Hi Steve

Average stars don't explode as hypernova/gamma-ray bursts. They usually puff up into red-giants and then become slowly cooling white-dwarfs.

Stars above 8 solar masses explode as supernova when their cores collapse after fusion-burning all the way to iron - iron can't fuse or fission without an energy input and so it's unable to resist the pressure from all the star's mass above it. Usually the core collapses into a ball of neutronium - a neutron star - but bigger stars can push past the neutronium limit can collapse into a black-hole. According to current theory that's the best explanation for a long-duration gamma-ray burst.

You might wonder if the stars are so heavy then why don't they live longer than the Sun? It's due to the way the pressure of all that mass squeezes on the star's core - it actually speeds up the fusion burning rate and causes a roughly (M)^4 relation between mass and luminosity. In otherwords a 2 solar mass star burns (2)^4 = 16 times faster and has 1/8th the lifespan. The heavier the star the closer the relationship gets to (M)^3 - thus a 20 solar mass star burns at 8,000 times the Sun's luminosity and lasts 1/400th as long. That's the kind of star that explodes as a gamma-burst.

Adam