In this galaxy--NGC 6946, a spiral some 17 million light-years away--supernovas are fairly common: astronomers have observed six there over the past 80 years, but none as odd as the one shown in the upper photo. In some ways it appears to be a new supernova--it is an incredibly bright source of X-rays. Yet it has none of the other signatures of a freshly blown-up star: the gas isn’t moving very quickly and the elements visible in it are typical of those found in more mature supernova remnants. To solve the mystery, William Blair, an astronomer at Johns Hopkins, trained the Hubble Space Telescope on the remnant. What he saw were two supernova remnants--one behind and to the left of the other--slamming together. The object was smeared out in ground-based observations, Blair says, but with Hubble you can see the structure that shows the interaction. In this case, the picture pretty much tells the story. The two massive stars that formed the remnants were probably twins, born 40 light-years apart at roughly the same time. No such supernova collision has ever been seen before, and Blair says by next century these two remnants will be completely merged into one giant bubble. It’s very short-lived on a cosmic scale, Blair says. That’s why we don’t see these things too often.