A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a supernova event. Neutron stars are the densest and tiniest stars known to exist in the universe; with a radius of only about 12-13 km (6 mi), they may have a mass of a few times that of the Sun. Neutron stars probably appear white to the naked eye.
Neutron 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 ~1.4 and about 2 solar masses with a surface temperature of ~6 x 105 Kelvin. 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),[b] which is comparable to 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. A normal-sized matchbox containing neutron star material would have a mass of approximately 5 billion tonnes or ~1 km³ of Earth rock.
In general, compact stars of less than 1.44 solar masses – the Chandrasekhar limit – are white dwarfs and compact stars weighing between that and 3 solar masses (the Tolman–Oppenheimer–Volkoff limit) should be neutron stars. The maximum observed mass of neutron stars is about 2 solar masses. Compact stars with more than 10 solar masses will overcome the neutron degeneracy pressure and gravitational collapse will usually occur to produce a black hole. The smallest observed mass of a black hole is about 5 solar masses. Between these, hypothetical intermediate-mass stars such as quark stars and electroweak stars have been proposed, but none have been shown to exist. The equations of state of matter at such high densities are not precisely known because of the theoretical and empirical difficulties.
Some neutron stars rotate very rapidly (up to 716 times a second, or approximately 43,000 revolutions per minute) and emit beams of electromagnetic radiation as pulsars. Indeed, the discovery of pulsars in 1967 first suggested that neutron stars exist. Gamma-ray bursts may be produced from rapidly rotating, high-mass stars that collapse to form a neutron star, or from the merger of binary neutron stars. There are thought to be on the order of 10^8 neutron stars in the galaxy, but they can only be easily detected in certain instances, such as if they are a pulsar or part of a binary system. Non-rotating and non-accreting neutron stars are virtually undetectable; however, the Hubble Space Telescope has observed one thermally radiating neutron star, called RX J185635-3754.
Listen to pulsar PSR B1937+21, which spins 642 times per second.