A black hole is formed when a massive star starts running out of nuclear fuel at its interior (mainly hydrogen and helium) and begins to collapse under its own gravity. Such a star may become a white dwarf or a neutron star, but if the star is sufficiently massive then it may continue shrinking eventually to the size of a tiny atom: this is the so-called “gravitational singularity”. A black hole refers to the region in space around the singularity in which the gravitational force is so strong that not even light can escape its pull.
