Hydrogen burning is the fusion of hydrogen into helium – the process by which all stars on the main sequence generate energy. Every star born with more than 0.08 solar mass "burns" hydrogen but the exact means by which hydrogen is converted to helium depends on the core temperature of the star, which in turn depends on the star's mass. Below 1.2 solar masses, hydrogen burning takes place almost exclusively via the proton-proton chain.

In stars heavier than 1.2 solar masses, in which the core temperature exceeds 18 million K, there is some contribution to hydrogen burning from the carbon-nitrogen cycle (also known as the carbon-nitrogen-oxygen cycle). Above 3 solar masses, a star converts hydrogen to helium almost exclusively by way of the CN cycle.