A star with a mass between about 1.5 and 3 solar masses that has collapsed under gravity to such an extent that it consists almost entirely of neutrons. Neutron stars are only about 10 kilometres across and have a density of 10 to the power of 17 kg/m3. They are formed in supernova explosions and observed as pulsars. Once nuclear fuel is exhausted in a star, the core starts to cool and the internal pressure falls, leading to contraction. This is a sudden and catastrophic event for stars of more than 1.8 solar masses, which implode until the pressure between neutrons balances the inward pull of gravity. In the resulting supernova, much of the original mass of the star is blown off into space. A stellar remnant of three solar masses or more will collapse into a black hole rather than a neutron star.