A neutron star is a type of compact star that can result from the gravitational collapse of a massive star after a supernova.
Neutron stars are the densest and smallest stars known to exist in the Universe; with a radius of only about 11–11.5 km (7 miles), they can have a mass of about twice that of the Sun.
Any main-sequence star with an initial mass of above 8 M☉ 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 via electron capture, 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.