When a star is on the main sequence, its basic source of energy is the conversion of hydrogen into helium. We start with four protons and end up with one 4He nucleus. However, it is unlikely that four protons will get close enough to directly form a 4He nucleus in a single reaction. There are different series of reactions that achieve this net result, and they will be discussed below.
We can calculate the energy released by converting four protons to one 4He by comparing their masses. We find that
This means that 0.007 of the mass of each proton is converted into energy. (The 007 will be familiar to James Bond fans.)
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