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 abbreviations 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.
     As an example, let's break the proton-proton cycle into its components. The start of this series of reactions is the collision of two protons, creating a deuteron (a hydrogen nucleus with one proton, as is normal, but with a neutron, as well, which is much less common). Since there are two positive charges on the two protons (one each), and only one on the deuteron, a positive charge must be carried off. In this case, that happens through the emission of a positron, or anti-electron -- a particle of antimatter. The reaction also produces a nearly massless particle, with an energy of 0.42 million electron-volts (MeV), called an electron neutrino (n_e):

(Main chain start) p⁺ + p⁺ -> p⁺n + e⁺ + n_e (0.42MeV)

     The positron produced in this reaction will almost immediately combine with a normal electron, resulting in the annihilation of the two particles (the total conversion of their mass into energy), and the creation of two gamma-ray photons, with a combined energy of 1.02 MeV:

(Main chain second reaction) e⁺ + e^- -> 2 g (1.02MeV)

     Occasionally, however (about once for every 400 reactions of the sort shown above), the collision of two protons involves a simultaneous collision with an electron. In this case, the energy normally produced by the positron-electron anniliation is not released as two gamma-rays, but transferred to the electron neutrino, giving it a much higher energy (equal to the combined energy of the two steps shown above):

(Side chain start) p⁺ + p⁺ + e^- -> p⁺n + n_e (1.44 MeV)

(more to follow)

The normal progression of the proton-proton cycle
(Wikimedia Commons image, altered by the author, for this discussion)

"Reading" from left to right, two positively charged protons collide, emitting a neutrino (n), 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 (g) 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 (a-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 loss of four protons and two electrons, in creating the alpha-particle.

(far more to follow)