Animation of a type II supernova, the explosive death of a massive star. Stars produce energy by fusing light elements into heavier ones, starting with hydrogen into helium. Stars with more than about eight solar masses can fuse the resulting elements together until the reactions produce nickel-56, which is the most stable nucleus. Any further fusion consumes rather than releases energy, so the nickel accumulates in an inert core, surrounded by layers still fusing lighter elements. When this core reaches a mass of about 1.4 solar masses it can no longer support itself against gravity, and it abruptly collapses. The collapse is only stopped when the core reaches the density of a nucleus, forming a neutron star, and the infalling layers rebound from this core and, powered by the release of gravitational energy, rebound in an enormous explosion. The resulting supernova remnant contains a wide variety of elements produced in the star and its sudden death, which enrich the surrounding space.