Supernova explosion. Image 1 of 4. Supercomputer simulation showing a supernova explosion of a 15- solar-mass star 0.05 seconds after core collapse. Supernovae occur when a massive star has run out of nuclear fuel and its core collapses to form a neutron star. The collapsed core triggers a shock wave that powers the supernova explosion. This simulation shows bubbles of hot, buoyant gas (light brown), 400 kilometres in diameter, that envelope the neutron star (unseen). In-falling cold gas bubbles are heated by neutrinos at the core. The heated gas rises and mixes with more in- falling cold gas resulting in turbulence. It is thought that neutrino-driven convection energy accompanies the shock wave to power the supernova explosion. Simulation created at the Max Planck Institute for Astrophysics, Germany. (For complete sequence, see images: R730/094 - R730/097).