A neutron star is the remnants of a massive star that has exploded in a supernova. After the explosion, the core collapses under gravity, causing protons and electrons to combine and form neutrons. This results in an incredibly dense object, where a sugar-cube-sized amount of material can weigh as much as a mountain. Neutron stars are typically about 1.4 times the mass of the Sun but only about 20 kilometers in diameter. They have strong magnetic fields and can rotate rapidly, sometimes emitting beams of radiation that can be detected as pulsars when they sweep past Earth.