Supermassive black hole (image: NASA/JPL-Caltech)
If nothing, not even light, can’t escape, such an object is called a black hole. The black hole has no material surface. The original matter of the star is shrunk to an infinitely dense point, called a singularity. The perimeter of the black hole is called the horizon. Everything which could happen beyond the horizon is trapped, and can only increase the mass of the hole. But a black hole is not a monster: it can only catch objects which come very near. If we replaced the Sun with a black hole, we hardly notice the difference.
There are fourth basic types of black holes:
1. Stellar black hole. It forms when a massive star runs out of nuclear fuel and collapses under its own weght (supernova).
2. Supermassive black hole. These holes hide in the cores of galaxies (including our own galaxy, the Milky Way). They have mass of millions of Suns. They can form when smaller black holes merge together. Because of their location in the centers of galaxies, close to many tightly packed stars and gas clouds, supermassive black holes continue to grow on a steady diet of matter.
3. An Intermediate-mass black hole. Mass of such hole varies from a few tens to a few millions of the mass of the Sun. It is not clear how such a black hole would form.
4. Mini black hole. These have the mass of an asteroid or less. None has ever been detected, but they could form under the extreme conditions.
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