Artist's impression of a gamma-ray burst. These bursts are extraordinarily high-energy events, created when a star in a distant galaxy explodes at the end of its life. This produces an intense stream of gamma rays that can last from a few seconds to a few hours. The violent burst then fades away, leaving a fainter afterglow that can be seen at X-ray, optical and radio wavelengths. Gamma-ray bursts emit such an enormous amount of energy that, when at their peak, they are the brightest and most powerful phenomena in the Universe. The source of such an extraordinary amount of energy is still uncertain, but there are several theories: jets escaping from the turbulent environment around a forming black hole, the merging of two compact objects such as neutron stars, or a beam of energy from a hypernova which is a very energetic type of supernova explosion following the death of an extremely massive star.