A neutron star is the remnants of a massive star that has undergone a supernova explosion. After the star exhausts its nuclear fuel, it collapses under its own gravity, causing protons and electrons to combine and form neutrons. This results in an incredibly dense object, where a sugar-cube-sized amount of neutron star material would weigh about a billion tons. Neutron stars are typically about 1.4 times the mass of the Sun but are only about 20 kilometers in diameter. They have extremely strong magnetic fields and can rotate very rapidly, sometimes spinning several hundred times per second. Some neutron stars emit beams of radiation, becoming known as pulsars when these beams are detected from Earth.