During most of a star's lifetime, nuclear processes in the core of a star generate electromagnetic radiation that prevents it from collapsing under its own gravitational pull. Once the nuclear fuel in the core is depleted, the radiation pressure that holds the star up decreases, and very soon, the star begins to collapse.

Depending on its mass, a star becomes either a white dwarf, a neutron star or a black hole when it dies. Stars with masses less than 3 solar masses collapse to form a white dwarf or a neutron star. Although nuclear fusion no longer goes on in these stars, white dwarfs and neutron stars continue to be held up by electron and neutron degeneracy pressures respectively. Very massive stars, on the other hand, are unable to support themselves against their own gravitational pull, and collapse to form black holes.

Einstein's theory of general relativity presents gravity as a manifestation of the curvature of spacetime. In 1916, just after the theory of general relativity was established, German physicist Karl Schwarzschild calculated what space surrounding a point mass would appear like. He found that at small radii surrounding the object, the solution to the Einstein equation behaved strangely. At r=0, in particular, there was a singularity where the curvature of spacetime was infinite. A black hole is the space around the singularity where gravity becomes so overpowering that nothing, including light and other forms of electromagnetic radiation escapes it.

When a massive star begins to burn out, an apparent event horizon forms. The apparent event horizon separates the light rays that might have a chance of escaping from those that are doomed to be sucked into the black hole forever. The light rays in the apparent event horizon, however, might still get sucked back in if more mass collapses, thus increasing the gravitational pull. The event horizon is a spherical shell around the singularity of infinite density which defines the surface of the black hole. The radius of the event horizon is known as the Schwarzschild radius. Once anything passes through the Schwarzschild radius, it is doomed for eternity, as it will never be able to resist the strong gravitational pull of the black hole.