What Is It?
Stellar nucleosynthesis is the process that involves the synthesis of nuclei due to nuclear reactions that take place in stars. These nuclei belong to various elements that are heavier than hydrogen. More particularly, the protons and neutrons of the lighter element nuclei take part in fusion reactions to form nuclei of much heavier elements.
A Little History on How the Theory Has Evolved
The first to suggest that nuclear fusion reactions were taking place in the stars was Arthur Eddington in 1920. The conversion of hydrogen to helium was the stellar source of energy. George Gamow later on developed a formula to calculate the probability of such reactions taking place (Gamow Factor).
Hans Bethe in 1939, proposed two processes used to convert hydrogen to helium. The first, proton-proton cycle, is taking place in stars with similar mass to our Sun. The second, CNO cycle (carbon-nitrogen-oxygen cycle), is taking place in much more massive stars. Later on, in 1946, Fred Hoyle suggested that even heavier nuclei are forming, such as carbon and iron. Many corrections on Hoyle's theory followed and A. G. W. Cameron and D. D. Clayton discovered and explained the further stages of nucleosynthesis.
Classification of Nucleosynthesis Types
Apart from the case of fusion reactions during the lifetime of a star, scientists have discovered that the formation of heavy elements takes place in other cases as well:
Explosive or Supernova Nucleosynthesis: The process in this case, is taking place during the explosion of a star. Elements such as silicon and nickel are formed through fast fusion.
Big Bang Nucleosynthesis: The synthesis that took place during the first three minutes of the Big Bang, led to the formation of protium (1H), deuterium (2H), helium-3 (3H) and helium-4 (4H). Twenty minutes after the Bing Bang the process stopped and therefore only elements lighter than beryllium or possibly boron were formed.
Cosmic Ray Spallation: The impact of fast protons or cosmic rays against the interstellar medium results in the spallation phenomenon. During this process the heavier nuclei of carbon or oxygen fragment into lighter elements such as Be, B, and Li, making these elements more abundant compared to the ratios that result by other types of nucleosynthesis.
Reaction Cycles During the Synthesis of Heavy Nuclei
Hydrogen Burning: This first stage begins with the proton-proton cycle where hydrogen is converted to helium. This process dominates in stars that have a similar mass to the Sun. When the energy is sufficient to overcome the electrostatic or Coulomb repulsion forces among the protons, the fusion takes place.
The second stage involves the carbon-nitrogen-oxygen cycle (CNO), in which the three elements are used as catalysts in order to fuse four protons and produce an alpha particle, two neutrinos and two positrons. After a cycle of reactions, the result is the formation of a helium nucleus. This cycle dominates in stars more massive than the Sun (> 1.3 Solar masses).
Helium Burning: The second stage begins after most of the hydrogen is burned. Helium burning is done either through the alpha process or the triple-alpha process. In the triple-alpha process three helium-4 nuclei or alpha particles, fuse to form a carbon nucleus. As soon as carbon is present, the alpha process begins where the formation of neon, oxygen and silicon takes place.
Burning of Heavier Elements: If the star is massive (> 8 Solar masses), another set of nuclear fusion reactions begins. These include the burning of carbon, neon, oxygen and silicon that lead to the formation of heavier elements and finally iron.
Production of Heavier Elements Than Iron: After the production of iron, the star collapses under its own gravity to a neutron star or a black hole. Other processes take place now including: the neutron capture (R-process and S-process) where heavier isotopes (nuclei with more neutrons) are produced and the proton capture (Rp-process) where heavier element nuclei are produced.
It is easy to see how stellar nucleosynthesis is responsible for all that we are—we are all star stuff!
- E. M. Burbidge, G. R. Burbidge, W. A. Fowler, F. Hoyle, Synthesis of the Elements in Stars, Rev. Mod. Phys. 29 (1957) 547
- D. D. Clayton, "Principles of Stellar Evolution and Nucleosynthesis", McGraw-Hill, 1968; University of Chicago Press, 1983
- Stellar Nucleosynthesis by ohio.edu