The crushed remnant of a star that exploded as a supernova. Stars that are born with about 8 to 20 times the mass of the Sun blast most of their material into interstellar space in titanic explosions, leaving only their crushed, dense cores. Neutron stars are named after their composition: neutrons. In a star with a core that is 1.4 to 3 times the mass of the Sun, the core collapses so completely that electrons and protons combine to form neutrons. A full bathtub of neutron-star material (instead of water) would weigh as much as two Mount Everests. A neutron star is about 10-15 miles (16-24 km) in diameter, with a liquid neutron core and a crust of solid iron. Some neutron stars, called pulsars, spin rapidly (from once a second to several hundred times per second) and generate powerful magnetic fields.