Stellar Nucleosynthesis

Nearly all matter is composed of "star stuff". According to the Big Bang theory, the early universe was hot enough to allow the nucleosynthesis of hydrogen, helium, and small amounts of lithium and beryllium. Deuterium, a common isotope of hydrogen, was also important as a reactant in many of the reactions required to form helium. The universe expanded and cooled too rapidly to form the heavier elements.

So where did all of the heavier stuff come from? It came from the cores of stars that formed later. The fusion processes in the cores of stars start with hydrogen fusion via the proton-proton chain and carbon-nitrogen-oxygen (CNO) cycle. This second process only occurs in modern stars enriched from the nucleosynthesis of earlier stars. Once the core "burns" up its fuel, there is no outward force to balance the pull of gravity and the star begins to collapse. This collapse raises the temperature in the core until it is hot enough to burn the "ash" from the previous reactions. With each stage, the ash gets heavier as new elements are forged. In the most massive stars, this chain leads to nickel-56, which later decays to iron-56 and does not release binding energy from fusion (in fact it absorbs energy); there are then no new fuel sources available. The star undergoes a violent collapse, giving birth to a supernova that spreads the newly formed elements throughout space to enrich the medium in which later stars will form.

This Demonstration shows paths taken by the primary fusion processes. Additional chains may occur that are not shown here. Individual arrows represent chains of reactions. Some processes involve multiple chains. Each arrow begins at the beginning of a chain and passes through end products generated in that chain. Protons and alpha particles (helium nuclei) are often reactants and are not shown for most processes.