A term that covers both white dwarfs and neutron stars, which are made up of degenerate matter. These stars are in an advanced state of evolution and have suffered extreme gravitational collapse. Normal atoms cannot exist under the conditions of very high pressure. In white dwarfs, the electrons and atomic nuclei collapse from the normal open atomic structure into a dense, compressed mass. A quantum-mechanical effect called degeneracy pressure counters further gravitational collapse. However, if the total mass of the star exceeds 1.4 times the mass of the Sun, the degeneracy pressure is insufficient to balance the gravitational force and a neutron star results. Electrons and nuclei combine into a form of matter consisting of tightly packed neutrons. See also: black hole.