Supernovae, Neutron Stars and Black Holes

A supernova is a rapid increase in the absolute magnitude of a star. For a short time a single supernova can be as bright as a whole galaxy.Core-collapse supernova. A very massive star becomes a red supergiant and fuses heavier and heavier elements in shells around its core, up to iron. Fusing iron does not release energy, so fusion stops and nothing supports the core against gravity. The core collapses in a fraction of a second and the outer layers rebound, blasting outwards in a supernova. What remains depends on the mass of the core:• core mass above about 1.4 solar masses: electrons and protons combine to form a neutron star
• core mass above about 3 solar masses: gravity is so strong that nothing can stop the collapse, forming a black holeA neutron star is made almost entirely of neutrons. It has a radius of only about 10 km and a density similar to that of an atomic nucleus (about 10¹⁷ kg m^-3). Neutron stars spin rapidly, and some emit beams of radiation detected as pulsars.A black hole is an object whose escape velocity is greater than the speed of light, so nothing, not even light, can escape from it. Supermassive black holes, millions or billions of times the mass of the Sun, are believed to exist at the centres of most galaxies.Gamma ray bursts are short, intense bursts of gamma rays lasting from seconds to minutes. They are thought to come from the collapse of supergiant stars to form neutron stars or black holes.