Figure 3
Collapsar model
Astronomers now believe that gamma-ray bursts occur when a massive star collapses to form a black hole or an unusual neutron star. According to the "collapsar" model of gamma-ray-burst formation, the inner core of certain rotating stars first collapses to form a black hole and an accretion disk (not shown). The disk and the black hole then interact to form two jets moving out from the star in opposite directions along the rotational axis. One jet is shown here travelling from left to right as it is about to burst through the parent star. Matter in these jets travels near the speed of light and is less dense than the star. This contrast in density helps to keep the jet focused along a small angle: low-density regions are shown in blue and high-density regions in yellow and white. After the jet erupts from the surface and travels hundreds of stellar radii, collisions within the jet produce gamma rays that are beamed in the direction of an observer, who is assumed here to be a few degrees off the axis. Other energetic phenomena may be seen by observers further off-axis. This simulation was performed by Weiqun Zhang, Stan Woosley and Alexander Heger at the National Energy Research Scientific Computing Center in the US.