Neutron stars are the densest and tiniest stars known to exist in the universe; although having only the radius of about 10 km (6 mi), they may have a mass of several times that of the Sun. Neutron stars probably appear white to the naked eye.
Neutron stars are the end points of stars whose inert core's mass after nuclear burning is greater than the Chandrasekhar limit for white dwarfs, but whose mass is not great enough to overcome the neutron degeneracy pressure to become black holes.
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.
The discovery of pulsars in 1967 suggested that neutron stars exist. Born in supernova explosions, these bodies are "only" ~12-13 kilometers by radius and spin around as rapidly as 642 times a second, or approximately 38,500 revolutions per minute. In contrast, the Sun's radius is about 60,000 times that.
A typical neutron star has a mass between ~1.4 and 3.2 solar masses with a surface temperature of ~6 x 105 Kelvin (see Chandrasekhar limit). 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 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.
In general, compact stars of less than 1.44 solar masses – the Chandrasekhar limit – are white dwarfs and a compact star weighing between that and 3 solar masses (the Tolman–Oppenheimer–Volkoff limit) should be a neutron star. Gravitational collapse will usually occur on any compact star between 10 and 25 solar masses and produce a black hole.
Between these, hypothetical intermediate-mass stars such as quark stars and electroweak stars have been proposed, but none have been shown to exist.
Rotating neutron stars are known as pulsars
Some neutron stars rotate very rapidly and emit beams of electromagnetic radiation as pulsars. 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 108 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.