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Stellar nucleosynthesis
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Stellar nucleosynthesis is the collective term for the nuclear reactions taking place in stars to build the nuclei of the heavier elements. (For other such processes, see nucleosynthesis.)
The processes involved began to be understood early in the twentieth century, when it was first realized that the energy released from nuclear reactions accounted for the longevity of the Sun as a source of heat and light. The prime energy producer in the sun is the fusion of hydrogen to helium, which occurs at a minimum temperature of 3 million kelvins.
HistoryIn 1920, Arthur Eddington, on the basis of the precise measurements of atoms by F.W. Aston, was the first to suggest that stars obtained their energy from nuclear fusion of hydrogen to form helium.
In 1928, George Gamow derived what is now called the Gamow factor, a quantum-mechanical formula that gave the probability of bringing two nuclei sufficiently close for the strong nuclear force to overcome the Coulomb barrier.
The Gamow factor was used in the decade that followed by Atkinson and Houtermans and later by Gamow himself and Teller to derive the rate at which nuclear reactions would proceed at the high temperatures believed to exist in stellar interiors.
In 1939, in a paper entitled "Energy Production in Stars", Hans Bethe analyzed the different possibilities for reactions by which hydrogen is fused into helium. He selected two processes that he believed to be the sources of energy in stars. The first one, the proton-proton chain, is the dominant energy source in stars with masses up to about the mass of the Sun. The second process, the carbon-nitrogen-oxygen cycle, which was also considered by Carl Friedrich von Weizsäcker in 1938, is most important in more massive stars. These works concerned the energy generation capable of keeping stars hot. They did not address the creation of heavier nuclei, however. That theory was begun by Fred Hoyle in 1946 with his argument that a collection of very hot nuclei would assemble into iron. Hoyle followed that in 1954 with a large paper outlining how advanced fusion stages within stars would synthesize elements between carbon and iron in mass.
Quickly, many important omissions to Hoyle's theory were added, beginning with the publication of a celebrated review paper in 1957 by Burbidge, Burbidge, Fowler and Hoyle (commonly referred to as the B²FH paper). This latter work collected and refined earlier researches into a heavily cited picture that gave promise of accounting for the observed relative abundances of the elements. Significant improvements were created by A. G. W. Cameron and by Donald D. Clayton. Cameron presented his own independent approach (following Hoyle) of nucleosynthesis. He introduced computers into time-dependent calculations of evolution of nuclear systems. Clayton calculated the first time-dependent models of the S-process, the R-process, the burning of silicon into iron-group elements, and discovered radiogenic chronologies for determining the age of the elements. The entire research field expanded rapidly in the 1970s.
Key reactionsThe most important reactions in stellar nucleosynthesis:
- Hydrogen burning:
- Helium burning:
- Burning of heavier elements:
- Production of elements heavier than iron:
- Neutron capture:
- Proton capture:
- Photo-disintegration:
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