Cosmos The Cosmic Journey

NEUTRON STARS

A neutron star is a type of stellar remnant that can result from thegravitational collapse of a massive star after a supernova. Neutron stars are the densest and smallest stars known to exist in theuniverse; with a radius of only about 12-13 km (7 mi), they can have a mass of about two times that of the Sun.

Radiation from the pulsar PSR B1509-58, a rapidly spinning neutron star, makes nearby gas glow in X-rays (gold, fromChandra) and illuminates the rest of thenebula, here seen in infrared (blue and red, from WISE).

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 pressuredue to the phenomenon described by thePauli exclusion principle, which states that no two neutrons (or any other fermionicparticles) can occupy the same place andquantum state simultaneously.

A typical neutron star has a mass between ~1.4 and about 3 solar masses (M☉) with a surface temperature of ~6×105 k.Neutron stars have overall densities of3.7×1017 to 5.9×1017 kg/m3 (2.6×1014 to4.1×1014 times the density of the Sun),which is comparable to the approximate density of an atomic nucleus of3×1017 kg/m3The neutron star's density varies from below 1×109 kg/m3 in the crust - increasing with depth - to above 6×1017 or8×1017 kg/m3 deeper inside (denser than an atomic nucleus). A normal-sized matchbox containing neutron star material would have a mass of approximately 5 billion tonnes or ~1 km3 of Earth rock.[citation needed]

In general, compact stars of less than 1.44 M☉ (the Chandrasekhar limit) are white dwarfs while compact stars weighing between that and 3 M☉ (the Tolman-Oppenheimer-Volkoff limit) should be neutron stars. The maximum observed mass of neutron stars is about 2 M☉. Compact stars with more than 10 M☉ will overcome theneutron degeneracy pressure andgravitational collapse will usually occur to produce a black hole.The smallest observed mass of a black hole is about 5 M☉. Between these, hypothetical intermediate-mass stars such as quark stars andelectroweak 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 seconor approximately 43,000 revolutions per minute) and emit beams of electromagnetic radiation aspulsars. 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 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.

Formation

Any main sequence star with an initial mass of around 10 M☉ or above has the potential to become a neutron star. As the star evolves away from the main sequence, subsequent nuclear burning produces an iron-rich core. When all nuclear fuel in the core has been exhausted, the core must be supported by degeneracy pressure alone. Further deposits of material from shell burning cause the core to exceed the Chandrasekhar limit. Electron degeneracy pressure is overcome and the core collapses further, sending temperatures soaring to over 5×109 K. At these temperatures, photodisintegration (the breaking up of iron nuclei into alpha particles by high- energy gamma rays) occurs. As the temperature climbs even higher, electrons and protons combine to form neutrons, releasing a flood of neutrinos. When densities reach nuclear density of 4×1017 kg/m3, neutron degeneracy pressure halts the contraction. The infalling outer atmosphere of the star is flung outwards, becoming a Type II or Type Ib supernova. The remnant left is a neutron star. If it has a mass greater than about 5 M☉, it collapses further to become a black hole. Other neutron stars are formed within close binaries.