The nature of these explosions has long been a mystery. Observations show that GRBs are one of two types: short- (less than a few seconds) and long-duration. Until 2003, it was suspected that two different types of cosmic event caused them. In 2003, ESO telescopes followed the aftermath of an explosion for one month. Their data showed that the light had similar properties to that of a supernova, and allowed astronomers to link long-duration GRBs with the ultimate explosions of massive stars (hypernovae). In 2005, ESO telescopes detected visible light after a short-duration burst and tracked this light for three weeks. The conclusion was that short-duration bursts could not be caused by a hypernova; instead, it is thought that they originate in the violent merges of neutron stars or black holes.