Infrared Astronomy

Infrared Astronomy, the detection and study of infrared radiation emanating from celestial objects. Infrared (IR) radiation has wavelengths longer than those of visible light, but shorter than those of radio waves, and is customarily considered to cover the wavelength range from 1 micron to 1 mm. Because large parts of the IR spectrum are blocked by the Earth’s atmosphere (principally by water vapour), observations are mainly conducted at high-altitude observatories (for example, Mauna Kea, or NASA’s Kuiper Airborne Observatory, which operates an IR telescope on a specially modified aircraft) or from orbiting satellites.

Sensitive astronomical IR detectors—generally, charged-couple devices attached to a reflecting telescope similar to that used for visible-wavelength astronomy—must be cooled to very low temperatures (using, for example, liquid helium, which maintains a temperature of between about 2 and 4 Kelvin, or -272.2° and -268.9° C, that is, very close to absolute zero) because all warm bodies emit IR radiation. The telescope itself and the atmosphere are at ambient temperatures and radiate strongly at IR wavelengths; in fact IR astronomy has been likened to trying to conduct optical astronomical observations in broad daylight.

While the atmospheres of the planets in our solar system reflect sunlight, they are also at low temperatures, which means they emit predominantly in the IR (as does the Earth). Hence, it is IR observations of the planets that have revealed much about their atmospheric composition, and which followed in detail the collisions of the remnants of Comet Shoemaker-Levy 9 with Jupiter in 1994. IR astronomy has also provided information concerning the temperatures of stars and the distribution of thermal energy in both galactic and intergalactic dust clouds. IR radiation is far less attenuated by interstellar matter—gas and dust clouds—than visual light, and so IR observations have revealed hidden aspects of the structure of the Milky Way, of its centre, and, in particular, of regions of star formation such as the Orion Nebula. The NICMOS near-IR camera was installed on the Hubble Space Telescope in 1997 and has enabled high-resolution IR images to be obtained of the centre of the Orion Molecular Cloud (OMC-1), which houses a massive young star that is completely hidden from view at visual wavelengths. NICMOS employed a solid nitrogen coolant (liquid nitrogen subsequently cooled to 40 K), which lasted for almost two years before its exhaustion terminated the operation of the instrument. In March 2002, during the fourth servicing mission of the Hubble Space Telescope, space shuttle astronauts installed a mechanical cryo-cooler (basically, a sophisticated refrigerator) in NICMOS, which restored the instrument to near-normal operation, and ensured that its future lifetime would not be limited by the coolant.

For many years the Earth’s atmosphere kept astronomers from exploring the full IR sky. Observations had to be made by means of brief rocket and balloon flights. The situation changed in 1983 with the year-long observations of the United States-Dutch-British Infrared Astronomical Satellite (IRAS); this satellite’s observations of the far-IR spectrum (close to radio wavelengths) have been used to create an IR map of almost the entire sky. This was followed up by the more powerful Infrared Space Observatory (ISO), which was launched by the European Space Agency in November 1995, and whose helium coolant allowed it to operate for more than two years. One of the key IRAS discoveries was of ultra-luminous IR galaxies that show evidence of mergers or interactions with other galaxies, thereby triggering intense bursts of star formation. Some of these galaxies emit 99 per cent of their radiation in the IR. The launch by NASA in 2003 of the Spitzer Space Telescope (originally known as the Space IR Telescope Facility/SIRTF) represents the last in its Great Observatories Program. It is the most sophisticated and powerful IR telescope yet put into space, and has sufficient coolant to operate for five or more years. Nevertheless, its primary mirror, at 0.85 m (33 in) diameter, is only slightly larger than IRAS and ISO. The next great advances in space-based IR astronomy are expected to be the European Space Agency’s Herschel Space Observatory (due for launch in 2007 to study longer wavelengths, known as submillimetre waves) and the 6.5-m (256-in) Next Generation Space Telescope (due for launch in 2008).

For even longer IR (and sub-millimetre) wavelengths the US Cosmic Background Explorer (COBE) was launched in 1989 to map the diffuse microwave background radiation that was left over from the Big Bang. This extremely cool (only 2.7 K) blackbody radiation was found to be not completely uniform in all directions but to exhibit very low amplitude fluctuations. These are believed to represent the forerunners to the largest structures (superclusters of galaxies) currently observed in the Universe.

The development of large-format arrays of detectors that are sensitive to the near IR has enabled true IR images to be made of regions within our own galaxy. IR observations over several years (exploiting the outstanding atmospheric seeing conditions on Mauna Kea in Hawaii and at La Silla in Chile) of stars in the centre of our galaxy have revealed that they are moving rapidly around a massive unseen object of approximately 3 million solar masses. This represents the best evidence yet that our galaxy harbours a massive black hole at its centre. IR surveys of hundreds of the faintest, most distant galaxies yet discovered have led to new theories of galactic evolution. The new IR array detectors are being used with instruments such as the 3.8-m (152-in) United Kingdom Infrared Telescope (UKIRT) at Mauna, the largest telescope dedicated to IR astronomy alone. In 1999 the Japanese Subaru telescope was inaugurated at the same observatory, with a primary mirror diameter of 8.3 m (327 in) and instrumentation optimized for infrared observations.

More recently, the US, UK, Canada, and a consortium of South American countries have completed the construction of a pair of identical 8-m (312-in) telescopes, the Gemini Observatory, one located in Hawaii (Mauna Kea), the other in Chile (Cerro Pachon). These single-mirror telescopes were polished to a very high specification so as to exploit the fact that the atmospheric seeing (turbulence) is better in the IR than in the optical range. The seeing effects can also now be compensated for using adaptive optics, in which a bright star in the target field is monitored with very high time resolution detectors so as to correct the signal for the rest of the field. In this way it is possible to reach the diffraction limit for the 8-m mirror, and thereby surpass even the spatial resolution of the Hubble Space Telescope. However, this can only be done when appropriate guide stars are available, and this greatly limits the applicability of this technique. Current research is under way to use powerful lasers to generate “artificial” guide stars in the sky by, for example, exciting sodium atoms in the upper atmosphere to radiate and thereby provide a reference signal.

See Astronomy; Cosmology; Heat.