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A massive star that undergoes a sudden, extreme increase in brightness across the electromagnetic spectrum, followed by a more gradual decrease lasting from several days to several months. Supernovae occur when a supergiant star collapses suddenly at the end of its life, condensing its core material into an extremely compact mass that then undergoes a slight rebound. The resulting shock wave sends all matter surrounding the core flying into space, leaving a neutron star or black hole at the site of the core's collapse. Supernovae may also occur when a white dwarf accretes material from a companion red giant star, resulting in an increase in mass that eventually triggers carbon fusion in the core of the white dwarf; the sudden increase in available fuel causes energy to be released in a violent explosion. In both cases the shock waves induce further fusion in the matter surrounding the collapsed core; the many elements resulting from this fusion and from the various other stages of nucleosynthesis over the lifetime of the star are scattered into space. These elements serve as the material from which new stellar and planetary systems are formed; in fact, every heavy element found on Earth is thought to have been the product of supernovae explosions. The last supernova to be observed in the Milky Way was seen in 1604 by Johannes Kepler and was used by Galileo, at his trial, as evidence against the presupposition that the universe never changes.

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