Stellar nucleosynthesis describes the nuclear reactions taking place in the centres of stars to build the nuclei of the heavier elements. Big Bang nucleosynthesis is the similar process that happened in the first few minutes of extreme pressures and temperatures (and before stars existed) after the Big Bang.

Prior to the early twentieth century, it was not known what produced the energy of the Sun. Various theories were postulated, and later dismissed including gravitational contraction, chemical reactions and radioactivity. None of these successfuly accounted for the amount of energy, nor the longevity of the Sun.

The prime energy producer in the Sun is the fusion of hydrogen to helium, which occurs at a minimum temperature of 3 million kelvin. Fusion involves the formation of a new atom from existing ones, as well as the release of energy in the form of electromagnetic radiation (e.g. heat, light, X-rays, gamma rays) and perhaps particles such as neutrinos, electrons, etc.

Schematic of the end products of fusion

The complete conversion of hydrogen to helium - commonly called the proton-proton chain is shown here:

The complete Proton-proton chain. H is converted into He along with 2 positrons, 2 neutrinos and 2 gamma rays.

The energy output of the Sun is traced back to Einstein's famous equation:

E = mc²

The two emitted gamma rays from the proton-proton chain have an energy = 0.43×10^-11 joules. To produce the Sun's luminosity a huge 6 × 10¹¹ kg of hydrogen must be converted into helium each second. However the Sun's mass of 2 × 10³⁰ kg has supported such a conversion for the last 4.6 billion years and it will continue for another 5 billion years!

Although in our Sun it is the proton-proton chain which dominates (91%), in other stars other fusion reactions are very important. Here are the main ones:

CNO cycle: A complex series of reactions in which the transformation carbon - nitrogen - carbon - nitrogen - oxygen - nitrogen - carbon facilitates the conversion of four protons to one helium nucleus (plus energy).

Helium "burning": Three helium nuclei fuse to create one carbon nucleus (plus energy). This is also called the "triple-alpha reaction".

Carbon "burning": Carbon is fused to form heavy elements (plus energy): in particular, iron is the final product of much carbon burning.

Hence nucleosynthesis is the creation of new (and heavier) elements due to fusion, with some liberated energy. The more positively-charged a nucleus is, the harder it will be for it to approach closely enough to another nucleus so that fusion can occur. More pressure (P) and temperature (T) will be needed.

Big Bang nucleosynthesis built H, He, Li and some Beryllium. All other elements were made inside the centres of stars by stellar nucleosynthesis.