| radio astronomy |
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The exploration of the universe through the detection of radio emission from celestial objects. The principal sources of cosmic radio emission are: the Sun, Jupiter, interstellar hydrogen and ionized gas, pulsars, quasars, and the cosmic background radiation of the universe itself. The frequencies used span a vast range, from 10 MHz to 300 GHz. There are several wavebands protected internationally against interference, such as 1,421 MHz (wavelength 21 centimetres), the natural frequency of atomic hydrogen. Radio telescopes are largely either single steerable dishes, up to 100 metres in diameter, or arrays of dishes linked to form radio interferometers. Single telescopes have poor angular resolution compared with optical telescopes, so they are mainly used in investigations where positional accuracy is not vital, such as the timing of pulsar signals or the mapping of large-scale distributions, such as the microwave background. Where structural detail is required, for example in the mapping of radio galaxies, it is essential to use an interferometer. Since its inception in the 1940s, radio astronomy has been directly responsible for the discovery of pulsars, quasars and the microwave background. See also: radio galaxy. |