We know that nuclear processes are very active in the Sun. The extreme high pressures, densities, and temperatures create an environment in which atomic nuclei are brought repeatedly into close proximity, resulting in frequent interactions. Both fission (the splitting of nuclei into multiple components via radioactive decay of unstable nuclei) and fusion (the combination of protons) are highly effective processes in the cores of stars. When either fission or fusion reactions take place in the Sun, the weight of the initial reactants is always slightly more than the weight of the final products, because small amounts of mass are converted into energy. These reactions release only small amounts of energy each time they occur, but that energy adds up quickly enough to power the entire Sun! We will review these mechanisms below.

• Fission
  Fission requires the presence of considerable unstable (radioactive) nuclei, found in heavy, high atomic number elements. In a fission process, an object is divided into parts (think of the word fissure, meaning a gap). Remember that the atomic number (the number of protons in each nucleus, defining its elemental composition) is written as a subscript for each atom in our reaction, while the total number of protons and neutrons is written as a superscript. In nuclear fission, a heavy atom can be divided into two atoms, with atomic numbers that sum to equal the atomic number of the initial radioactive atom. A few gamma rays (high-energy photons, designated by the symbol ) will be released in the process. Fission requires the presence of substantial amounts of radioactive materials as initial reactants, and as these materials are relatively rare in stars, fission does not product as much energy as fusion does for powering the Sun.

• Fusion
  Fusion requires lots of protons, or the nuclei of hydrogen atom without companion electrons. The word fusion relates to the verb to fuse, meaning to combine. In nuclear fusion via the proton-proton chain, for example, four individual protons are combined within a stellar core to form a helium atom. Two of the protons stay in their initial form, giving us an atomic number of two (and thus defining the final element as helium). The two remaining protons transform into neutrons, and two positrons (positively charged electrons, designated as e) are released, so that charge is conserved. As with the fission process, energy is released in the form of gamma-rays, and a few neutrinos () are also produced.