The universe is full of galaxies of many sizes and compositions. It is thought that most if not all massive galaxies have an immense black hole, known as a supermassive black hole, at their center. In some cases, the center of the galaxy is extremely luminous, brighter than all light sources in the rest of the galaxy combined and emitting a great deal of energy along the entire electromagnetic spectrum. Such galaxies are known as active galaxies, and their centers are known as active galactic... więcej
Salem Press Encyclopedia of Science, 2019. 2p.