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 tons of hydrogen in its core every second.