Neutron stars are a class of very compact astrophysical objects which are remnants of massive stars that collapse after exhausting their capacity for thermonuclear burning in their interiors. The typical mass of a neutron star is about one and a half times that of the Sun (which is about 4.4x1030 lb, or 2x1030 kg), while its radius is only about 6 mi (10 km)--one hundred thousand times smaller than the solar radius. Such a combination exists through compression of matter in the interior of the star to a density greater than 2.7 x 1016 lb per cubic inch (1015 g per cubic cm) 2 cm, several times the density in atomic nuclei. Unlike most naturally occurring systems in equilibrium, where matter exists in the form of atoms surrounded by electrons, matter in the cores of neutron stars is believed to be a uniform mixture of nucleons, electrons and possibly other species...