Stars like the Sun (and stars less massive than the Sun) generate energy by the fusion of hydrogen into helium via the proton-proton process. For stars more massive than around two times the mass of the Sun, another energy generation process becomes important. (Note, however, that we still consider the fusion of hydrogen into helium--it is simply the process through which the burning occurs which is different)
We introduce what is referred to as the Carbon-Nitrogen-Oxygen (CNO) cycle.
Well, note that the initial reaction involves a carbon nucleus (with 6 protons) and a hydrogen nucleus (1 proton). Because of the large electrical charge of the carbon nucleus, there is a stronger electrical repulsion. Consequently, a higher temperature is needed in order for the reaction to take place.
