Hydrogen burning is the fusion of 4 hydrogen nuclei (protons) into a single helium nucleus (2 protons + 2 neutrons). The fusion process takes place via a series of reactions. Exactly which reactions take place in a given star depends on its mass, and therefore its core temperature and density. In the Sun, this process is called the proton-proton chain. In stars somewhat more massive than the Sun, a different sequence of reactions, called the C-N-O cycle (Carbon-Nitrogen-Oxygen), serves to fuse hydrogen into helium.
Stars on the Main Sequence produce their energy via hydrogen burning.
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