neutron star

Very small, ‘superdense’ star composed mostly of neutrons. They are thought to form when massive stars explode as supernovae, during which the protons and electrons of the star's atoms merge, owing to intense gravitational collapse, to make neutrons. A neutron star has a mass two to three times that of the Sun, compressed into a globe only 20 km/12 mi in diameter.

If its mass is any greater, its gravity will be so strong that it will shrink even further to become a black hole. Being so small, neutron stars can spin very quickly. The rapidly flashing radio stars called pulsars are neutron stars. The flashing is caused by a rotating beam of radio energy similar in behaviour to a lighthouse beam.