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While smaller stars use the proton-proton chain to create helium and release energy, very massive stars use the carbon-nitrogen-oxygen (or CNO) cycle to create helium. CNO cycle comes in 'hot' and 'cold' varieties depending on the mass of the star in which it occurs.
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Unlike the latter, the CNO cycle is a catalytic cycle. Theoretical models suggest that the CNO cycle is the dominant source of energy in stars more massive than about 1.3 times the mass of the Sun. The proton–proton chain is more important in stars the mass of the Sun or less.In stars the primary constituents are hydrogen and helium, but there are usually (much) smaller amounts of heavier elements present. In particular there can be Carbon (C), Nitrogen (N), and Oxygen (O) ions. If these are present, they can participate in the sequence of reactions.
