They begin to bunch up and collide, heating them up to temperatures at which X-rays are emitted, which is about one million Kelvins. Such X-rays have been detected in binary star systems where the source is not visible. No black hole has yet been positively identified. By the late 1980s, many astrophysicists were conjecturing that many, if not all, galaxies of substantial size may contain a black hole in their center. The astrophysical evidence for black holes has become so overwhelming in recent years that astronomers now treat these once-fanciful objects as normal components of the universe.
Recent evidence indicates a staggering number of black holes exist in the universe.
They include the millions of ordinary black holes believed to be peppering each of the estimated 50 billion to 100 billion galaxies in the universe up to giant black holes in the centers of most galaxies with masses that are billions of times greater than the sun's. Normally such X rays are produced by an accretion disk, a dense, hot disk of gas that forms as the gas from a normal star spirals into a compact object. The companion in Cygnus X-1, because of its massiveness, is thought likely to be a black hole rather than a white dwarf or neutron star. The strongest evidence yet is immense clouds containing water vapor that have been observed circling in the center of the galaxy NGC 4258. By tracking the motion of the water-vapor clouds with powerful radio telescopes, astrophysicists can chart the gravitational field of the black hole at the center.