The study of the Universe in the infrared part of the spectrum, at wavelengths of 1–300 μm. Infrared astronomy is hampered by the Earth's atmosphere, which is opaque and bright throughout much of the infrared band due mainly to water vapour and carbon dioxide. Another source of interference is warmth from a telescope's surroundings, including the telescope itself, which peaks around 10 μm. Ground-based infrared astronomy is restricted to the few infrared windows in the Earth's atmosphere, especially in the near-infrared 1–5 μm region, and around 10 μm. Even then, infrared telescopes are placed on high, dry mountain tops. High-altitude balloons and aircraft have also been used, notably the Kuiper Airborne Observatory (KAO). But unimpeded viewing of the infrared sky requires telescopes in space, such as the Infrared Astronomical Satellite (IRAS), the Infrared Space Observatory (ISO), the Spitzer Space Telescope, and the Herschel Space Observatory.
Prominent infrared sources include red giants and supergiants with dust shells, H II regions, the galactic centre, star-forming regions, and active galaxies. Many active galaxies emit the bulk of their energy in the infrared, and the infrared Iuminosity of spiral galaxies has become a key element in the Tully–Fisher relation method of measuring extragalactic distances. Infrared waves can readily penetrate interstellar dust, and infrared astronomy has played an important role in the study of obscured regions such as the galactic disk and dark nebulae. Spectroscopy at infrared wavelengths is an important source of information about interstellar molecules.
The type of detector used depends on the wavelength to be detected. At near-infrared wavelengths, photovoltaic detectors (such as indium antimonide) are common, while at far-infrared wavelengths bolometers are used. Arrays of detectors are used for imaging. Infrared detectors are cooled by liquid helium (to 4 K) or liquid nitrogen (to 77 K) to reduce thermal noise.
Subjects: Astronomy and Astrophysics.