long gamma-ray burst
When astronomers timed many gamma-ray bursts, they found the durations were not random but split into two clumps. The longer clump — bursts lasting more than about two seconds, and often tens of seconds — are the long gamma-ray bursts. These are the ones tied to the death of a single, very massive star, and they are the more commonly detected of the two kinds.
The leading model is the collapsar: a star many tens of times the mass of the Sun runs out of fuel, and its core collapses straight into a black hole. As the surrounding star falls inward, a swirling disk forms around the new black hole and launches twin jets that bore out through the star at nearly the speed of light. Internal collisions within the jet produce the gamma rays; the long duration reflects the time the engine keeps feeding, roughly the time it takes material to fall in. Because they trace dying massive stars, long bursts are found in galaxies busy with star formation, and they sit near the most active star-forming regions.
Long gamma-ray bursts matter because they are signposts of the most massive stars and the births of black holes, visible clear across the universe — we have seen them from when the cosmos was only a few percent of its present age. Many long bursts are also accompanied, days later, by the light of a supernova in the same spot, directly linking these bursts to the explosive collapse of massive stars. They are, in effect, the brightest funeral flares of the biggest stars.
In 1998, a long gamma-ray burst was followed weeks later by an unusually bright supernova in the very same spot — the first hard evidence that long bursts are the death cries of massive stars collapsing into black holes, with the burst and the supernova two faces of one catastrophe.
Bursts lasting seconds to minutes, from a massive star collapsing to a black hole — a collapsar.
The two-second split between long and short bursts is a useful rough boundary, not a sharp law — duration overlaps, and astronomers confirm a burst's true family using its spectrum and host galaxy, not the clock alone.