They're not completely sure, but: > There are at least two different types of progenitors (sources) of GRBs: one responsible for the long-duration, soft-spectr

They're not completely sure, but: > There are at least two different types of progenitors (sources) of GRBs: one responsible for the long-duration, soft-spectrum bursts and one (or possibly more) responsible for short-duration, hard-spectrum bursts. The progenitors of long GRBs are believed to be massive, low-metallicity stars exploding due to the collapse of their cores. The progenitors of short GRBs are still unknown but mergers of neutron stars is probably the most popular model as of 2007. http://en.wikipedia.org/wiki/Gamma-ray_burst_progenitors http://en.wikipedia.org/wiki/Gamma-ray_burst_progenitors