A star containing about 1½ solar masses of material compressed into a volume approximately 6 mi (10 km) in radius. (1 solar mass equals 4.4 × 10³⁰ lbm or 2.0 × 10³⁰ kg.) Neutron stars are one of the end points of stellar evolution and are the final states of stars that begin their lives with considerably more mass than the Sun. The density of neutron star material is 10¹⁴ to 10¹⁵ times the density of water and exceeds the density of matter in the nuclei of atoms. Neutron stars are pulsars (pulsating radio sources) if they rotate sufficiently rapidly and have strong enough magnetic fields.