A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event.
Such stars are composed almost entirely of neutrons, which are subatomic particles without net electrical charge and with slightly larger mass than protons. Neutron stars are very hot and are supported against further collapse by quantum degeneracy pressure due to the phenomenon described by the Pauli exclusion principle.
This principle states that no two neutrons (or any other fermionic particles) can occupy the same place and quantum state simultaneously.
A typical neutron star has a mass between about 1.4 and 3.2 solar masses (see Chandrasekhar Limit), with a corresponding radius of about 12 km.
In contrast, theSun's radius is about 60,000 times that. Neutron stars have overall densities of 3.7×1017 to 5.9×1017 kg/m3 (2.6×1014 to 4.1×1014 times the density of the Sun), which compares with the approximate density of an atomic nucleus of 3×1017 kg/m3.
The neutron star's density varies from below 1×109 kg/m3 in the crust, increasing with depth to above 6×1017 or 8×1017 kg/m3 deeper inside (denser than an atomic nucleus). This density is approximately equivalent to the mass of a Boeing 747 compressed to the size of a small grain of sand.
In general, compact stars of less than 1.44 solar masses – the Chandrasekhar limit – are white dwarfs, and above 2 to 3 solar masses (the Tolman–Oppenheimer–Volkoff limit), aquark star might be created; however, this is uncertain. Gravitational collapse will usually occur on any compact star between 10 and 25 solar masses and produce a black hole.
Some neutron stars rotate very rapidly and emit beams of electromagnetic radiation as pulsars.