Neutron stars
Gravitationally collapsed cores of massive stars, second only to black holes.
A neutron star is the gravitationally collapsed core of a massive supergiant star, resulting from a supernova explosion and gravitational collapse that compresses the core past white dwarf density to that of atomic nuclei. Surpassed only by black holes, neutron stars are the second-smallest and second-densest known class of stellar objects, with a radius on the order of 10 kilometers and a mass of about 1.4 solar masses. They are thought to number around one billion in the Milky Way, though many have cooled and are difficult to detect; most detected neutron stars are pulsars or part of a binary system.
These objects form when a main-sequence star with an initial mass greater than eight solar masses exhausts its nuclear fuel, developing an iron-rich core. Once the core exceeds the Chandrasekhar limit, electron-degeneracy pressure fails, triggering collapse. Temperatures soar, causing photodisintegration of iron nuclei and electron capture, which produces neutrons and a flood of neutrinos. When density reaches nuclear density, strong force repulsion and neutron degeneracy pressure halt contraction, and the outer envelope is flung outward by neutrino flux, creating a supernova. If the remnant exceeds about 2.3 solar masses, it becomes a black hole instead. During collapse, the star retains its angular momentum but its radius shrinks drastically, resulting in extremely rapid rotation, with periods ranging from about 1.4 milliseconds to 30 seconds. The surface gravity is immense, with escape velocity exceeding half the speed of light, and tidal forces near the surface can cause spaghettification. Neutron stars cool over time and no longer actively generate heat, but they can evolve through collisions or accretion. In binary systems, they can pull gas from a companion, and such systems are central to gravitational wave astronomy; the merger of binary neutron stars produces gravitational waves and is associated with kilonovae and short gamma-ray bursts.
- radius
- ~10 km (6 miles)
- mass
- ~1.4 solar masses (M☉)
- density
- ~4×10^17 kg/m³ (nuclear density)
- rotation_period
- from about 1.4 ms to 30 s
- escape_velocity
- over half the speed of light
- known_for
- second-densest stellar objects; source of gravitational waves; pulsars
Lore & Background
Neutron stars are the gravitationally collapsed cores of massive supergiant stars, resulting from a supernova explosion that compresses the core past white dwarf density to that of atomic nuclei. They are the second-smallest and second-densest known stellar objects, surpassed only by black holes, with a radius on the order of 10 kilometers and a mass of about 1.4 solar masses. Their formation begins when a main-sequence star with an initial mass greater than eight solar masses exhausts its nuclear fuel, leaving an iron-rich core that collapses under gravity. Electron-degeneracy pressure is overcome, temperatures rise above 100 billion kelvins, and photodisintegration breaks iron nuclei into alpha particles. Electrons and protons combine into neutrons via electron capture, releasing neutrinos. When density reaches nuclear density, strong force repulsion and neutron degeneracy pressure halt the collapse, and the outer envelope is flung outward by a neutrino flux, leaving the neutron star. If the remnant exceeds about 2.5 solar masses, it becomes a black hole. Neutron stars retain most of their parent star’s angular momentum, resulting in very high rotation speeds, with observed periods from 1.4 milliseconds to 30 seconds. Their immense surface gravity, typically 10^11 to 10^12 times Earth’s, gives an escape velocity over half the speed of light, causing tidal forces that can spaghettify infalling matter. Most detected neutron stars are pulsars or part of binary systems, where they can accrete gas from a companion. The merger of binary neutron stars produces gravitational waves and is associated with kilonovae and short gamma-ray bursts, as observed in the 2017 event GW170817.
Reader's Guide
Neutron stars are central to gravitational wave astronomy. The merger of binary neutron stars produces gravitational waves and is associated with kilonovae and short gamma-ray bursts. Neutron stars also serve as natural laboratories for fundamental physics, as their extreme densities—ranging from nuclei in a sea of electrons in the outer crust to possibly exotic QCD matter in the inner core—allow study of the strong interaction and equation of state under conditions unattainable on Earth. Understanding the nature of matter in neutron star layers and phase transitions remains a major unsolved problem in physics.
Did You Know?
- Neutron stars have an escape velocity of over half the speed of light.
- A matchbox-sized amount of neutron-star material would weigh approximately 3 billion tonnes.
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