Active galactic nucleus (AGN)

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. See also active galaxies, black holes , "The Energy Machine" .