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.
Department of Astrophysics,
IMAPP, Radboud University,
P.O. Box 9010,
6500 GL Nijmegen, The Netherlands
Institute for Theoretical Physics (ITP),
Max-Planck-Institut für Radioastronomie (MPIfR),
Auf dem Hügel 69, D-53121