Danish astronomer Tycho Brahe witnessed a supernova explosion on November 11, 1572. He didn’t know what it was, but he documented the suddenly brilliant star in the constellation of Cassiopeia (the Queen), which then slowly faded away. Modern astronomers believe Brahe had witnessed a type 1a supernova, the death of a star 1 billion times brighter than the sun. Astronomers think they discovered its companion star, solving a 400-year-old mystery.