According to theory, there might be three types of black holes: stellar, supermassive, and miniature black holes — depending on their size. These black holes have also formed in different ways. Supermassive black holes likely exist in the centers of most galaxies, including our own galaxy, the Milky Way. They can have a mass equivalent to billions of suns.
In the outer parts of galaxies (where our solar system is located within the Milky Way) there are vast distances between stars. However, in the central region of galaxies, stars are packed very closely together. Because everything in the central region is tightly packed to start with, a black hole in the center of a galaxy can become more and more massive as stars orbiting the event horizon can ultimately be captured by gravitational attraction and add their mass to the black hole.
By measuring the velocity of stars orbiting close to the center of a galaxy, we can infer the presence of a supermassive black hole and calculate its mass. Perpendicular to the accretion disk of a supermassive black hole, there are sometimes two jets of hot gas. These jets can be millions of light years in length.
They are probably caused by the interaction of gas particles with strong, rotating magnetic fields surrounding the black hole. Observations with the Hubble Space Telescope have provided the best evidence to date that supermassive black holes exist.
The exact mechanisms that result in what are known as miniature black holes have not been precisely identified, but a number of hypotheses have been proposed. The basic idea is that miniature black holes might have been formed shortly after the "Big Bang," which is thought to have started the Universe about 15 billion years ago.
Very early in the life of the Universe the rapid expansion of some matter might have compressed slower-moving matter enough to contract into black holes. Some scientists hypothesize that black holes can theoretically "evaporate" and explode.
The time required for the "evaporation" would depend upon the mass of the black hole. Very massive black holes would need a time that is longer than the current accepted age of the universe. Only miniature black holes are thought to be capable of evaporation within the existing time of our universe. For a black hole formed at the time of the "Big Bang" to evaporate today its mass must be about 1015g (i.e., about 2 trillion pounds), a little more than twice the mass of the current Homo sapien population on planet Earth.
During the final phase of the "evaporation," such a black hole would explode with a force of several trillion times that of our most powerful nuclear weapon. So far, however, there is no observational evidence for miniature black holes.
