There are two basic parts to a black hole:

1. The singularity 2. The event horizon.

The event horizon is the "point of no return" around the black hole. It is not a physical surface, but a sphere surrounding the black hole that marks where the escape velocity is equal to the speed of light.

The hole part of the BLACK_hole is called Singularity. i.e. that the whole mass of the BLACK_hole is concentrated in a single point in the space.

1. Stellar BLACK_holes

When a star burns through the last of its fuel, it may collapse, or fall into itself. For smaller stars, up to about three times the sun's mass, the new core will be a neutron star or a white dwarf. In less than a second, the iron core, which is about the size of the Earth, shrinks to a neutron core with a radius of about 10 kilometers. The outer layers of the star fall inward on the neutron core, thereby crushing it further. The core heats to billions of degrees and explodes (supernova), thereby releasing large amounts of energy and material into space. The shock wave from the supernova can initiate star formation in other interstellar clouds. But when a larger star collapses, it continues to compress and creates a stellar BLACK_holes.

According the Harvard-Smithsonian Center for Astrophysics, "the Milky Way contains a few hundred million" stellar BLACK_holes.

2. Supermassive BLACK_holes

Small BLACK_holes populate the universe, but their cousins, supermassive BLACK_holes, dominate. Supermassive BLACK_holes are millions or even billions of times as massive as the sun, but have a radius similar to that of Earth's closest star. Such BLACK_holes are thought to lie at the center of pretty much every galaxy, including the Milky Way.

Supermassive BLACK_holes may be the result of hundreds or thousands of tiny BLACK_holes that merge together. Large gas clouds could also be responsible, collapsing together and rapidly accreting mass. A third option is the collapse of a stellar cluster, a group of stars all falling together.

3. Intermediate BLACK_holes

Scientists once thought BLACK_holes came in only small and large sizes, but recent research has revealed the possibility for the existence of mid-size, or intermediate, black holes (IMBHs). Such bodies could form when stars in a cluster collide in a chain reaction. Several of these forming in the same region could eventually fall together in the center of a galaxy and create a supermassive BLACK_holes.

In 2014, astronomers found what appeared to be an intermediate-mass black hole in the arm of a spiral galaxy.

4. Miniature BLACK_holes

Miniature BLACK_holes also called quantum mechanical BLACK_holes or mini BLACK_holes, are hypothetical tiny BLACK_holes, for which quantum mechanical effects play an important role. It might have formed during the dawn of our universe. Between 10 and 20 billion years ago, all matter and energy was compressed into a single point. Then this tiny point exploded (the Big Bang) and expanded rapidly.