Animation of an expanding supernova remnant (SNR). A supernova is the explosive death of a massive star. When it runs out of fuel at the end of its life the outer layers collapse under gravity, before rebounding off the extremely dense core. This leads to a huge release of energy and the star's constituents, which are thrown outwards at high speeds. The event produces temperatures and densities high enough to fuse lighter elements into very heavy elements. The nuclear reactions during the explosion produce the majority of the elements heavier than nickel in the universe. SNRs are the visible remnants of these events, glowing as the ejected matter ploughs into the interstellar medium, heating it and emitting light.