|a star that has collapsed under its own gravity to a diameter of about 10 to 15 km. It is composed mostly of neutrons, has a mass of between 1.4 and about 3 times that of the sun, and a density in excess of 1017 kilograms per cubic metre|
A celestial object consisting of an extremely dense mass of neutrons, formed at the core of a supernova, where electrons and nuclei are compressed together so intensely by the force of gravity that protons and electrons merge together into neutrons. Though their mass is close to that of the Sun, the density of neutron stars is much higher—about 3 × 1011 kilograms per cubic centimeter (by comparison, the density of steel is 7.7 grams per cubic centimeter). Neutron stars are typically about 10 km across, and rotate very rapidly. Due to the spinning of electrically charged protons and electrons at their surfaces, their rotation gives rise to strong magnetic fields. The existence of neutron stars was predicted in the 1930s but was not confirmed until the discovery of the first pulsar in 1967. See more at pulsar.