A neutron star is the collapsed core of a large star (10–29 solar masses). Neutron stars are one of the smallest and densest celestial objects known to exist in the Universe.[1] With a radius of only about 11–11.5 km (7 miles), they can, however, have a mass of about twice that of the Sun. They result from the supernova explosion of a massive star, combined with gravitational collapse, that compresses the core past the white dwarf star density to that of neutrons. Neutron stars are composed almost entirely of neutrons, which are subatomic particles with no net electrical charge and with slightly larger mass than protons. They are supported against further collapse by neutron degeneracy pressure, a phenomenon described by the Pauli exclusion principle. If the remnant has too great a mass, between 1.4 and 2-3 solar masses, it will continue to collapse into a form called a black hole. Neutron stars are very hot and typically have a surface temperature around 6×105 K.[2][3][4][5][a] They are so dense that a normal-sized matchbox containing neutron-star material would have a mass of approximately 13 million tonnes, or a 2.5 million m3 chunk of the Earth.[6][7] The density of the star is comparable to that of the nucleus of an atom. They have strong magnetic fields, between 108 and 1015 times that of Earth's. The gravitational field at the neutron star's surface is about 2×1011 times that of the Earth's.

As the star's core collapses, its rotation rate increases as a result of conservation of angular momentum, hence neutron stars rotate at up to several hundred times per second. As they do so, they can emit beams of electromagnetic radiation that makes them detectable as pulsars. Indeed, the discovery of pulsars in 1967 first suggested that neutron stars exist. The radiation from pulsars is thought to be primarily emitted from regions near their magnetic poles. If the magnetic poles do not coincide with the rotational axis of the neutron star, the emission beam will sweep the sky, and when seen from a distance, if the observer is somewhere in the path of the beam, it will appear as pulses of radiation coming from a fixed point in space. The rotation of neutron stars can be very rapid; up to 716 times a second[8][9] has been detected, which is approximately 43,000 revolutions per minute, giving a linear speed at the surface on the order of 0.165 c.

There are thought to be around 100 million neutron stars in the Milky Way, a figure obtained by estimating the number of stars that have undergone supernova explosions.[10] However, most are old and cold, and neutron stars 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. The sudden collapse of rapidly-rotating high-mass stars, or the merger of binary neutron stars, may be the source of gamma-ray bursts. Soft gamma repeaters are conjectured to be a type of neutron star with very strong magnetic fields, known as magnetars, or alternatively, neutron stars with fossil disks around them.[11]