When a massive object, such as a star, has burned through all of its fuel, it is no longer able to support itself against the inward pull of gravity by releasing energy through nuclear reactions. The star then begins to collapse in on itself under the force of its own gravity.

If the mass of the star is less than about three times that of the sun, the collapse will be halted by the Pauli exclusion principle, resulting in the formation of a dense neutron star. However, if the mass of the star is greater than about three times that of the sun, the collapse will continue until all matter is compressed into a single point, known as a singularity, and a black hole is formed.