Infrared (IR) astronomy studies radiation that arrives from celestial objects at infrared
wavelengths, longer than optical
and shorter than radio
wavelengths. Astronomers divide this region of the electromagnetic spectrum into three smaller ones: the near, mid, and far infrared. They correspond, respectively, to wavelength ranges of 1 to 5, 5 to 25, and 25 to 350 microns. (One micron equals 10,000 angstroms, and one angstrom equals 10¦x£ m.) These divisions relate to the varying ability of Earth's atmosphere to transmit IR radiation and to the types of celestial objects that can be observed. The near IR is mainly useful for star observation, the mid IR for warm interstellar dust, and the far IR for cool dust. Using IR detectors, astronomers can observe far cooler celestial objects than they can with optical devices, and IR radiation is less dimmed by interstellar dust than is light.InstrumentsPrior to about 1960, astronomers could only detect and photograph IR radiation in the very near infrared, using conventional photographic plates (see astrophotography) and photoelectric photometers (see photometry). Objects in the near IR became available for study by means of lead sulfide (PbS) photoconductive solid-state detectors, in widespread use for IR by the late 1960s. The devices change their electrical conductivity proportional to the intensity of radiation, producing a measurable change in electric current.To observe longer wavelengths, the detectors used are composed of the semiconductor germanium, doped with other materials (see charge-coupled device). Sensitivity to various wavelength regions depends on the doping material employed. By combining sensors in a rectangular array, IR detectors can provide images with many pixels and hence resolutions comparable to those achieved in optical astronomy, particularly in the near and mid IR ranges. This technology was developed for military systems and only later became more generally available. Prototypes of the near-infrared camera and multi-object spectrograph (NICMOS), planned for installation on the Hubble Space Telescope, have been in use since 1990.ObservatoriesAlthough it is possible to make IR
observations with the detectors attached to conventional reflecting telescopes, IR astronomy has benefited from the construction of large telescopes specifically designed for use at IR wavelengths. An important attribute of these instruments is their ability to alternately measure the IR source and the sky background in rapid succession. Without the subtraction of the background radiation, most such observations would be hopelessly inaccurate.Ground-based astronomers are faced with a serious problem because water and carbon dioxide molecules in the atmosphere strongly absorb IR radiation at many wavelengths. Thus they are able to observe only in the gaps between the molecular absorption bands, known as atmospheric windows. To optimize observing conditions, ground-based IR observatories are constructed at very high and dry locations such as Kitt Peak National Observatory, Arizona; Mauna Kea Observatory, Hawaii; Cerro Tololo Inter-American Observatory, Chile; and Siding Spring Observatory, Australia. Also, the University of Wyoming's Infrared Michelson Array (IRMA) incorporates a pair of movable IR telescopes that can function as an interferometer of varying size. Another approach has been to send aircraft, carrying infrared telescopes, on short observing missions at altitudes of about 12,500 m (41,000 ft), above most of the atmosphere's water vapor.In January 1983 the Dutch-U.S. Infrared Astronomy Satellite (IRAS; see IRAS) entered space, the first satellite built to survey the sky at IR wavelengths. Its main instrument was a 60-cm (24-in) Cassegrain telescope, using detectors operating at wavelengths of 100, 60, 24, and 12 microns. IRAS operated through most of 1983, discovering about 250,000 new IR sources. The IRAS Sky Survey Atlas published in 1992 used improved imagrocessing techniques to show sharp, undistorted IR images. In November 1995 the European Space Agency launched the Infrared Space Observatory (ISO) for a nearly two-year mission. Its camera, spectrometers, and photometer cover infrared wavelengths ranging from 240 to 3 microns. As with IRAS, ISO is maintained at cryogenic temperatures by superfluid helium to keep its own infrared emissions at a minimum.Infrared ObjectsDuring the formative years of IR astronomy, various catalogs and surveys were published of interesting IR objects that merited study. Because a star having the relatively cool surface temperature of 3,000 K will have a peak in its energy distribution at an IR wavelength of about 2 microns, it is not surprising that a large number of the sources in these surveys were cool stars. Many of the remaining objects showed an unexpectedly large amount of IR radiation, commonly referred to as IR excess. In surveying such objects, IRAS found regions where new stars are being formed, both in our own and in neighboring galaxies. It also observed many more IR objects than ground-based astronomers are able to detect, including enormous numbers of very dim, low-mass galaxies and, in our own Galaxy, solid (possibly planetary) objects around nearby stars (see planets and planetary systems).IR observations within the solar system have produced results that include comet and asteroid discoveries. Even prior to the landing of probes on the Moon, IR observations provided data sufficient to determine the existence and properties of lunar dust. Observations of asteroids permitted the accurate determinations of the diameters of these small bodies. IR observations also indicated that Jupiter emits more radiation than it receives from the Sun, suggesting that Jupiter may be undergoing a slow gravitational contraction that releases the observed radiation excess.