A [[galaxy]] which emits EM radiation (in the [[radio]], optical, and/or X-ray wavebands) that cannot be attributed to the normal constituents of a galaxy, such as stars, gas and [[dust]]. In some types of active galaxy, such as [[Seyfert galaxies ]]and [[radio-quiet]] [[quasar]]s, the unusual activity is confined to the [[active galactic nuclei]] (AGN), the central region of the active galaxy. In others, such as [[radio galaxies]] and [[radio-loud]] [[quasar]]s, the results of the activity can be detected on much larger scales as well. Only a small fraction of [[galaxy]]s are active. [[Seyfert galaxies]] [[../Chapters/Topic 3 Mapping the Universe]] [[Docent/S284 Astronomy]]