Stellar nucleosynthesis

Stellar nucleosynthesis is the collective term for the nuclear reactions taking place in stars to build the nuclei of the heavier elements.

The processes involved began to be understood early in the 20th century, when it was first realised that the energy released from nuclear reactions accounted for the longevity of the Sun as a source of heat and light.

Contents

1 History 2 Key Reactions 3 References 4 External links

History

In 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.

Later, many important details were added to Bethe's theory, like the publication of a celebrated paper in 1957 by Burbidge, Burbidge, Fowler and Hoyle. This latter work collected and refined earlier researches into a coherent picture that accounted for the observed relative abundances of the elements.

Key Reactions

The most important reactions in stellar nucleosynthesis are:

• Hydrogen burning:
  • The proton-proton chain
  • The carbon-nitrogen-oxygen cycle
• Helium burning:
  • The triple-alpha process
  • The alpha process
• Burning of heavier elements:
  • Carbon burning process
  • Neon burning process
  • Oxygen burning process
  • Silicon burning process
• Production of elements heavier than Iron:
  • Neutron capture:
    • The R-process
    • The S-process
  • Proton capture:
    • The P-process

References

• H. A. Bethe, Energy Production in Stars, Phys. Rev. 55 (1939) 103; online edition (subscription needed)
• H. A. Bethe, Energy Production in Stars, Phys. Rev. 55 (1939) 434-456; online edition (subscription needed)

External links

• How the Sun Shines by John N. Bacall