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.
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!
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).
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.
|Reaction...||becomes significant at|
|Proton-proton reaction||8 million Kelvin (K)|
|CNO cycle||20 million K|
|triple-alpha reaction||100 million K|
|carbon "burning"||600 million K|