A neutron star is the collapsed core left behind when a massive star explodes as a supernova. With roughly the mass of the Sun packed into a sphere about twenty kilometers across, its matter is so dense that a sugar-cube-sized piece would weigh as much as a mountain. Neutron stars spin rapidly, carry titanic magnetic fields, and appear as pulsars and magnetars. When two of them collide they forge heavy elements like gold and platinum and ripple spacetime with gravitational waves.