The Sun shines because of the process of fusion where four protons fuse to form an alpha particle , two positrons (e⁺), and two neutrinos , that is, 4p + 2e⁺ + 2. In this fusion process of hydrogen nuclei into helium nuclei, the latter also known as alpha particles, the fusion can be accomplished through two different series of principal reactions: 98.5% of the energy generat ion in the present day Sun comes from the proton-proton chain (p-p chain); 1.5% of the solar energy output is due to the Carbon-Nitrogen-Oxygen cycle (CNO cycle). The p-p chain and the CNO cycle are shown in Table 1-3; there the third column indicates the percentage of the solar terminations of the p-p chain in each reaction. Since the dependence of the energy generation rate (compare equation (8)) on the temperature is quite different between p-p chain and CNO cycle, the p-p chain dominates at low temperature (T 18 10⁶K) and the CNO cycle does not become important until high temperature is reached. At the present stage in the evolution of the Sun, the CNO cycle is believed to play a rather small role in the energy and neutrino production budget (Bahcall 1989).

    In the first reaction of the p-p chain, a proton decays into a neutron in the immediate vicinity of another proton. The two particles form a heavy variety of hydrogen known as deuterium, along with a positron and an electronneutrino. There is a second reaction in the p-p chain producing deuterium and a neutrino by involving two protons and an electron. This reaction (pep-reaction) is 230 times less likely to occur in the solar core than the first reaction between two protons (pp-reaction). The deuterium nucleus produced in the pp- or pep-reaction fuses with another proton to form helium-3 and a gamma ray. About 88% of the time the p-p chain is completed when two helium-3 nuclei react to form an helium-4 nucleus and two protons, which may return to the beginning of the p-p chain. However, 12% of the time, a helium-3 nucleus fuses with a helium-4 nucleus to produce beryllium-7 and a gamma ray. In turn the beryllium-7 nucleus absorbs an electron and transmutes into lithium-7 and an electronneutrino. Only once for every 5000 completions of the p-p chain, beryllium-7 reacts with a proton to produce boron-8 which immediately decays into two helium-4 nuclei, a positron and an electronneutrino.

    The net result of either the p-p chain or CNO cycle is the production of helium nuclei and minor abundances of heavier elements as ⁷Be, ⁷Li, ⁸Be, ⁸B (in the case of the p-p chain) or ¹³N, ¹⁴N, ¹⁵N (in the case of the CNO cycle). The energy generated by thermonuclear reactions in the form of gamma rays is streaming (actually, diffusing) toward the solar surface, thereby getting scattered, absorbed and reemitted by nuclei and electrons. On their way outward, the high-energy gamma ray is progressively changed to x-ray, to extreme ultraviolet ray, to ultraviolet ray and finally emerges mainly as visible light from the solar surface and radiates into outer space. Only the weakly interacting neutrinos can leave the solar core with almost no interaction with solar matter. However, the chlorine experiment of Davis and collaborators, the Japanese Kamiokande experiment, and the GALLEX experiment at Gran Sasso to detect solar neutrinos show that the Sun emits fewer of these elusive particles than the standard solar model predicts (Iben 1969; Lande 1989; Hirata et al. 1991; Anselmann et al. 1992 a,b). Since the beginning of the 70's this deficit challenges current understanding of solar and neutrino physics and of the process by which the Sun shines. The mystery of the missing solar neutrinos is commonly referred to as the "solar neutrino problem" (Bahcall and Davis 1982).