The more mass contained by the object the deeper its potential well and the faster the material in the accretion disc will swirl around it. Thus, the more heating the disc will suffer and the more energy it will release in the form of electromagnetic radiation. Accretion discs are, primarily, found around protostars and black holes. A galactic nucleus in which some kind of extraordinary activity is liberating vast amounts of energy in the form of electromagnetic radiation. Sometimes the energy released by the nucleus is so great that it outshines all the other stars in the galaxy combined.
Most astronomers believe that, at the centre of an active galactic nuclei, lies a black hole .
A minority believe that AGN can be explained as starbursts and multiple supernovae. 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.