Single stars that are more than five times as massive as the Sun can continue generating energy by fusion after having depleted their hydrogen supplies because their gravitational potential energy enables them to build up extremely high pressures and temperatures deep in their interior. In this way, they can successively create elements as heavy as iron in their cores. It is thought, however, that reactions involving iron fusion result in the catastrophic collapse of the stellar core. The outer layers of such a heavyweight star explode violently as a supernova, shining 109 times more brightly than a normal star for several months. After the supernova event, the core may remain as a neutron star.
Many neutron stars emit radio radiation in the form of short pulses at very regular intervals.
Such stellar objects are commonly called pulsars.