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Active galaxy

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Active galactic nucleus
An active galactic nucleus (AGN) is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion – and possibly all – of the electromagnetic spectrum. Such excess emission has been observed in the radiomicrowavesinfraredopticalultra-violetX-ray and gamma ray wavebands. A galaxy hosting an AGN is called an active galaxy. The radiation from an AGN is believed to be a result of accretion of mass by a supermassive black hole at the centre of its host galaxy. AGN are the most luminous persistent sources of electromagnetic radiation in the universe, and as such can be used as a means of discovering distant objects; their evolution as a function of cosmic time also puts constraints on models of the cosmos.

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Active galaxy
A galaxy in which some kind of unusual, perhaps violent, phenomenon is releasing energy. The energy is usually released from the galactic nucleus and is not related to the way in which energy is normally released by stars. Active galaxies comprise ten per cent of the population of galaxies which can be viewed by astronomers from Earth. Many different classifications of active galaxies now exist such as Seyfert galaxies , quasars , radio galaxies and blazars. A unified theory of active galaxies exists which attempts to explain all types of active galaxy as being intrinsically the same kind of object. See also active galactic nucleus.

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