When stars reach the end of their lives, they go through the giant or supergiant and supernova phases and depending on the mass of the stellar remnant the star may become one of three things.
If the mass of the stellar remnant is less than about 1.4 solar masses, the remnant becomes a white dwarf.
If the mass of the stellar remnant is between about 1.4 and 3 solar masses, it becomes a neutron star.
Neutron stars are made up entirely of neutrons. Quantum physics predicts they exert a pressure which support the star against further collapse. This is only sufficient up to a point however. If the mass of the stellar remnant is more than about 3 solar masses, the neutron degeneracy pressure is insufficient. The force of gravity is stronger. Gravity wins and there is no known force to support the star against total collapse to a singularity..This singularity is so called because we know extremely little about it..
Event Horizon
A black hole is so called because the hole itself appears opaque and black to the eye. Every black hole has what is called an “event horizon”. The event horizon is the “surface” of the hole, the point of no return. On the diagram below, the event horizon is at the centre of the gas disk. Matter may orbit the hole as it would orbit any central mass, but if it get closer to the central singularity than the event horizon then it disappears from view forever, and must proceed to the singularity.
Accretion Disc
Some black holes may be isolated in space. In this case they are invisible and hard to detect. If a black hole exists inside a gas cloud, then a disk of gas may form around the black hole, orbiting it. The gas closest to the hole orbits the fastest, and friction in the gas cloud heats the gas, making it visible. When the gas falls inside the event horizon it disappears forever.
