Active Galaxy

An active galactic nucleus (AGN) is a compact region at the centre of a galaxy which has a much higher than normal luminosity over some or all of the electromagnetic spectrum (in the radio, infrared, optical, ultra-violet, X-ray and/or gamma ray wavebands). A galaxy hosting an AGN is called an active galaxy. The radiation from AGN is believed to be a result of accretion on to the supermassive black hole at the centre of the 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 provides constraints on cosmological models.
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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.