Editing Active galactic nucleus (section)

{{short description|Compact region at a galaxy's center with abnormally high luminosity}}

An '''active galactic nucleus''' ('''AGN''') is a compact region at the center of a [[galaxy]] that emits a significant amount of energy across the [[electromagnetic spectrum]], with characteristics indicating that this luminosity is not produced by the [[star]]s. Such excess, non-stellar emissions have been observed in the [[radio waves|radio]], [[microwave]], [[infrared]], [[visible spectrum|optical]], [[ultra-violet]], [[X-ray]] and [[gamma ray]] wavebands. A galaxy hosting an AGN is called an '''active galaxy'''. The non-stellar radiation from an AGN is theorized to result from the [[accretion (astrophysics)|accretion]] of matter by a [[supermassive black hole]] at the center of its host galaxy.

Active galactic nuclei 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 [[cosmology|models of the cosmos]].

The observed characteristics of an AGN depend on several properties such as the mass of the central black hole, the rate of gas accretion onto the black hole, the orientation of the [[accretion disk]], the degree of [[Extinction (astronomy)|obscuration]] of the nucleus by [[Cosmic dust|dust]], and presence or absence of [[Astrophysical jet|jets]].

Numerous subclasses of AGN have been defined on the basis of their observed characteristics; the most powerful AGN are classified as [[quasar]]s. A [[blazar]] is an AGN with a jet pointed toward the Earth, in which radiation from the jet is enhanced by [[relativistic beaming]].