A chain of nuclear reactions inside stars that converts hydrogen into helium, with the associated release of energy. In the reaction, four hydrogen nuclei (protons) fuse to form one nucleus of helium, with the production of a number of intermediate nuclei such as deuterium and isotopes of lithium, beryllium, and boron. At temperatures below 18 million K the proton–proton reaction is more important than the carbon–nitrogen cycle, and thus operates chiefly in stars of less than 2 solar masses.