Neutron star, animation. A neutron star forms when the core of a dying star collapses into a super_dense state only a few kilometres across. The star is composed mainly of neutrons, formed when protons and electrons are forcibly combined during the collapse. Neutron stars rotate extremely rapidly, up to several thousand times per second and are extremely hot, with surface temperatures in the millions of degrees Celsius. Some neutron stars produce regular beams of radio emissions known as pulsars.