The proton-proton chain is a series of nuclear fusion reactions by which stars convert hydrogen to helium. The general result of the proton-proton chain is the conversion of four hydrogen nuclei (protons) into one helium nucleus, with the emission of energy and other products. The complete chain releases a net energy of 26.732 MeV, but 2.2 percent of this energy (0.59 MeV) is lost to the neutrinos that are produced. The proton-proton chain is not a chain reaction in the normal sense, but rather a series of reactions where the product of one reaction is the starting material of the next reaction. The chain has two branches, p-p I and p-p II, which dominate at different temperatures. The p-p I branch is dominant at temperatures of 10 to 18 MK, while the p-p II branch occurs only at relatively high temperatures and is of interest because such energetic neutrinos were detected in a large-scale experiment using tetrachloroethylene.