The Proton-Proton Cycle
     The proton-proton cycle is a series of nuclear reactions which convert hydrogen nuclei (protons) into helium nuclei (alpha-particles). All Main Sequence stars do this in one way or another. For stars like the Sun the process almost always begins with the collision of two protons, hence its name, the "proton-proton" cycle. In massive stars the process almost always involves carbon atoms catalyzing the reactions, and in the process being changed from carbon to nitrogen to oxygen and at the end, back to carbon; hence its name, the "carbon" cycle, or using the atomic symbols of the heavy atoms involved, the "CNO" cycle.
     Each "cycle" can be represented as a "chain" of reactions proceeding one after the other. Sometimes only one such chain is involved in a particular reaction, but usually there are several ways in which particles can combine, which result in more or less the same end. In such a case the chain which occurs most of the time (and usually produces the most overall energy) is the main chain, and those which occur less frequently are side chains.
     (For the rest of the discussion that was originally incorporated in the notes for the particular lecture referenced above see Main and Side Chains in Stellar Nucleosynthesis; however, much more was covered in class, as summarized by the illustration and caption below, and will be covered in more detail on this or the other page at a later date.)

     "Reading" from left to right two positively charged protons collide, emitting a neutrino (ν), a positron (e+) or anti-electron, and a deuteron (np+). The positron soon collides with a normal electron (e-) and suffers an annihilation reaction in which all of the mass and energy of motion are converted into a single gamma-ray (γ) photon. In the next step the deuteron collides with another proton, emitting another gamma-ray and forming a light helium nucleus (np+p+). Finally, under normal circumstances the light helium nucleus collides with another light helium nucleus, creating a normal helium nucleus (nnp+p+) or alpha-particle (α-particle) and emitting two protons. Looking at the two chains, top and bottom, that create the light helium nuclei, six protons are used up and two are returned, for a net conversion of four protons and two electrons into the alpha-particle.