What was the biggest supernova to ever occur?

Probably the brightest one ever seen was in the year 1054. The Chinese astronomers at the time noted that a "guest star" appeared in the constellation of Taurus. It was so bright it could be seen in the daytime! That made it brighter than Venus. Some American Indians may have seen it too-- some drawings on the side of a rock in the U.S. desert look like an artist's rendition of the event, with a bright star near the crescent Moon.

Now, almost a thousand years later, we have identified the spot where this occurred. The Crab Nebula is one of the most studied objects in the sky. We can even see it expanding slowly (to us!). If you run that expansion backwards and figure out when it started, you get about the year 1100! So we know it's the same object that the Chinese saw.

But not all supernovae are bright. One star that blew up in the year 1066 didn't get very bright at all, even though it was pretty close by as those things go. We think now that the brightness of a supernova depends a lot on what the star was doing right before it blew up. When a heavy star explodes, it's really the innermost core of it that blows up. A lot of the light we see is due to the explosion interacting with the outer part of the star. Now some stars have a wind of gas that they always blow (the sun has one too, called the solar wind). If that wind is strong, a lot of the outer part of the star blows away before it explodes. If that happens, there is nothing for the core to light up, and the supernova doesn't get very bright! On the other hand, when a star does have that thick layer, the core explosion has to fight its way through it, losing energy (just like we use up energy to walk up a flight of stairs). So the more energetic explosions probably aren't the brightest ones! It's sort of the opposite of what you expect .

One more thing: the typical supernova is bright for a couple of months or so. During that time, the amount it puts out every second is equal to the amount of energy the Sun produces in its entire lifetime! Supernovae are incredibly violent and energetic events, which is why we can see them from so far away!

There is a vast amount of information about supernovae on the web. My web page has a good starting point, with pictures and descriptions of supernova 1987A, the last bright supernova, and links to other sites as well.