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Gamma ray bursters (GRBs) appear as bursts of gamma rays coming from points randomly scattered in the sky. These bursts are very brief, lasting between a few milliseconds to a few hundred seconds. For a long time there were hundreds of theories about what caused them, but very little evidence for any of these theories, since nothing was ever seen at the location where one of these bursts occurred. Their random distribution eventually made a convincing case that they occurred not within our solar system or within our galaxy, but much farther away. Given this, it was clear that they must be extraordinarily powerful. Starting in the late1990s, astronomers made a concerted effort to catch gamma ray bursters in the act, focusing powerful telescopes to observe them in the visible and ultraviolet spectrum moments after a burst was detected. These efforts paid off in1999 when one was seen to emit visible light for as long as a day after the burst occurred. A redshift measurement of z =1.6 indicated that the gamma ray burster was about10,000 million light years away. If the burst of gamma rays was omnidirectional, this would mean that its power was about10^16 times that of our sun—for a very short time.
A more detailed observation of a burst on March3,2003 convinced many astrophysicists that at least some gamma-ray bursters are so-called "hypernovae". A hypernova is an exceptionally large supernova formed by the nearly instantaneous collapse of the core of a very large star, at least10 times the mass of the sun, which has already blown off most of its hydrogen. Such stars are called Wolf-Rayet stars. The collapse of such a star need not be spherically symmetric, so the gamma ray burst could be directional, reducing the total power needed to explain the brightness we see here (if the burst happened to point towards us).
