A neutron star is formed by the gravitational collapse of a massive star after a supernova. It has a mass of 1-3 times the sun's mass but is only about 20 km in diameter, making it incredibly dense. The first neutron star was discovered in 1967 by Jocelyn Bell, a graduate student who discovered the first pulsar, a type of neutron star. Neutron stars come in different types, including pulsars which emit beams of electromagnetic radiation and magnetars which have extremely powerful magnetic fields.