A star that has shrunk under its own gravity during a supernova event, so that most of its material has been compressed into neutrons only (the protons and electrons have been crushed together until they merge, leaving only neutrons). Neutron stars are very hot, quite small (typically 20 to 30 kilometres in diameter), extremely dense, have a very high surface gravity and rotate very fast. A pulsar is a kind of highly-magnetized rapidly-rotating neutron star.