The electromagnetic spectrum consists of far more than just visible light and radio waves. Optical and radio astronomy are the oldest and best-established branches of astronomy, but since the 1970s there has been a virtual explosion of observational techniques covering the rest of the electromagnetic spectrum. Today, all portions of the spectrum are studied, from radio waves to gamma rays, to maximize the amount of information available about astronomical objects. As noted earlier, the types of astronomical objects that can be observed differ quite markedly from one wavelength range to another. Full-spectrum coverage is essential not only to see things more clearly, but even to see some things at all.
Because of the transmission characteristics of Earth's atmosphere, astronomers must study most wavelengths other than optical and radio from space. The rise of these "other astronomies" has therefore been closely tied to the development of the space program.
Infrared studies are a very important component of modern observational astronomy. Generally, infrared telescopes resemble optical telescopes (indeed, many optical telescopes are also used for infrared work), but their detectors are sensitive to longer-wavelength radiation. Although most infrared radiation is absorbed by the atmosphere (primarily by water vapor), there are a few windows in the high-frequency part of the infrared spectrum where the opacity is low enough to allow ground-based observations (see Figure 3.11). 
(Sec. 3.3) Some of the most useful infrared observing is done from the ground, even though the radiation is somewhat diminished in intensity by our atmosphere. Because of its 4-km altitude, Mauna Kea (Figure 5.13a) is one of the finest locations on Earth for both optical and infrared ground-based astronomy. The thin air at this high altitude guarantees less atmospheric absorption of incoming radiation and hence a clearer view than is possible from sea level.
As with radio observations, the longer wavelength of infrared radiation often enables us to perceive objects partially hidden from optical view. As a terrestrial example of the penetrating properties of infrared radiation, Figure 5.26 shows a dusty and hazy region in California, hardly viewable optically, but easily seen using infrared radiation.
Figure 5.26 An optical photograph (a) taken near San Jose, California, and an infrared photo (b) of the same area taken at the same time. Longer-wavelength infrared radiation can penetrate smog much better than short-wavelength visible light.
Astronomers can make still better infrared observations if they can place their instruments above most or all of Earth's atmosphere. Improvements in balloon-, aircraft-, rocket-, and satellite-based telescope technologies have made infrared research a very powerful tool with which to study the universe (see Figure 5.27). However, as might be expected, the infrared telescopes that can be carried above the atmosphere are considerably smaller than the massive instruments found in ground-based observatories. The most advanced facility to function in this part of the spectrum is the Infrared Astronomy Satellite (IRAS), shown in Figure 5.27(b). Launched into Earth orbit in 1983 but now inoperative, this British—Dutch—U.S. satellite housed a 0.6-m mirror with an angular resolution as fine as 30". (As usual, the resolution depended on the precise wavelength observed.) Its sensitivity was greatest for radiation in the 10- to 100- µm range. During its 10-month lifetime (and long afterward—the data archives are still heavily used even today), IRAS contributed greatly to our knowledge of clouds of Galactic matter destined to become stars, and possibly planets. These regions of interstellar gas are composed of warm gas that cannot be seen with optical telescopes or adequately studied with radio telescopes. Because much of the material between the stars has a temperature between a few tens and a few hundreds of kelvins, Wien's law tells us that the infrared domain is the natural portion of the electromagnetic spectrum in which to study it. (Sec. 3.4) Throughout the text we will encounter many findings made by this satellite about comets, stars, galaxies, and the scattered dust and rocky debris found among the stars.
Figure 5.27 (a) A gondola containing a 1-m infrared telescope (lower left) is readied for its balloon-borne ascent to an altitude of about 30 km (100,000 feet), where it will capture infrared radiation that cannot penetrate the atmosphere. (b) An artist's conception of the Infrared Astronomy Satellite, placed in orbit in 1983. This 0.6-m telescope surveyed the infrared sky at wavelengths ranging from 10 to 100 µm. During its 10 months of operation it greatly increased astronomers' understanding of many different aspects of the universe, from the formation of stars and planets to the evolution of galaxies.
Figure 5.28(a) shows an IRAS image of the Orion region. At about 1' angular resolution, the fine details of the Orion nebula evident in the visible portion of the earlier image (Figure 5.23) cannot be perceived. Nonetheless, astronomers can extract much useful information about this object and others like it from infrared observations. Star-forming clouds of warm dust and gas and extensive groups of bright young stars, completely obscured at visible wavelengths, are seen.
Figure 5.28 (a) This infrared image of the Orion Nebula and its surrounding environment was made by the Infrared Astronomy Satellite. The whiter regions denote greater strength of infrared radiation; the false colors denote different temperatures, descending from white to red to black. (b) The same region photographed in visible light. The labels 
and 
refer, respectively, to Betelgeuse and Rigel, the two brightest stars in the constellation. Note how the red star Betelgeuse can be seen in the infrared (part a), but the blue star Rigel cannot.
Unfortunately, by Wien's law, telescopes themselves also radiate strongly in the infrared unless they are cooled to nearly absolute zero. The end of IRAS's mission came not because of any equipment malfunction or unexpected mishap but simply because its supply of liquid helium coolant ran out. IRAS's own thermal emission then overwhelmed the radiation it was built to detect. In November 1995 the European Space Agency launched the 0.6-m Infrared Space Observatory (ISO). It is now in orbit, refining and extending the groundbreaking work begun by IRAS. NASA plans to deploy the 0.85-m Space Infrared Telescope Facility (SIRTF) in 2002. Both instruments are designed to operate at wavelengths in the 3- to 200- µm range.
To the short-wavelength side of the visible spectrum lies the ultraviolet domain. This region of the spectrum, extending in wavelength from 400 nm (4000 Å, blue light) down to a few nanometers ("soft" X-rays), has only recently begun to be explored. Because Earth's atmosphere is partially opaque to radiation below 400 nm and is totally opaque below about 300 nm (in part because of the ozone layer), astronomers cannot conduct any useful ultraviolet observations from the ground, not even from the highest mountaintop. Rockets, balloons, or satellites are therefore essential to any ultraviolet telescope—device designed to capture and analyze this high-frequency radiation.
One of the most successful ultraviolet space missions was the International Ultraviolet Explorer (IUE). Placed in Earth orbit in 1978 and shut down for budgetary reasons in late 1996. (see Interlude 18-1). Like all ultraviolet telescopes, its basic appearance and construction are quite similar to optical and infrared devices. Several hundred astronomers from all over the world have used IUE to explore a variety of phenomena in planets, stars, and galaxies. In subsequent chapters we will learn what this relatively new window on the universe has shown us about the activity and even the violence that seems to pervade the cosmos. The Hubble Space Telescope (Interlude 5-1), best known as an optical telescope, is also a superb ultraviolet instrument.
Figure 5.29(a) shows an image of a supernova remnant—the remains of a violent stellar explosion that occurred some 12,000 years ago—obtained by the Extreme Ultraviolet Explorer (EUVE) satellite, which was launched in 1992. EUVE operates at the short-wavelength (1—50 nm) end of the ultraviolet range, making it sensitive to phenomena involving high temperatures (hundreds of thousands, even millions, of kelvins) or other energetic events. Since its launch EUVE has mapped out our local cosmic neighborhood as it presents itself in the far ultraviolet and has radically changed astronomers' conception of interstellar space in the vicinity of the Sun.
Figure 5.29 (a) A camera on board the Extreme Ultraviolet Explorer satellite captured this image of the Vela supernova remnant, the result of a massive star blowing itself virtually to smithereens. The release of energy is prodigious, and the afterglow lingers for millennia. The glowing field of debris shown here lies some 1500 light years from Earth. Based on the velocity of the outflowing debris, astronomers estimate that the explosion itself must have occurred about 12,000 years ago. For scale, the inset shows the angular size of the Moon. (b) This false-color image of the spiral galaxy M74 was made by an ultraviolet telescope aboard the Astro payload carried by a space shuttle in 1990 and 1995.
An alternative means of placing astronomical payloads into (temporary) Earth orbit is provided by NASA's space shuttles. In December 1990 and again in March 1995, a shuttle carried aloft the Astro package of three ultraviolet telescopes. An Astro image of a nearby galaxy is shown in Figure 5.29(b). Astronomical shuttle missions offer a potentially very flexible way for astronomers to get instruments into space, without the long lead times and great expense of permanent satellite missions like HST.
High-energy astronomy studies the universe as it presents itself to us in X-rays and gamma rays—the types of radiation whose photons have the highest frequencies and hence the greatest energies. How do we detect radiation of such short wavelengths? First, it must be captured high above Earth's atmosphere because none of it reaches the ground. Second, its detection requires the use of equipment basically different in design from that used to capture the relatively low energy radiation discussed up to this point. The basic difference in the design of high-energy telescopes comes about because X and gamma rays cannot be reflected easily by any kind of surface. Rather, these rays tend to pass straight through, or be absorbed by, any material they strike. When X-rays barely graze a surface, however, they can be reflected from it in a way that yields an image, although the mirror design is fairly complex (see Figure 5.30). For gamma rays (with wavelengths of less than about 0.01 nm), no such method of producing an image has yet been devised. Present-day gamma-ray telescopes simply point in a specified direction and count photons received.
Figure 5.30 The arrangement of mirrors in an X-ray telescope allows X-rays to be reflected at grazing angles and focused to form an image.
In addition, X-ray and gamma-ray detection methods using photographic plates or CCD devices do not work well. Instead, individual X-ray and gamma-ray photons are counted by electronic detectors on board an orbiting device, and the results are then transmitted to the ground for further processing and analysis. Furthermore, the number of photons in the universe seems to be inversely related to frequency. Trillions of visible (starlight) photons reach the detector of an optical telescope on Earth each second, but hours or even days are often needed for a single gamma-ray photon to be recorded. Not only are these photons hard to focus and measure, they are also very scarce.
Figure 5.31(a) is a photograph of the second High-Energy Astronomy Observatory (HEAO-2, also known as the Einstein Observatory). Launched in 1978, this was the first X-ray telescope capable of forming an image of its field of view. During its 2-year lifetime, this spacecraft made major advances in our understanding of high-energy phenomena throughout the universe; its observational database is still heavily used. The most recent major X-ray satellite is the German ROSAT (short for Röntgen Satellite, after Wilhelm R öntgen, the discoverer of X-rays), launched in 1991 by a European Ariane rocket. With more sensitivity, a wider field of view, and better resolution than Einstein, ROSAT is providing high-energy astronomers with new levels of observational detail (Figure 5.32). Even more powerful will be NASA's Advanced X-ray Astrophysics Facility (AXAF), another long-duration orbiting observatory (in the spirit of IUE and HST) that is scheduled to become operational in 1999 (see Figure 5.31b).
Figure 5.31 (a) HEAO-2, also known as the Einstein Observatory, the first imaging X-ray telescope. (b) AXAF, the Advanced X-ray Astrophysics Facility,is a much larger device, shown here being assembled in 1998.
Gamma-ray astronomy is the youngest entrant into the observational arena. As just mentioned, imaging gamma-ray telescopes do not exist, so only fairly coarse (1° resolution) observations can be made. Nevertheless, even at this resolution, there is much to be learned. Cosmic gamma rays were originally detected in the 1960s by the U.S. Vela series of satellites, whose primary mission was to monitor illegal nuclear detonations on Earth. Since then, several X-ray telescopes have also been equipped with gamma-ray detectors. By far the most advanced instrument is the Gamma Ray Observatory (GRO), launched by space shuttle in 1991. This satellite can scan the sky and study individual objects in much greater detail than previously attempted. Figure 5.33 shows GRO in low Earth orbit, along with a false-color gamma-ray image of a highly energetic outburst in the nucleus of a distant galaxy.
Figure 5.32 An X-ray image of the Orion region, taken by the ROSAT X-ray satellite. (Compare with Figures 1.6, 5.23, and 5.28.) Objects that are most intensely emitting X-rays are colored blue, including the three stars of Orion's belt and the glowing nebula below them at bottom center in the photograph.
Figure 5.33 (a) This photograph of the 17-ton Gamma-Ray Observatory (also called the Compton Observatory, after an American gamma-ray pioneer) was taken by an astronaut during the satellite's deployment from the space shuttle Atlantisover the Pacific Coast of the United States. (b) A typical false-color gamma-ray image—this one showing a violent event in the distant galaxy 3C279, also known as a "gamma-ray blazar."
