Scheme of branch I of p–p chain. (Credit: Wikimedia Commons)

A fusion reaction that takes place in the cores of Sun-like stars, in which hydrogen is gradually transformed into helium. The proton–proton chain consists of a series of reactions during which protons fuse creating deuterium, positrons, neutrinos and helium nuclei. Other branches of these reactions create lithium, boron or beryllium. The ignition temperature of the p–p chain is 15 million kelvin and it is a relatively inefficient, slow reaction. Because the Sun is huge, it nevertheless burns 500 million metric tonnes of hydrogen in its core every second.