extremely small, extremely dense star roughly double the sun in mass but only a few miles in radius. According to current theories, the core of a neutron star is composed of ELEMENTARY PARTICLES and is surrounded by a fluid composed primarily of NEUTRONS squeezed in close contact. The fluid in turn is encased in a rigid, extremely dense crust a few hundred miles thick. Evidence of the existence of neutron stars is provided by PULSARS, radio sources that fluctuate in intensity in a manner that indicates that they may be rotating neutron stars. Neutron stars are believed to be the dead stellar corpses remaining after the SUPERNOVA explosion of an intermediate-mass star. See GRAVITATIONAL COLLAPSE; STELLAR EVOLUTION.