Animation of a pulsar, a rapidly rotating neutron star that emits two beams of radiation. The two beams are emitted from its magnetic poles, which are not necessarily aligned with its rotational axis. If one of the beams is directed towards Earth, it appears as a periodic pulse of intense radiation. Pulsars generally rotate in less than a second. Like all neutron stars, they are formed in a supernova, the explosive death of a massive star. The supernova ejects the outer layers of the giant star, which form an expanding cloud of debris that can be illuminated by the pulsar's radiation. The core collapses to form a neutron star, a body with the density of an atomic nucleus. Neutron stars are typically about 12 kilometres in diameter with a mass twice that of the Sun.