682 IMAGING SCIENCE IN ASTRONOMY 65 (a) E. V. Sayre and H. N. Lechtman, Stud. Conserv. 13, force for innovation and discovery in astronomical 161–185 (1968); (b) M. W. Ainsworth et al., Art and Autora- imaging. diography: Insights into the Genesis of Paintings by Rem- Classical astronomy — for example, the search for new brandt, Van Dyck, and Vermeer, The Metropolitan Museum solar system objects and the classification of stars — is of Art, NY, 1982; (c) C. O. Fischer et al., Nucl. Instrum. Meth- still largely conducted in the optical wavelength regime ods A 424, 258–262 (1999); (d) Images and supporting text (400–700 nm). This has been the case, of course, since provided by Ward Laboratory, Cornell University, Ithaca, humans first imagined the constellations, noted the NY 14853-7701. appearance of ‘‘wandering stars’’ (planets), and recorded 66. D. L. Glackin and E. P. 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Asmus, Byte Magazine March (1987). For most of us, our eyes provide our first, fundamental 80. P. Clogg, M. Diaz-Andreu, and B. Larkman, J. Archaeological contact with the universe. It is interesting to ponder Sci. 27, 837 (2000). how humans would conceive of the universe if we had 81. P. Clogg and C. Caple, Imaging the Past, British Museum nothing more in the way of imaging apparatus at our Occasional Paper, London, 1996, p. 114. disposal, as was the case for astronomers before Galileo. In contrast to the complex cosmologies currently pondered in modern physics, most of which involve an expanding universe shadowed by the afterglow of the Big Bang, the ‘‘first contact’’ provided by our eyes produces a model of IMAGING SCIENCE IN ASTRONOMY the universe that is entirely limited to the Sun, Moon, and planets, the nearby stars, and the faint glow of JOEL H. KASTNER the collective background of stars in our own Milky Rochester Institute of Technology Way galaxy and a handful of other, nearby galaxies. Rochester, NY From this simple thought experiment, it is clear that the bulk of the visible radiation arriving at Earth is emitted by stars. INTRODUCTION But the apparent predominance of visible light from the Sun and nearby stars is in fact merely an accident The vast majority of information about the universe is of our particular position in the universe, combined collected via electromagnetic radiation. This radiation is with the evolutionary adaptation that gave our eyes emitted by matter distributed across tremendous ranges maximal sensitivity at wavelengths of electromagnetic in temperature, density, and chemical composition. Thus, radiation that are near the maximum of the Sun’senergy more than any other science, astronomy depends on output. The Sun provides by far the majority of the innovative methods to extend image taking to new, visible radiation arriving at Earth strictly by virtue unexplored regions of the electromagnetic spectrum. To of its proximity. The brightest star in the night sky, bring sufficient breadth and depth to their studies, Sirius (in the constellation Canis Major), actually has astronomers also require imaging capability across a vast an intrinsic luminosity about 50 times larger than that of range in spatial resolution and sensitivity, with emphasis the Sun, but is about 8.6 light years distant (a light year on achieving the highest possible resolution and signal is the distance traveled by light in one year, 9 × 1012 km; gain in a given wavelength regime. This simultaneous the Sun is about 8 light minutes from Earth). In turn, quest for better wavelength coverage and ever higher Sirius is only about one-ten-thousandth as luminous as spatial resolution and sensitivity represents the driving the star Rigel (in the neighboring constellation Orion), IMAGING SCIENCE IN ASTRONOMY 683 but Sirius appears several times brighter than Rigel because it is about 50 times closer to us. Like the Sun, which has a surface temperature of about 6,000 K, most of the brightest stars have surfaces within the range of temperatures across which hot objects radiate very efficiently (if not predominantly) in the visible region. Representative stellar surface temperatures are 3,000 K for reddish Betelgeuse, a red supergiant in Orion; 10,000 K for Sirius; and 15,000 K for the blue supergiant Rigel (Fig. 1). Thermal Continuum Emission The tendency of objects at the temperatures of the Sun and stars to emit in the visible can be understood to first order via Planck’s Law, which describes the wavelength dependence of radiation emitted by a perfect blackbody. The peak of the Planck function lies within the visible regime for an object at a temperature of 6,000 K. This same fundamental physical principle tells us that objects much hotter or cooler than the Sun should radiate predominantly at wavelengths much shorter or longer than visible, respectively. Indeed, for a perfect blackbody, the peak wavelength of radiation is given by Wien’s displacement law (2), 0.51 λ(cm) ∼ .(1) T(K) This relationship between the temperatures of objects and the wavelengths of their emergent radiation allows us to understand why Betelgeuse appears reddish and Rigel appears blue (Fig. 2). 106 Figure 2. Wide-field photograph of Orion, illustrating the difference in color between the relatively cool star Betelgeuse 5 (upper left) and the hot star Rigel (lower right). The large, red 10 Rigel Betetgeuse Deneb object at the lower center of the image, just below Orion’sbelt, 104 is the Orion Nebula (see Fig. 7). (Photo credit: Till Credner, Main sequence Superglants AlltheSky.com) See color insert. 1) Spica = 103 Red glants Capella Aldebaran 102 Vega The same, simple relationship also provides powerful Arcturus insight into astrophysical processes that occur across a Sirius A 10 Pollux very wide range of energy regimes (Fig. 3). The lowest Altair Procyon A energies and hence longest (radio) wavelengths reveal 1 Sun ‘‘cold’’ phenomena, such as emission from dust and gas in optically opaque clouds distributed throughout interstellar 10−1 Intrinsic brightness (Sun White dwarts space in our galaxy. At the highest energies and hence shortest wavelengths (characteristic of X rays and γ rays), −2 10 astronomers probe the ‘‘hottest’’ objects, such as the Sirius B − explosions of supermassive stars or the last vestiges 10 3 Procyon B of superheated material that is about to spiral into a 40,000 20,000 10,000 6,000 4,000 3,000 2,000 black hole. Stars' surface temperature (K) Figure 1. The Hertzsprung–Russell diagram. The diagram Nonthermal Continuum Emission shows the main sequence (Sun-like stars that are fusing hydrogen Certain radiative phenomena in astrophysics do not to helium in the cores), red giants, supergiants, and white dwarfs. In addition, the positions of the Sun, the twelve brightest strongly depend on the precise temperature of the material stars visible from the Northern Hemisphere, and the white and are instead sensitive probes of material density and/or dwarf companions of Sirius and Procyon are indicated [Source: chemical composition (3,4). For example, the emission NASA (http://observe.ivv.nasa.gov/nasa/core.shtml.html)]. See from ‘‘jets’’ ejected from supermassive black holes at color insert. the centers of certain galaxies (Fig. 4) is said to be 684 IMAGING SCIENCE IN ASTRONOMY Figure 3. Schematic diagram showing various regimes of the electromagnetic spectrum in terms of temperatures corresponding to emission in that regime. The diagram also illustrates the wavelength ‘‘niches’’ of NASA’s four orbiting ‘‘Great Observatories.’’ [Source: NASA/Chandra X-Ray Center (http://chandra.harvard.edu)]. See color insert. ‘‘nonthermal’’ because its source is high-velocity electrons emission at one of these specific wavelengths1 is both that orbit around magnetic field lines. Other, similar necessary and sufficient to determine the presence of examples are the emission from filaments of ionized gas that element or molecule.