Nucleosynthesis is the process of creating new atomic nuclei from preexisting nucleons (protons and neutrons). The primordial preexisting nucleons were formed from the quark-gluon plasma of the Big Bang as it cooled below ten million degrees. This first process may be called nucleogenesis, the genesis of nucleons in the universe. The subsequent nucleosynthesis of the elements occurs primarily either by nuclear fusion or nuclear fission.

History

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Historically 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 hard calculations for the conditions in stellar cores. Hans Bethe first gave a quantitative description of this process in the years immediately before World War II. Fred Hoyle's original work on nucleosynthesis of heavier elements in stars (including a detailed mechanistic analysis for the production of carbon) occurred just after World War II, but this work was in search of a way to produce heavier elements from hydrogen in stars, in the steady state model of cosmology. Subsequently, Hoyle's picture was expanded by creative contributions by E. Margaret Burbidge, Geoffrey Burbidge, William A. Fowler, Alistair G. W. Cameron, and Donald D. Clayton, and then by many others.

Processes

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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, helium burning, carbon burning, neon burning, oxygen burning and silicon burning. These processes are able to create elements up to iron and nickel (⁶²Ni is the isotope with the highest binding energy). 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.

Types of nucleosynthesis

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The types of nucleosynthesis known of are:

• Big Bang nucleosynthesis (see link for details), which occurred within the first three minutes of the universe is responsible for much of the abundance ratios of ¹H (protium), ²H (deuterium), helium-3 (³He), and helium-4 (⁴He), in the universe ^*. Although ⁴He continues to be produced by other mechanisms (such as stellar fusion and alpha decay) and trace amounts of ¹H continue to be produced by spallation and certain types of radioactive decay (proton emission and neutron decay), most of the mass of these isotopes in the universe, and all but the insignificant traces of the ³He and deuterium in the universe produced by rare processes such as cluster decay, are thought to have been produced in the Big Bang. The nuclei of these elements, along with some ⁷Li, are believed to have been formed when the universe was between 100 and 300 seconds old, after the primordial quark-gluon plasma froze out to form protons and neutrons. Because of the very short period in which Big Bang nucleosynthesis occurred before being stopped by expansion and cooling, no elements heavier than lithium could be formed. (Elements formed during this time were in the plasma state, and did not cool to the state of neutral atoms until much later).

• Stellar nucleosynthesis occurs in stars. It is responsible for generation of elements between helium and iron by nuclear fusion processes. Stars are the nuclear furnaces in which H and He are fused into heavier nuclei. Of particular importance is carbon, because its formation from He is a bottleneck in the entire process. 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 (⁵⁶Fe and ⁶²Ni). The products of stellar nucleosynthesis are generally distributed into the universe as planetary nebulae or through the solar wind.

• Explosive nucleosynthesis including supernova nucleosynthesis, produces most of the heavy elements present in the universe. In explosive environments such as supernovae further nucleosynthesis processes can occur, such as the r process (in which elements heavier than iron and nickel are produced by rapid absorption of free neutrons) and the rp process (which involves the rapid absorption of free protons).

• Cosmic ray spallation produces some of the lightest elements present in the universe (though not significant deuterium). Most notably spallation is believed to be responsible for the generation of all or almost all of ³He and the elements lithium, beryllium and boron. This process results from the impact of cosmic rays against the interstellar medium, fragmenting carbon, nitrogen and oxygen nuclei present in the cosmic rays. Note that Be and B are not significantly produced in stellar fusion processes, because the instability of any ⁸Be formed from two ⁴He nuclei prevents simple 2-particle reaction building-up of these elements.

Theories of nucleosynthesis are tested by calculating isotope abundances and comparing with observed results. Isotope abundances are typically calculated by calculating the transition rates between isotopes in a network. Often these calculations can be simplified as a few key reactions control the rate of other reactions.

References

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• E. M. Burbidge, G. R. Burbidge, W. A. Fowler, F. Hoyle, Synthesis of the Elements in Stars, Rev. Mod. Phys. 29 (1957) 547 (article at the Physical Review Online Archive (subscription required)).
• C. E. Rolfs, W. S. Rodney, Cauldrons in the Cosmos, Univ. of Chicago Press, 1988, ISBN 0226724573.
• D. D. Clayton, "Principles of Stellar Evolution and Nucleosynthesis", McGraw-Hill, 1968; University of Chicago Press, 1983, ISBN 0226109526

Astrophysics | Nuclear physics | Nucleosynthesis

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