A neutron star is the dense remnant of a massive star that has undergone a supernova explosion. After the star exhausts its nuclear fuel, it collapses under its own gravity, compressing protons and electrons to form neutrons. These stars are incredibly small, typically about 20 kilometers in diameter, yet they can have a mass greater than that of the Sun. Neutron stars are known for their extreme density and strong gravitational fields. A sugar-cube-sized amount of neutron star material would weigh about 6 billion tons on Earth. Some neutron stars also emit beams of radiation, becoming known as pulsars when these beams sweep across the sky like a lighthouse.