The Role of Images in Astronomical Discovery Read online

  telescopes for photographic surveys of galaxies (Fig. 3.10). The name of the telescope optics

  design recognizes German optician Bernhard W. Schmidt (1879–1935), who invented the

  concept. The Schmidt design combines a Cassegrain reflector configuration and a large

  corrector plate, which results in a compact telescope that gives a super-wide field of view.

  Schmidt telescopes, although of small aperture, provide significantly larger fields of view

  than even the Ritchey–Chrétien design. Fields of view several times the angular size of the

  Moon can be imaged in a single shot. Schmidt telescopes were most suitable for Zwicky’s

  work because galaxy groupings were of larger angular sizes than the fields of view of large

  telescopes, such as the Mount Wilson 60-inch and 100-inch.

  Zwicky had found a “surprisingly large number of rather widely separate galaxies which

  appeared connected by luminous intergalactic formations.”60 These structures were very

  60 F. Zwicky, Multiple Galaxies, Ergebnisse der exakten Naturwissenschaften, 1956, Vol. 29, pp. 344–385.

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  Part I – Images and the Cosmos

  faint, showing barely above the brightness level of the sky background. A very tight light

  bucket was needed to detect them. In terms of light baffling, the powerful Mount Wilson

  100-inch was like an open door; it could not record the faint intergalactic features until all

  sources of stray light were eliminated. Hence, it was very challenging to bring out faint

  features at the limit of the photographic emulsion. Moreover, it was extremely difficult to

  reproduce the elusive features on a positive print and even more challenging to get them

  to come out in the printed journals. These limitations led Zwicky to draw the features by

  hand. He used simple sketches to highlight “multiple galaxies” and the faint filamentary

  structures that connected some of the galaxies, structures that he interpreted correctly as

  the result of inelastic collisions.61

  Zwicky also employed a sequence of drawings to illustrate how interacting galaxies

  could produce bridges and tails of stars as material was torn apart from the parent galaxies.

  In a seminal drawing, he described the transfer of momentum and the formation of tidal

  tails as two galaxies pass by each other and interact (Fig. 3.11; see also Fig. 6.8). These

  examples from Zwicky’s work represent a fine example of the use of sketching to illustrate

  a new phenomenon, based on the detection of features then at the very limit of instrument

  capabilities. The sketches were somewhat a reversal from the Worthington problem: While

  Worthington used photography to show that his earlier drawings had idealized the shapes

  of splashing drops, Zwicky employed old-style drawing to highlight real features barely

  visible on the original photographic plates.

  It is compelling that for his 1953 article in Physics Today, Zwicky chose to show a

  sketch of the double galaxy Messier 51.62 For the Swiss–American Carnegie Observatories

  astronomer François Schweizer, Zwicky’s “sketch shows faint details that became visible to

  most folks only later when the new Kodak IIIa-J plates showed significantly fainter details

  than the old 103a-0 or IIa-O plates did.”63

  Today’s astronomers, professional and amateur, have pushed the art and science of

  astrophotography to new heights.64 Researchers using images obtained with the Hubble

  Space Telescope and modern ground-based telescopes are now producing superb colour

  images by combining images obtained in filters of different wavelength passes. These

  images are used to derive important scientific measurements. The images are also a pow-

  erful means to convey the beauty of astronomical objects and to share the excitement of

  discovery with a larger public. Additionally, “when processed correctly, an attractive and

  evocative picture brings out the scientific content within.”65 Amateur astronomers have

  also caught up spectacularly with their efficient equipment and advanced image processing

  with computers. They now produce images of a quality that were not even dreamt of by

  professional astronomers of a few decades ago.66

  61 F. Zwicky, Multiple Galaxies, Ergebnisse der exakten Naturwissenschaften, 1956, Vol. 29, pp. 366–370.

  62 F. Zwicky, Luminous and Dark Formations of Intergalactic Matter, Physics Today, 1953, Vol. 6, pp. 7–11.

  63 Private e-mail note to the author (Sept. 12, 2014).

  64 T. A. Rector, et al., Image-Processing Techniques for the Creation of Presentation-Quality Astronomical Images, Astronomical Journal, 2007, Vol. 133, pp. 598–611.

  65 R. Villard and Z. Levay, Creating Hubble’s Technicolor Universe, Sky & Telescope, 2002, September issue, pp. 28–34.

  66 R. Gendler, Forays into Astronomical Imaging: One Person’s Experience and Perspective, Astronomy Beat, Astronomical Society of the Pacific, 2011, No. 79, August 30, pp. 1–6.

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  3. From Celestial Snapshots to Photographing the Realm of Galaxies

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  Fig. 3.11 Drawing sequence by Zwicky highlighting the dynamical phases of an interacting pair of

  galaxies and the formation of tidal tails. From Zwicky (1956), Ergebnisse der exakten Naturwis-

  senschaften.

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  Part I – Images and the Cosmos

  Fig. 3.12 Each of the four PanSTARRS cameras is equipped with a mosaic of 64 x 64 CCDs, spread-

  ing over an area of about 40 cm and providing a total of 1.4 gigapixels. Credit: Institute for Astronomy

  University of Hawai’i.

  Fig. 3.13 Schematic of the LSST camera. Credit: LSST Corp./National Science Foundation.

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  3. From Celestial Snapshots to Photographing the Realm of Galaxies

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  A new phase of systematic photography of the sky is beginning. The 1.8-m Panoramic

  Survey Telescope and Rapid Response System (PanSTARRS) and the 8-m Large Synoptic

  Survey Telescope (LSST) will transform our view of the universe by fully exploiting the

  time domain (Fig. 3.12). Located in the northern hemisphere on Haleakala, Maui Hawai’i,

  the PanSTARRS is a 1.8-m wide-field telescope, which can observe the entire available sky

  several times each month. Built by the US Department of Energy for the southern hemi-

  sphere (site of Cerro Pachón, Chile), the 2.8-ton camera of the 8-m LSST blows the mind.

  Providing a 3.5-degree field of view with 189 sixteen megapixel CCDs, a single image has

  a total of 32 gigapixels (Fig. 3.13). The LSST science plan is to image most of the sky

  visible from the southern hemisphere through several filters a few times a month.

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  4

  Portraying “Nebulae” for the Mind

  Often the most effective way to describe, explore and summarize a set of

  numbers – even a very large set – is to look at pictures of these numbers.

  Edward R. Tufte 1

  Photographs and naked-eye drawings of the Milky Way, however, must

  picture somewhat different portions of the stellar world.

  Cornelis Easton 2

  When one becomes more familiar with the ordering of the hundred-odd

  chemical substances wit
hin the table, the symmetries seem so obvious,

  the sequences so natural, that most people find hard to imagine a time

  when this object did not exist . . .

  Michael D. Gordin 3

  What is the Role of Abstract, or Representational, Images in Unveiling

  the Underlying Physics of “Nebulae”?

  Ebenezer Porter Mason (1819–1840) was a young American astronomer who died from

  tuberculosis when only 21 years old. He is little known. John Herschel admired Mason’s

  accurate and methodical work on “nebulae.” He wrote, “Mr. Mason, a young and ardent

  astronomer, a native of the United States of America, whose premature death is the more to

  be regretted, as he was (as far as I am aware) the only other recent observer who has given

  himself, with the assiduity which the subject requires, the exact delineations of nebulae,

  and whose figures I find at all satisfactory.”4

  Mason was working with college friends to learn more about “nebulae.” To conduct

  their pioneering work, Mason and Yale College friends built a 30-cm reflector, at the time

  the largest telescope in the Americas. Mason’s goal was to advance the study of “nebulae”

  1 E. T. Tufte, The Visual Display of Quantitative Information, Cheshire: Graphic Press, 1983, p. 9.

  2 C. Easton, A Photographic Chart of the Milky Way and the Spiral Theory of the Galactic System, The Astrophysical Journal, 1913, Vol. 37, p. 105.

  3 M. D. Gordin, A Well-Ordered Thing, Dmitrii Mendeleev and the Shadow of the Periodic Table, New York: Basic Books, 2004, p. xvii.

  4 J. Herschel, Results of Astronomical Observations Made During the Years 1834, 5, 6, 7, 8 at the Cape of Good Hope, London: Smith, Elder & Co., p. 1847.

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  4. Portraying “Nebulae” for the Mind

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  beyond just description and cataloguing. He was involved in geodesic work, participating

  in the field survey of the Maine–Canadian border. As we will see later, his knowledge of

  topography may have influenced his approach to astronomical work, and nebular observing

  in particular. He applied the technique of contour lines to his study of “nebulae.” It was a first

  step in representational imaging to study “nebulae” which would become a most powerful

  tool of twentieth-century astrophysics.

  Representing the Immense with Synoptic Imaging

  The celestial vault is immense and unfathomable. As observing tools, telescopes, photo-

  graphic plates and camera systems improved and became more powerful, the number of

  astronomical objects recorded increased phenomenally. Let us recall the Carte du ciel and

  Astrographic Catalogue project. Already in 1918, Heber Curtis had completed a rapid pho-

  tographic survey of the sky with the 36-inch Crossley telescope at the Lick Observatory. He

  had inferred the number of spiral galaxies in the observable volume of the Crossley to be

  at least one million.5 Beyond the sheer number, astronomical photography revealed a stag-

  gering amount of details and features. Astronomers also found many new forms of sidereal

  objects: asteroids in huge numbers, patches of darkness or “empty regions,” groups and

  clusters of objects, nebulosities of many kinds and shapes and galaxies in ever increasing

  numbers.

  It is no surprise that researchers felt the need to integrate the vast amount of informa-

  tion in a more synthetic form, which could be visualized differently. Synoptic charts and

  maps came to the rescue; they were effective means to summarize, average and distil large

  amounts of information. As implied by Edward Tufte’s quote, new images were created,

  but they were images of the mind and for the mind, not of a natural phenomenon.6

  Synoptic charts and maps are called non-homomorphic representations, as opposed to

  direct, homomorphic, images that the eye sees.7 Geographical maps are the simplest and

  most familiar expressions of non-homomorphic representations; they may show contours of

  equal heights to emphasize and quantify the geographical relief of the Earth’s continents or

  seafloors. Meteorologists make extensive use of synoptic maps of temperature and pressure

  distributions across land and sea for weather forecasts. There are also maps of hours of

  sunshine, height of snow or rainfall, foliage coverage and much else that integrate data

  over long periods of time.

  Non-homomorphic representations are visual forms, visual “languages,” that integrate

  or summarize large quantities of data, which can also be handled in tabular form. Images

  of transformed data are created to impress on the mind and to convey quantified informa-

  tion in a single synthetic view. Dmitrii Mendeleev’s periodic table of chemical elements

  may be considered as one of the finest and most powerful non-homomorphic scientific

  5 H. D. Curtis, The Number of the Spiral Nebulae, Publications of the Astronomical Society of the Pacific, 1918, Vol. 30, 159–161.

  6 E. T. Tufte, op. cit., 1983.

  7 P. Galison, Image & Logic, A Material Culture of Microphysics, Chicago: University of Chicago Press, 1997, p. 19.

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  Fig. 4.1 Synoptic chart of the solar magnetic field assembled from individual magnetograms covering one full solar rotation in April 2013. Light shading shows the positive magnetic regions, and dark shading the negative regions. Credit: National Solar Observatory Integrated Synoptic Program.

  4. Portraying “Nebulae” for the Mind

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  representations of modern chemistry.8 Maps of compiled data are now extensively used

  and produced; they serve many practical purposes. In the social or economic sciences, non-

  homomorphic representations help visualize vast sums of data: demographic indicators of

  population, natural resources, or the spread of endemic diseases, etc.

  Because of their efficient summarizing power, non-homomorphic representations have

  been used extensively in the physical and natural sciences to synthesize or illustrate complex

  sets of data. They average several features in order for the intellect to process large quan-

  tities of information and to comprehend reality at a higher level. The photograph might

  raise an unneeded barrier for the neophyte. Geological maps and geological sections are

  highly complex, abstract and formalized kinds of representations.9 Photographs of the ter-

  rain would not necessarily or easily reveal the richness and complexity of the geomorpho-

  logical or stratigraphic landscape. The maps and charts are used to highlight features and

  guide the researcher or the student through the complex natural scenery.10,11 Remarkable

  insights into geohistory have emerged by using higher-level representations.

  Mapping the Heavens by Counting

  In astronomy, the role of synoptic images has been to bring together or to average quantities

  of data or to provide a unifying picture of the systems or phenomena observed. For example,

  synoptic maps of the solar surface magnetic fields have been very useful for visualizing the

  large-scale magnetic properties of the Sun, for revealing the reversal of the overall magnetic

  field polarity every 11 years, and for making sense of the full solar activity 22-year cycle

  (Fig. 4.1). On a larger scale, objects external to the Milky Way appear much
more numerous

  at high galactic latitudes. Edwin Hubble produced a “zone of avoidance” map to illustrate

  the obscuring material in the main plane of our Milky Way (Fig. 4.2; see Plate 6.1).12 This

  obscuration limits our viewing ability in several directions as dust clouds hide a significant

  part of the universe at distances greater than a few hundred parsecs.

  An early example of a synoptic representation in astronomy was the 1785 outline of the

  Milky Way star system by William Herschel (Fig. 4.3). His schematic model illustrated his

  ambitious approach of “constructing the heavens.” Herschel observed countless stars, at

  whatever the direction he pointed his telescope. Being an astute observer, he noticed dif-

  ferences in the distribution of stars across the sky, finding variations subtler than the naked

  eye could perceive. Consequently, Herschel set himself the staggering task of reproducing

  the full three-dimensional distribution of the stars as he could see them with the aid of his

  telescopes. He decided to count stars in different directions, to make a census of all the stars

  in each given beam.

  8 M. D. Gordin, A Well-Ordered Thing, Dmitrii Mendeleev and the Shadow of the Periodic Table, New York: Basic Books, 2004.

  9 M. J. S. Rudwick, The Emergence of a Visual Language for Geological Science 1740–1840, History of Science, 1976, Vol. XIV, p. 159.

  10 T. Sharpe, The Birth of the Geological Map, Science, 2015, Vol. 347, pp. 230–232.

  11 M. J. S. Rudwick, Earth’s Deep History: How It Was Discovered and Why It Matters, Chicago: University of Chicago Press, 2014, p. 140–142.

  12 E. P. Hubble, The Distribution of Extra-Galactic Nebulae, The Astrophysical Journal, 1934, Vol. 79, pp. 8–76.

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  Part I – Images and the Cosmos

  Fig. 4.2 Dust in the plane of the Milky Way absorbs stellar light and blocks our view to distant

  portions of the universe. Very few galaxies can be seen in that part of the sky, hence the appellation

  of “zone of avoidance.” The zone of avoidance is sketched against the distribution of galaxies across

  the sky. From Hubble (1934), The Astrophysical Journal. C

  AAS. Reproduced with permission.