Nucleosynthesis

Nucleosynthesis is the science of astrophysical processes which are responsible for the abundances of the elements and their isotopes in the universe. The astrophysical sites of interest are the Big Bang and stellar objects, either during their stable (hydrostatic) evolution and wind ejection or during explosions like novae and supernovae, or possibly other events, where binary stellar systems are involved. The understanding of these environments requires in general hydro (fluid/gas) dynamics, thermodynamics and energy transport. Nuclear abundances and energy generation are determined by thermonuclear reactions.

In a typical Nucleosynthesis process, heavier chemical elements are produced from the fusion or joining of lighter chemical elements (e.g., hydrogen or helium nuclei) in thermonuclear reactions in stellar interiors. There are four main domains involved in nucleosynthesis: the early stages of the big bang; the energy-generating reactions inside stars; reactions during nova and supernova explosions; and cosmic ray collisions within the interstellar medium.

The first nucleosynthesis reactions to be discovered were the proton–proton reaction and the carbon–nitrogen–oxygen cycle. Both of these were suggested in 1938 to be sources for the energy of the Sun, the former by Hans Bethe, and the latter, independently of each other, by Bethe and Carl von Weizsäcker. These processes both convert hydrogen into helium-4, the proton–proton reaction being most important for stars with masses up to that of the Sun and the carbon–nitrogen–oxygen cycle becoming dominant within more massive stars.

In the mid 50s, Geoffrey and Margaret Burbidge, Sir William Fowler and Sir Fred Hoyle identified most of the remaining reactions inside stars that synthesize the elements up to iron, though only the most massive stars are able to follow the whole sequence. The next stage after helium formation is not the generation of beryllium-8, because that decays back to two helium-4 nuclei in 2 x 10^-16 s, but it is the Triple-α process producing carbon-12. The triple- α process involves two helium nuclei combining and then a third being added before the beryllium-8 can decay. It requires a minimum temperature of about 10⁸ K before it will start since the nuclear reactions involved are slightly endothermic (requiring the supply of energy) and therefore need the input of thermal energy to get them to occur. The triple- α process overall is exothermic: more energy is released by the process than is used in starting it, because the carbon-12 nucleus that is the result of the reaction is in an excited state and emits a high-energy gamma ray.

Once carbon-12 has been produced, helium-4 nuclei are added to it (a process known as α-capture) to produce, successively, oxygen-16, neon-20, magnesium-24, silicon-28, sulphur-32 and argon-36. For the Sun, the sequence probably comes to a halt with the production of oxygen-16.  α-capture can also start from other elements and this is particularly significant for the nitrogen-14 left by the carbon–nitrogen–oxygen cycle, leading to ?uorine-18 and neon-22. As the temperature rises, carbon and oxygen start to combine directly and eventually, at about 3 x 10⁹ K, silicon-28 combines with itself to produce nickel-56. The latter is a radioactive element and decays to cobalt-56 and then to the stable isotope iron-56, although those decay processes will occur outside the star, because the production of nickel at the star’s centre occurs immediately prior to its explosion as a supernova. There are many more nuclear reactions than are listed here, and some result in the release of neutrons. Those neutrons combine with the existing nuclei to produce intermediate elements and isotopes.

Questions to Ponder

• How are elements heavier than iron formed?
• What is Stellar Nucleosynthesis?