The stars aren't fixed and unchanging, unlike what many ancient people thought. Once in a while, a star appears where there wasn't one before, and then it fades away in a matter of days or weeks.
In 1572, the famous Danish astronomer Tycho Brahe observed a new star in the constellation Cassiopeia. After reporting the event in his work 'De Nova Stella' or 'On the New Star,' astronomers came to associate the word nova with stellar explosions.
Very massive stars - those more than eight times the mass of our Sun - explode in dramatic supernova explosions, like the ones people observed in 1054 and 1572.
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