observer. To read William Herschel’s papers is to retrace a good part of the process Wilson

  has described.

  Unaware of the nature and distances of “nebulae,” the Herschels and other early

  observers did not know they were most often observing galaxies when looking at these

  celestial blurs. They were puzzled, often confused. Our predecessors considered “nebulae”

  either as patches of a dispersed sidereal substance, or some sort of clouds or assemblies of

  faint distant stars. For centuries, they could not figure out either of these alternatives. True

  nebulae, like the Orion Nebula, are gaseous and dusty. They concentrate along the band

  of the Milky Way in the thousands, and they are much less abundant than the extragalactic

  “nebulae” or galaxies; the latter appear to avoid the plane of the Milky Way. This is because

  our view through the plane of our galaxy is blocked by interstellar dust, microscopic grains

  of graphite and silicates that stop starlight from objects, or from being visible, further away.

  As per Wilson’s stumbling discovery process, generations of astronomers grappled with

  the reality of the nebular images and tried to discern the nature of these mysterious objects.

  The history of the discovery of galaxies illustrates the complicated interplay between the

  “mishmash” of observations and the patterns of thought as hypotheses are constructed.

  8 M. Hoskin, The Construction of the Heavens: William Herschel’s Cosmology, Cambridge: Cambridge University Press, 2012.

  9 E. O. Wilson, The Diversity of Life, Cambridge, MA: The Belknap Press of Harvard University Press, 1992/Allen Lane, 1993.

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  Part I – Images and the Cosmos

  Furthermore, new knowledge from other fields is often needed to solve puzzles in one

  field. The nature of true nebulae (fluorescent clouds of hot gas) came to be correctly under-

  stood only after distinguishing them from galaxies by their apparent relative receding or

  approaching velocities, morphologies and spectra. In the 1860s, spectra of stars and “nebu-

  lae” came to be partly understood following the work of German physicist Gustav Kirchhoff

  (1824–1887) and chemist Robert Bunsen (1811–1899). The full explanation of the spectra

  and the nature of gaseous nebulae came much later from the findings of atomic physics in

  the 1920s and 1930s. More on this in Chapter 5.

  As Wilson points out so well, observing is fundamental to the natural sciences. The

  practices of observing are a key part of the scientific process. If observations are important,

  they are not a neutral source of information about nature. Stephen Jay Gould liked to quote

  Charles Darwin: “How can anyone not see that all observations must be for or against

  some view if it is to be of any service?”10 Observing and viewing are complex mental and

  cognitive processes that need to be understood.

  Observing has been a study in itself. For example, manuals on how and what to observe,

  or how to use a given instrument for observing, have been and continue to be important

  guides to scientists of all epochs. The application of common practices has been crucial

  in viewing objects, obtaining images, describing them and interpreting them. “The col-

  lective empiricism institutionally launched by seventeenth-century academies such as the

  Academia Naturae Curiosorum or the Royal Society of London depended on the recruit-

  ment and reciprocal calibration of observers in correspondence networks. This was most

  obvious in the case of weather observers, who were encouraged to standardize their instru-

  ments, hours of observation, and recording forms, but it also held for astronomy, anatomy,

  and natural history.”11 These practices attained maturity only after a time. Ancient scholars

  or naturalists did not necessarily use such standard practices. It is important to remember

  this to avoid the trap of historicism especially when we look at centuries of a scientific

  quest.

  Shining Through a Horn

  The Greek–Egyptian mathematician astronomer Claudius Ptolemy of Alexandria (c. 90–

  168) was the first to mention objects in the sky (apart from the Sun and the Moon) that were

  not a star, a planet nor a comet. He identified five “nebulous stars.” These “nebulous stars”

  were the stellar open clusters h & χ Perseus (Messier 34), Praesepe (Messier 44) in Cancer,

  Messier 7 in Scorpius, and the star pair ν Sagittarius.12 For example, Ptolemy described the

  star λ Orionis, Meissa, as “the nebulous star in the head of Orion.”13 One thousand five

  hundred years later, Galileo showed that Meissa was simply a pack of a few unresolved

  10 S. J. Gould, Why Darwin?, The New York Review of Books, April 14, 1996, Vol. XLIII, No. 6, p. 10.

  11 L. Daston, On scientific observation, Isis, March 2008, Vol. 99, No. 1, p. 102.

  12 More details in P. Kunitzsch, A Medieval Reference to the Andromeda Nebula, The Messenger, 1987, Vol. 49, pp. 42–43.

  13 The Almagest: in Ptolemy’s Almagest, Translated and annotated by G. J. Toomer, Princeton: Princeton University Press, 1998, p. 382.

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  stars appearing to cluster around a brighter star. For Galileo and several of his contempo-

  raries, mesmerized by telescopic viewing, all nebulous patches were expected to be resolved

  into stars given enough telescopic power.

  While Meissa fell flat as a true nebulous object, a fuzzy patch in Andromeda, not listed

  by Ptolemy, turned out to be much more promising. Persian astronomer Abd al-Rahman

  ibn Umar al-Sufi (903–986) wrote about the “little cloud” of Andromeda in his Book of

  the Constellations of the Fixed Stars of 964; al-Sufi gave descriptions of the 48 Ptolemaic

  constellations, and included two drawings of each constellation, one “as seen in the sky” and

  one “as seen on the celestial globe.”14,15 Medieval astronomers also noticed the nebulous

  patch in the constellation of Andromeda. Indeed under a dark sky and a moonless night,

  the fuzzy object is visible to the naked eye (easily distinguishable with binoculars). Now<
br />
  recognized as the Andromeda Galaxy, it is one of the closest galaxies and one of the largest

  spirals in the local universe. Unsurprisingly, the object has occupied a special place in the

  development of our understanding of the world of galaxies. Apart from our own Milky

  Way, only four galaxies are visible to the naked eye: they are Messier 31 in Andromeda

  (see Plate 1.1, Fig. 0.4), Messier 33 in the Triangle, and the two Magellanic Clouds (see

  Plate 6.5). Nevertheless, al-Sufi’s short and astute remark about a peculiar object in the

  northern constellation was followed by a long lapse of silence.

  Early in the seventeenth century, astronomical observations were dramatically improved

  by the use of the telescope probably invented in Italy and improved by northern Euro-

  pean opticians.16 Italian astronomer Galileo Galilei and English astronomer Thomas Har-

  riot (1560–1621) were the first astronomers known to have observed the sky with telescopes

  of their own making. However, observing through the primitive Galileo-type refractor was

  a real challenge. Initially there was questioning about the reality of features seen through

  the assembly of the multiple glass meniscus and within the very narrow field of view of

  early telescopes. It is easy to understand why a skeptic priest would raise some doubt

  about astronomers’ assertions. However, most observers quickly admitted and concluded

  that what they where seeing with the magnifying optics was “true.”17

  German astronomer Simon Marius (1573–1624) was the first western individual known

  to have observed a “nebula” with the naked eye and with a small telescope in late 1612,

  and to have reported on his experience.18 Marius had spent a few months in Prague in 1601

  working and learning from Tycho Brahe, and from Johannes Kepler. We find his observing

  notes and comments about the Andromeda “cloud” in the preface to his Mundus Jovialis

  of 1614: “It was then visible to the naked eye, and appeared through the telescope to be

  composed of rays of light ( radii albicantes), increasing in brightness as they approached the

  14 P. Kunitzsch, Sufi: Abu al-Husayn Abd al-Rahman ibn Umar al-Sufi, in Biographical Encyclopedia of Astronomers, T. Hockey, V. Trimble, and T. R. Williams (editors), New York: Springer, 2007.

  15 For a fine reproduction of the “cloudlike spot” in the Andromeda constellation, see Fig. 3.1 in H. Nussbaumer and L. Bieri, Discovering the Expanding Universe, Cambridge: Cambridge University Press, 2009, p. 28.

  16 Henry C. King, The History of the Telescope, Mineola: Dover Publications, Inc., 2003.

  17 M. Biagoli, Galileo, Courtier: the Practice of Science in the Culture of Absolutism, Chicago: University of Chicago Press, 1993, pp. 96–101.

  18 H. Nussbaumer and L. Bieri, Discovering the Expanding Universe, Cambridge: Cambridge University Press, 2009, pp. 27–28.

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  Part I – Images and the Cosmos

  centre, which was marked by a dull, pale light, – ‘ in centre est lumen obtusum et pallidum.’

  Its diameter was a quarter of a degree, and it resembled the light of a candle, at some

  distance, shining through horn.”19 This is a fine description of the object as it can be seen

  today through a good set of binoculars or a small telescope.

  Despite the fact that Galileo scanned great portions of the sky with his telescopes, we

  have no evidence that he observed the Andromeda “nebula.” It is likely that Galileo sur-

  mised that with greater optical power the “cloud” would resolve into stars as he had shown

  for Meissa. The Florentine astronomer may not have been alone in ignoring the “cloud” in

  Andromeda. Marius was caught in a nasty priority dispute with Galileo about who had first

  discovered the moons of Jupiter, resolved the Milky Way into stars, and found sunspots.

  The German expressed his surprise that the cloud had been noticed neither by the perspica-

  cious Tycho Brahe (1546–1601), nor by the ancient Greek astronomer Hipparchus (c. 190–

  c. 120 BC), the two greatest visual astronomical observers of pre-telescopic astronomy.

  The enraptured Marius went overboard and claimed to have resolved the stars of several

  “nebulae.” This did not help his credibility and, unfortunately, trash hides the gems in his

  contentious work.20 Nevertheless, it is puzzling that the fuzzy patch in Andromeda was not

  reported more often, nor inscribed on the many celestial maps of the times.21

  Astronomers using telescopes became more intrigued as they found more and more

  “nebulae” across the sky. With improved telescopes, Swiss astronomer Jean-Philippe de

  Chéseaux (1718–1751) and French astronomer Guillaume le Gentil (1725–1792) added a

  few dozen objects, some of which we now know to be star clusters that were unresolved

  due to the limited power of their instruments. Le Gentil also left a sketch of the Orion Neb-

  ula. A few wrote vivid descriptions of their appearance and tried to figure out what they

  were. English astronomer and mathematician Edmund Halley (1656–1742) was intrigued

  by what he called “nebulous stars” and described them as space “through which a lucid

  medium is diffused, that shines with its own proper lustre.”

  We were still in the era of positional astronomy, the branch of astronomy going back to

  antiquity, and to which Newtonian mechanics and the use of the telescope were then giving

  a new life and a new degree of certainty. Studying the clockwork of planetary orbits was

  serious business compared to the frailty of speculative nebular work. Sidereal astronomy

  was quite different, so new and so flimsy that many did not even bother with it, especially

  when faced with the mysterious foggy patches.

  Comet Fanatics

  It was soon realized that when viewed through rudimentary telescopes, comets and “neb-

  ulae” could easily be confused. The difference is that comets moved daily across the sky,

  changing rapidly in brightness. In contrast, “nebulae” are stationary and do not vary in

  19 G. P. Bond, An account of the nebula in Andromeda, Memoirs of the American Academy of Arts and Sciences, New Series, 1848, Vol. 3, pp. 75–86.

  20 S. L. Jaki, The Milky Way, An Elusive Road for Science, New York: Science History Publications, 1972, p. 111.

  21 D. Schultz, The Andromeda Galaxy and the Rise of Modern Astronomy, New York: Springer, 2012.

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  brightness. Comets belong to the solar system; “nebulae” do not. Still, when a presumed

  comet is first spotted, it might just be an existing “nebula”. French astronomer Charles

  Messier (1730–1817) wanted to prevent false alarms. A passionate comet observer, Messier

  discovered 13 of them. To warn other comet chasers not to be fooled, he provided a list of

  “nebulae”, objects that resemble comets but are not. The purpose of his catalogue was a

  sort of black list. Do not bother about the fixed nebulous patches: learn about their location

  but just ignore them!

  Messier’s catalogue included many objects that we now know to be galaxies. Oddly, the

  catalogue remains to this day a magnificent and useful compendium of deep-sky objects.

  Like ornithologists compiling sighting lists of birds observed, many amateur astronomers

  of today aim to put the whole set of Messier objects un
der their belt when they get their

  first telescope. The objects are generally relatively easy to photograph with the sophis-

  ticated equipment of today, and many amateurs produce spectacular images of Messier

  objects.22

  If Messier and other comet chasers like Pierre Méchain (1744–1804), who added several

  objects to Messier’s list, had little interest in “nebulae” – as these were just distractions

  and had to be skipped over – others cared about “nebulae.” Investigative and broad in his

  approach, French astronomer Nicolas Louis de Lacaille (1713–62) paid attention. He was

  one of the greatest observers of the eighteenth century, a pioneer in exploring the sky of the

  southern hemisphere. Ahead of Messier, Lacaille had catalogued 42 nebulae and clusters of

  the southern sky and had proposed an informative and practical classification of “nebulae.”

  He divided these objects into three groups: “nebulae of the first class” (or nebulae without

  stars), “nebulous stars in clusters,” and “stars accompanied by nebulosity.” Lacaille was on

  the right track. As noted by Swedish astronomer Peter Nilson, Lacaille’s simple depiction

  and nomenclature happen to agree approximately with the modern division of nebulous

  objects into globular clusters, open clusters and gaseous nebulae.23

  From Music to the Construction of Heavens

  A huge and unexpected step in deep-sky probing came from remarkable siblings: two Ger-

  man musicians who had migrated to England to earn a better living and who became

  outstanding self-made astronomers. William and Caroline Herschel swiftly advanced to

  take rank among the better-known astronomers of their time.24 Starting in 1773, William’s

  interests shifted from music to mathematics and lenses; he became obstinately passionate

  about them. William started building reflecting telescopes based on the use of copper–tin

  alloy for the reflecting primary mirror in the early 1770s. With the help of his younger

  brother Alexander and teams of local artisans, William became very skillful at their

  22 R. Gendler, Forays into Astronomical Imaging: One Person’s Experience and Perspective, Astronomy Beat, August 30, No. 79, 2011.

  23 Peter Nilson presents a concise summary of the history of surveying and cataloging ‘nebulae’ and galaxies in the Uppsala General Catalogue of Galaxies, Royal Society of Science of Uppsala, 1973.

 

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