A supernova is a vast explosion in which an entire star is destroyed. After the explosion, extremely bright light is emitted for several days. Supernovas appear a billion times brighter than the Sun. Sometimes, a supernova explosion can go on for weeks or even months. Supernovas mostly occur in distant galaxies. The last supernova to take place in the Milky Way occurred in 1604. It was observed by the famous astronomer Johannes Kepler. The brightest supernova to be recorded so far is 1993J in the galaxy M81. It was seen on 26 March 1993. But because the stars are so far away, we may see a supernova explosion long after it takes place.