A long time ago (1987) 160,000 light years away ... a star exploded. We've been watching the ensuing explosion ever since. I made a little sprite animation out of this image showing the supernova's progress over almost 20 years.
It's a truly beautiful phenomenon, and I think it's a great way to get a perspective of just how big stars are - when the explode they explode for decades and you can see them from hundreds of thousands of lightyears away. Amazing. Way more information at Wikipedia and the source of the sprite image Australia Telescope Outreach and Education