Galaxies are collections of billions of stars orbiting together as islands in the universe. These types of galaxies are called spiral galaxies. Another type of galaxy, called an elliptical galaxy, appears like a fuzzy ball or egg-shaped collection of stars, with out the familiar disk. Normal galaxies , like our own Milky Way, are fairly plan. Active galaxies on the other hand, exhibit some remarkable features.
Active galaxies are usually many, many times brighter than their normal galaxy counterparts.
Most of this extra light comes from the central core, or nucleus of the galaxy. Astronomers have surmised that a very massive black hole, up to a billion times the mass of our Sun, sits at the center of these galaxies. The intense gravitational attraction of the black hole is pulling the gas, dust and stars in the central core of these galaxies into itself. As the material falls into the black hole, it is heated to tremendous temperatures, and emits a large amount of radiation, particularly ultraviolet light .