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!