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A neutron star is the gravitationally collapsed core of a massive supergiant star.

It results from the supernova explosion of a massive star—combined with gravitational collapse—that compresses the core past white dwarf star density to that of atomic nuclei. Most neutron stars that have been detected are pulsars or a part of a binary system. Neutron stars in a binary system with a main sequence star can pull in large amounts of gas from its companion, a process called accretion. The study of neutron star systems is central to gravitational wave astronomy.

Any main-sequence star with an initial mass of greater than 8 M☉ (eight times the mass of the Sun) has the potential to become a neutron star. Observations of gravitational waves from neutron star merger GW170817, which is thought to have generated a black hole shortly afterward, have refined the mass limit estimate for a non-rotating neutron star to ~2.17 M☉. As the core of a massive star is compressed during a Type II supernova or a Type Ib or Type Ic supernova, and collapses into a neutron star, it retains most of its angular momentum.

A presumptive neutron star equation of state would encode information about the structure of a neutron star and would explain how matter behaves at the extreme densities found inside neutron stars.

Using these relations, one can constrain the neutron star equation of state when gravitational waves from binary neutron star mergers are observed. For example, the LIGO detection of the binary neutron star merger GW170817 provided limits on the tidal deformability of neutron star binaries, ruling out whole families equations of state.

If the radius of the neutron star is 3GM/c2 or less, then the photons may be trapped in an orbit, thus making the whole surface of that neutron star visible from a single vantage point, along with destabilizing photon orbits at or below the 1 radius distance of the star. EB is the gravitational binding energy of the observed neutron star of mass of M with radius R,

As the density of the neutron star increases, the nuclei break down, and the neutron pressure of the star becomes dominant.

If the axis of rotation of the neutron star is different from the magnetic axis, external viewers will only see these beams of radiation whenever the magnetic axis point towards them during the neutron star rotation.

As a neutron star ages, its rotation slows (P increases); eventually, the rate of rotation will become too slow to power the radio-emission mechanism, so radio emission from the neutron star can no longer be detected.

The merger of binaries containing two neutron stars, or a neutron star and a black hole, has been observed through the emission of gravitational waves.

Binary systems containing neutron stars often emit X-rays, which are emitted by hot gas as it falls towards the surface of the neutron star. As the neutron star accretes this gas, its mass can increase; if enough mass is accreted, the neutron star may collapse into a black hole.

The energy source of the pulsar is the rotational energy of the neutron star. The energy source is gravitational and results from a rain of gas falling onto the surface of the neutron star from a companion star or the interstellar medium.

Magnetar: a neutron star with an extremely strong magnetic field (1,000 times more than a regular neutron star), and long rotation periods (5 to 12 seconds)

X-ray burster: a neutron star with a low-mass binary companion from which matter is accreted resulting in irregular bursts of energy from the surface of the neutron star Electroweak star: currently a hypothetical type of extremely heavy neutron star, in which the quarks are converted to leptons through the electroweak interaction, but the gravitational collapse of the neutron star is prevented by radiation pressure.

Quick Facts

• Using these relations, one can constrain the neutron star equation of state when gravitational waves from binary neutron star mergers are observed.
• A presumptive neutron star equation of state would encode information about the structure of a neutron star and would explain how matter behaves at the extreme densities found inside neutron stars.
• Observations of gravitational waves from neutron star merger GW170817, which is thought to have generated a black hole shortly afterward, have refined the mass limit estimate for a non-rotating neutron star to ~2.17 M☉.
• Electroweak star: currently a hypothetical type of extremely heavy neutron star, in which the quarks are converted to leptons through the electroweak interaction, but the gravitational collapse of the neutron star is prevented by radiation pressure.
• As the neutron star accretes this gas, its mass can increase; if enough mass is accreted, the neutron star may collapse into a black hole.

Source material: Wikipedia - "Neutron star". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.