A supernova is the explosion of a star at the end of its life. When this happens, huge amounts of matter and energy are released in the explosion, giving rise to a bright new object in the sky. This start remnant remains visible for a few weeks and then dies away.

Supernova 2005ap shown with four nearby galaxies (A, B, C, and D). The photo on the right was taken two and half months later than the one on the left.

Because they are explosions, you know astronomers will keep a league table of the most powerful explosions. And so it goes that a few days ago, the most powerful supernova explosion was detected. Called SN 2005ap, the paper describes the measurements that put the explosion's peak brightness at more than 100 billion times the brightness of the Sun and placing the supernova at 4.7 billion light-years away from us. To re-phrase the distance measurement, the explosion happened 4.7 billion years ago, but only now are we seeing it because light needs time to travel that distance.

I heard about the news from Bad Astronomy Blog, which also has a lot more details about the science behind supernova explosions and measurements.