P-P Chains

The p-p chain is the series of nuclear reactions converting four protons to one He nucleus and releasing an energy of about 6.6 MeV/nucleon or 5 10 erg/gr. This provides the main source of energy for Main sequence ▶ stars of less than 1.3 M. while in more massive stars the ▶CNO cycle dominates. The first reaction of the series (p + p ! D + e + ne + 0.42 MeV) involves a slow, weak interaction (the conversion of a proton into a neutron), which is responsible for the long lifetimes (>10 Gyr) of the Sun and other low mass stars.