An illustration for Scientific American in late 2002, this shows a gamma-ray burst, or GRB. These are the brightest explosions in the known Universe, now thought to arise from the merger of two black holes or from the collapse of a super-massive star — an event called a hypernova. The event, whatever it is, leaves a black hole surrounded by a disc of gas, which is in turn embedded in a halo of unused material. As material from the disc spirals to its doom in the black hole, it is heated to extremely high temperatures and emits powerful radiation. This material spews outwards in a jet and slams into the halo material, creating shockwaves. These are what produce the conspicuous gamma-ray emission.