On August 17th 1885 the French astronomer Prof L. Gully at Rouen in France saw a star near the centre of the Andromeda galaxy, it just about reached naked eye brightness it could just be seen without binoculars or telescopes, but this was no ordinary star, this was a supernova. A supernova is a star that destroys itself in a massive explosion.
The star was designated as S Andromeda, at this time astronomers did not know how far away the Andromeda Spiral as it was then called was away from the Sun. It was assumed to be a spiral group of stars within our galaxy. Therefore they could not work out how bright the star had really become. S Andromeda was in fact the first extra galactic supernova to have been discovered.
At this time stars that suddenly appeared in the sky were referred to as nova or ‘new stars’ (nova means new in Latin), but S Andromeda was different and although the astronomers at the time did not know it,but this was a special nova, a ‘supernova’, although the term would not be introduced until the 1930s.
When later the distance to the Andromeda Galaxy as we now call it was worked out, the modern estimate is about 2.2 million light years we know that S Andromeda was about 1.6 billion times brighter than the Sun.