Stars of at least 100 solar masses or so can reach core temperatures (and, thus, core photon energies) great enough that pair production (where a very-high-energy photon strikes another particle, transforming the photon into a matched particle-antiparticle pair - usually an electron-positron pair, though other types are possible - and causing the other particle to recoil slightly) starts to occur in earnest. As this is an endothermic (energy-absorbing) process, it reduces the temperature and pressure within the star’s core, causing the star to start to collapse under its own weight.
One of three things can now happen:
- In high-metallicity stars, and in low-metallicity stars up to ~130 MS, the resultant increase in core temperature and pressure halts the collapse before it can do anything irrevocable, and the star heats up and expands back out again, blowing off significant mass in the process. These pulsations will continue until either the star becomes too small and cold for much pair production or it explodes for some other reason.
- In low-metallicity stars from ~130 to ~250 MS, the collapse compresses and heats the interior of the star quickly and vigorously enough that the resulting increase in core reaction rate and energy release is sufficient to unbind the entire star.
- In low-metallicity stars exceeding ~250 MS, the collapse is so rapid, and the rise in core temperature so great, that an increasingly-large fraction of the photons produced have energies high enough to cause photodisintegration (where an extremely-high-energy photon is absorbed by an atomic nucleus, causing the nucleus to break apart into two or more smaller pieces), robbing the star’s core of energy and causing the entire star to collapse directly into a black hole.
Both pair production and photodisintegration are endothermic (energy-absorbing) processes, and, thus, tend to cause the star to collapse. However, their eventual effects on the collapsing star are different; pair production causes only a partial collapse, followed by runaway thermonuclear fusion and a supernova explosion, while photodisintegration causes a complete collapse into a black hole, with nothing escaping.
Why does core energy loss resulting from pair production result in a partial collapse, runaway fusion, and a supernova, while core energy loss resulting from photodisintegration results in a complete collapse to a black hole? Why don’t these two endothermic processes either both result in a partial collapse with resulting explosive fusion, or both result in total collapse to a black hole?