The process by which the energy associated with electromagnetic radiation is transferred as the radiation interacts with matter. In radiative transfer, photons are continually being absorbed and re-emitted by matter. In simple terms, the law of conservation of energy requires that the energy emerging from a medium is equal to the amount entering it, plus any energy emitted by the medium itself but minus any energy absorbed. This can be expressed mathematically, but real solutions are difficult because of the complexity of the way in which matter and radiation interact in practice.

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.

The distribution across the sky of the radio emission from an extended radio source, expressed as radio flux density per unit solid angle as a function of position. It can be displayed as a contour map, or computer processed to produce an image similar to an optical photograph.

A galaxy that is an intense source of radio emission. About one galaxy in a million is a radio galaxy. The radio emission is synchrotron radiation from electrons travelling at speeds close to that of light. In Cygnus A, often regarded as the prototype radio galaxy, two huge clouds of radio emission, disposed symmetrically on each side of a disturbed elliptical galaxy, span more than three million light years. It seems unlikely that the huge energy output can be generated by normal nuclear reactions in stars. Mechanisms in which black holes act as a kind of central engine have been suggested. Radio galaxies are closely related to quasars, many of which have similar radio properties.

A radio telescope in which two or more separate antennas observe the same object simultaneously. The signals received by a pair of antennas are fed into a receiver that multiplies the voltages. The amplitude and phase of the correlated output depend on the distribution of radio emission from the source being studied. Little can be learned from a single measurement of this kind but, if the spacing and orientation of the interferometer are changed, the correlated voltage varies and can be analysed by computer to generate maps showing the distribution of radio brightness on the sky. This technique is fully exploited in Earth rotation synthesis. See also: aperture synthesis, very-long-baseline interferometry.

Any natural source of cosmic radio emission. In cosmology, it has a more restricted meaning and refers only to radio galaxies and quasars. See also: radio astronomy, source counts.

An expression used in the early years of radio astronomy when the resolution of observations was too poor to enable astronomers to match radio sources to visible objects. It was assumed, quite wrongly, that many of these sources were stars; subsequently many were shown to be radio galaxies. True radio stars are very rare.

An instrument for the collection, detection and analysis of radio waves from any cosmic source. All such telescopes consist of a radio antenna feeding an amplifier and a detector. The large range of frequencies covered by radio astronomy means that radio telescopes vary greatly because different techniques are used for different parts of the spectrum. A fundamental problem in radio astronomy is obtaining adequate angular resolution. A telescope with a diameter of 100 wavelengths has a resolving power of only 1°. To reach a resolution of half an arc second, comparable to that of a good optical telescope, a diameter of 50,000 wavelengths constructed to an accuracy of a tenth of a wavelength is required. At a wavelength of 21 centimetres, the diameter of dish needed is 100 kilometres! Single steerable dishes are used mainly for studies of interstellar matter, through the twenty-one centimetre line, and variable sources, such as pulsars. Fully steerable dishes are limited to apertures of about 100 metres by the weight of the structure. The higher angular resolution needed to map structure in objects such as radio galaxies and quasars is obtained by linking arrays or networks of telescopes to form a radio interferometer. See also: aperture synthesis, radio astronomy, very-long-baseline interferometry.

In radio astronomy, a map of the distribution of radio emission processed to make an image similar in appearance to an optical photograph.

A radio telescope designed for mapping the distribution of radio emission from the Sun.

Any instrument for measuring the total amount of electromagnetic radiation received from an object. In infrared astronomy, the term is applied to a device designed to measure only infrared flux. In radio astronomy, a radiometer is a detector able to measure with great accuracy the total radio energy received.

The line joining an orbiting body to its centre of motion at any instant, directed radially outwards. For a circular orbit, the centre of motion coincides with the centre of the circle; for a parabolic or hyperbolic orbit, the centre of motion is the focus, and for an elliptical orbit it is one of the two foci.

English name for the constellation Aries.

The same as exit pupil.

A telescope eyepiece consisting of two similar plano-convex lenses, convex sides facing and separated by a distance equal to two-thirds the sum of their focal lengths. This simple design suffers from chromatic aberration and, for astronomy, the Kellner eyepiece is preferred.