Initially, population III stars were dependent on the proton-proton chain for energy production. However, over time, increasing quantities of 12C were produced through the triple alpha process. This 12C catalysed the more rapid formation of 4He via the CNO cycle, increasing the rate of energy production inside the star and heating it up; see fig. 1.
The production of 12C by the first stars led to the CNO cycle starting up prematurely in later generations of stars. The additional energy it produced would blow larger stars apart as they formed, making it impossible for stars as large as population III stars to ever form again.
After a few million years, population III stars ran out of H/He fuel and couldn’t produce the energy needed to support themselves. As population III stars were so massive, they typically ended their lives as black holes or pair-instability supernovae.
As population III stars heated up, energy produced by exothermic nucleosynthetic reactions was black-body radiated into the interstellar medium.
The emergence of the hot population III stars marked an end to the cosmic dark ages and the initiation of a reionisation epoch. The radiation they produced reionised the interstellar medium, increasing its opacity and flooding the universe with photons. Importantly, the soft 11.2-13.6 eV UV radiation emitted by the hot population III stars acted to destroy the fragile bonds between hydrogen atoms. This suppressed the hydrogen cooling mechanism and so made it more difficult for new population III stars to form.