There are two main processes constantly going on in massive stars: nuclear fusion (which tends to blow the star's hydrogen outward from the star's center) and gravitation (which tends to pull all hydrogen back in the direction it had come). These two processes balance one another until all the star's hydrogen is exhausted, allowing gravitation to take over. Once gravitation dominates, the star becomes unstable and starts to collapse. Once the star starts to collapse, it does not stop, and the star (and ultimately its atoms) will cave inward upon itself, resulting in the formation of a black hole (Hewitt 186).

However, not all stars upon gravitational collapse form black holes. A star less than 1.4 times the mass of the sun will become a white dwarf. A star between 1.4 and 3 times the mass of the sun will become a neutron star. It's only those stars greater than 3 times the mass of the sun that become black holes upon collapse.

Additionally, a black hole can be formed by compression through external forces. This type of black hole is called a primordial black hole.