Loterre: Astronomy: active galactic nucleus

galactic and extragalactic astronomy > galaxy > galactic nucleus > active galactic nucleus

high-energy astrophysics > active galactic nucleus

galactic and extragalactic astronomy > galaxy > active galaxy > active galactic nucleus

active galactic nucleus

An active galactic nucleus (AGN) is one that gives off much more energy than can be explained purely in terms of its star content. AGN are found at the heart of active galaxies, including quasars, Seyfert galaxies, blazars, and radio galaxies. In addition to their great energy output, they can be highly variable. Some quasars vary in brightness over a few weeks or months, while some blazars show changes in X-ray output over as little as three hours. These fluctuations place strict limits on the maximum size of the energy source, because an object cannot vary in brightness faster than it takes light to travel from one side of its energy-producing region to the other. The rapid flickering of AGN means that they draw their energy from a small volume, in some cases less than one light day across. Furthermore, observations of the orbital motion of stars and other material around AGN show that a large mass, ranging up to several billion solar masses, is concentrated within its "engine room." This leads to the almost unavoidable conclusion that the central engine is a supermassive black hole. Since a black hole, by definition, emits nothing, the radiation from an AGN is believed to come from material heated to several million degrees in an accretion disk before tumbling into the black hole or, in some cases, being shot away in twin jets along the central engine's spin axis. (Encyclopedia of Science, by David Darling, https://www.daviddarling.info/encyclopedia/A/AGN.html)

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