The central region of an [[active galaxy]]: often abbreviated to AGN. The active nucleus is the region in which an active [[galaxy]] differs most strongly from a normal galaxy; they often exhibit strong optical and X-ray emission and show bright [[emission lines]]. The activity in the active nucleus is thought to be powered by [[accretion]] of matter from within the galaxy on to a [[black hole]]with a mass 10$^6$–10$^9$ times that of the Sun. The active nucleus is sometimes called the [[central engine]]. [[../Chapters/Topic 3 Mapping the Universe]] [[Docent/S284 Astronomy]]