Gamma-ray bursts are the most energetic and luminous events known to occur in the Universe. Short-lived flashes of gamma rays that typically last from a a tenth of a second to less than an hour, gamma-ray bursts may for a brief period of time outshine entire galaxies. The explosions are believed to be caused by the collapse of massive stars, the collision of neutron stars, or the merging of a neutron star and a black hole.