Two main categories of black holes have been observed. Stellar-mass black holes are the remnants of individual stars that originally were many times more massive than the sun. We can detect them when they're in binary systems, with another star rapidly orbiting the black hole. Supermassive black holes are found in the centers of galaxies and have masses that are millions to billions of times greater than that of the sun. These black holes accumulated their enormous masses over billions of years by swallowing up gas, stars and even other black holes. When gas is spiraling toward a black hole, it heats up and radiates tremendous amounts of energy. Such "active" black holes that are consuming large quantities of gas are called quasars, and they are among the most luminous objects in the universe.