An intense burst of gamma-ray radiation, lasting from a few milliseconds to a few tens of minutes. Gamma-ray bursts were initially detected in the late 1960s by US Vela satellites designed to monitor nuclear explosions, and have since been studied in detail by specialized satellites. The bursts are distributed uniformly over the sky, indicating that they originate in distant galaxies and fall into two types, short and long, suggesting two distinct mechanisms. Short bursts last from a few milliseconds up to 2 seconds; these are believed to be caused by the merger of two compact objects such as black holes, neutron stars, or a white dwarf and a neutron star. Long bursts last from a few seconds to over a thousand seconds and are believed to be caused by massive stars exploding as a supernova or hypernova, with the collapsing core of the star forming a rapidly rotating black hole. In both scenarios, an outflow of gas creates shells that collide internally at close to the speed of light, producing the burst of gamma rays. These gamma rays are highly directional and hence we can detect only those bursts emitted towards us. The combined shells expand and collide with the surrounding gas and dust of the interstellar medium, heating it so that it begins to emit afterglow radiation at X-ray wavelengths. As the gas cools the afterglow becomes visible at optical, infrared, and radio wavelengths, and this emission may remain detectable for days to years. See also soft gamma-ray repeater.