The formation of dead stars

The types of stars described in Chapters 13 and 14 are held up by the thermal pressure of hot gas, the source of energy to provide the pressure being nuclear energy generation in their cores. As evolution proceeds off the main sequence, up the giant branch and towards the final phases when the outer layers of the star are ejected, the nuclear processing continues further and further along the route to using up the available nuclear energy resources of the star. The more massive the star, the more rapidly it evolves and the further it can proceed along the path to the formation of iron, the most stable of the chemical elements. An intriguing question is whether or not the star is disrupted by the various ‘flashes’ which are expected to take place as new regimes of nucleosynthesis are switched on, for example, at the points E and G in Fig. 13.19. In the most massive stars, M ≥ 10M⊙, it is likely that the nuclear burning can proceed all the way through to iron, whereas, in less massive stars, the oxygen flash, which occurs when core burning of oxygen begins, may be sufficient to disrupt the star. In any case, at the end of these phases of stellar evolution, the core of the star runs out of nuclear fuel, and it collapses until some other form of pressure support enables a new equilibrium configuration to be attained.

The possible equilibrium configurations which can exist when the star collapses are white dwarfs, neutron stars and black holes.