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Main Sequence Stars
Main sequence refers to those stars that are in the prime of their life, with active fusion taking place within their cores. These stars are characterized by a direct relationship between their mass and luminosity. These stars are all one "type" of star, differentiated not by type but by size, or mass. The Sun is a relatively small main sequence star. The size of the Sun has been defined as one solar mass. Main sequence stars that will eventually become black holes are much larger than the Sun, typically greater than 20 to 30 solar masses.
Death of a Star
Once a main sequence star exhausts its available fuel, there are three primary potential outcomes for the dying star: a white dwarf star, a neutron star, or a black hole. Small main sequence stars, such as the Sun, are destined to become white dwarfs. Medium main sequence stars, generally between eight and 20 to 30 solar masses, are destined to become neutron stars or pulsars. Very large main sequence stars, greater than 20 to 30 solar masses, will become black holes. However, no concrete boundary exists regarding the size required to transition from a neutron star to black hole.
White Dwarfs
Without the force generated by their fusion reactions to counteract the forces of gravity, main sequence stars collapse. The result is a rapid expansion of their outer layers, forming what is known as a red giant, and the formation of a very small, dense core. Once the matter in this core is compressed to the level of electrons, the collapse is halted by electron degeneracy. This atomic force counters the force of gravity, and the core stabilizes as a white dwarf. This occurs in cores of less than 1.44 solar masses, known as the Chandrasekhar limit.
Neutron Stars
For medium main sequence stars, the collapse results in a supernova. If the core left behind exceeds 1.44 solar masses, the matter will be compressed so much that electrons and protons will combine to form neutrons. At this point, the core's collapse is halted by neutron degeneracy, creating a neutron star. This occurs in cores of less than three to five solar masses.
Black Holes
For massive main sequence stars, the collapse results in a hypernova. If the core left behind exceeds three to five solar masses, the force of gravity will overcome even the neutron degeneracy. Beyond this point, there is no force left that can resist gravity. The core will collapse to a singularity, forming a stellar black hole. The merging of two binary neutron stars can also produce a stellar black hole.