In astronomy, a compact star (sometimes called a compact object) is a star that is a while dwarf, a neutron star or a black hole. Our Galaxy is populated by billions of white dwarfs, a few hundred million neutron stars and probably by a few hundred thousand black holes. Of all these objects, only a very tiny fraction has been detected so far by astronomical instruments, just a few thousand while dwarfs, about 2000 neutron stars, and only a few dozen black holes. Of all these objects, only black holes can appreciably grow in mass. Its is one of the great successes of the last 15 years that it could be shown that practically every center of galaxies harbors a supermassive black hole with a mass in the range of one million to a few billion solar masses. The visible Universe therefore contains at least 1(X) billion supermassive black holes. Only about 100,000 of these objects have now been detected as quasars and only about 50 as mass centers of nearby galaxies. Black holes of varying mass are also thought to be the driver behind gamma bursters.