Nucleosynthesis is the process of creating new atomic nuclei from preexisting
nucleons (protons and neutrons). The primordial nucleons themselves were formed from the quark-gluon
plasma of the
Big Bang as it cooled below ten million degrees. A few minutes afterward, starting with only
protons and
neutrons, nuclei up to
lithium and
beryllium (both with mass number 7) were formed but only in relatively small amounts. This first process of
primordial nucleosynthesis may also be called nucleogenesis. The subsequent nucleosynthesis of the elements (including all
carbon, all
oxygen, etc.) occurs primarily in
stars either by
nuclear fusion or
nuclear fission.
The first ideas were that the chemical elements were created at the beginnings of the universe, but no successful picture could be found. Arthur Stanley Eddington first suggested in 1920 that stars obtain their energy by fusing hydrogen to helium, but this idea was not generally accepted because it lacked nuclear mechanisms. In the years immediately before World War II Hans Bethe first provided those nuclear mechanisms by which hydrogen is fused into helium. However, neither of these early works on stellar power addressed the origin of the elements heavier than helium. Fred Hoyle's original work on nucleosynthesis of heavier elements in stars occurred just after World War II (see Ref. list). This work attributed production of heavier elements from hydrogen in stars during the nuclear evolution of their compositions. Hoyle's work explained how the abundances of the elements increased with time as the galaxy aged. Subsequently, Hoyle's picture was expanded during the 1960s by creative contributions by William A. Fowler, Alistair G. W. Cameron, and Donald D. Clayton, and then by many others. The creative 1957 review paper by E. M. Burbidge, G. R. Burbidge, Fowler and Hoyle (see Ref. list) is a well-known summary of the state of the field in 1957. That paper defined new processes for changing one heavy nucleus into others within individual stars, processes that could be documented by astronomers.
There are a number of astrophysical processes which are believed to be responsible for nucleosynthesis in the universe. The majority of these occur within the hot matter inside stars. The successive nuclear fusion processes which occur inside stars are known as hydrogen burning (via the proton-proton chain or the CNO cycle), helium burning, carbon burning, neon burning, oxygen burning and silicon burning. These processes are able to create elements up to iron and nickel, the region of the isotopes having the highest binding energy per nucleon. Heavier elements can be assembled within stars by a neutron capture process known as the s process or in explosive environments, such as supernovae, by a number of processes. Some of the more important of these include the r process which involves rapid neutron captures, the rp process which involves rapid proton captures and the p process (sometimes known as the gamma process) which involves photodisintegration of existing nuclei.
Of particular importance is carbon, because its formation from He is a bottleneck in the entire process. Carbon is produced by the triple-alpha process in all stars. Carbon is also the main element used in the production of free neutrons within the stars, giving rise to the s process which involves the slow absorption of neutrons to produce elements heavier than iron and nickel (57Fe and 62Ni). Carbon and other elements formed by this process are also fundamental to life.
