What is a supernova?

A supernova is the explosion caused when a massive star, at least eight times the Sun's mass, exhausts its fuel (or dies) and then collapses. If the original star is less than 20 solar masses, the supernova will leave behind a neutron star, or remnant. Heavier stars will collapse into black holes. Supernova explosions are among the most energetic events in the universe, and are a rare sight. The last supernova seen in our galaxy, the Milky Way, was in 1604.