An extremely small, superdense object believed to be formed when a massive star undergoes a Type II supernova explosion. During the explosion, the core of the massive star collapses under its own gravity until, at a density of about 1017 kg/m3, electrons and protons are so closely packed that they combine to form neutrons. The resultant object, consisting only of neutrons, is supported against further gravitational collapse by the degeneracy pressure of the neutrons, provided its mass is not greater than about 2 solar masses (the Oppenheimer–Volkoff limit). If the object were more massive it would collapse further into a black hole. A typical neutron star, with a mass a little greater than the Sun's, would have a diameter of only about 30 km, and a density such that the mass of the entire human race would occupy the volume of a sugar cube. The greater the mass of a neutron star, the smaller its diameter. Neutron stars are believed to have an interior of superfluid neutrons (i.e. neutrons behaving like a fluid of zero viscosity), surrounded by a solid crust about 1 km thick composed of elements such as iron. Pulsars are spinning, magnetized neutron stars. Massive X-ray binaries are also thought to contain neutron stars.
Subjects: Astronomy and Astrophysics.