I just saw a show on, I think it was NOVA, about research being done on Gamma Ray Bursts. While no one seems quite ready to say for sure, one of the more widely accepted theories seems to be that these are very very large stars that are suddenly and catastrophically collapsing into black holes, releasing a enormous burst of gamma ray energy in the process. The show indicated that all of these bursts, of which they detect almost one a day now, are located very far away in distant galaxies, and yet they are so powerful that they still outshine any other gamma ray source in the sky when they occur. It was suggested that the energy release from such an event, if it were to occur within our own galaxy, even several hundred light years away, would be enough to effectively broil and sterilize the surface of the earth.
What I am curious about, is why these events seem relatively common outside our own galaxy and yet never occur closer to home. Not that I'm complaining, mind you. But it does make me a little nervous- a star exlploding so violently that the gamma radiaiton could fry us even from several 100 light years distance?