A gamma-ray burst is the most energetic explosion known, releasing in seconds more energy than the Sun will emit in its entire lifetime. Long bursts, lasting more than about two seconds, come from the collapse of massive rapidly spinning stars into black holes; short ones come from merging neutron stars. The energy is beamed into narrow jets, so we only see the small fraction pointed our way. They were discovered by accident in the 1960s by satellites built to watch for clandestine nuclear tests.