Stellar nucleosynthesis

The word synthesis refers to making something. Stellar nucleosynthesis means making nuclei of atoms in stars. Hydrogen and helium were created during the big bang, with very small traces of larger nuclei. Some helium is also created via fusion inside stars, and almost all of the atoms of heavier elements that exist in the universe today were fused in stars. As we have seen, elements up to iron could be created in stellar cores. Elements more massive than iron can be created during the supernova process. When the shock wave moves outward through the envelope of the collapsing star, the resulting high temperatures can cause nuclei to fuse.

  • Nuclei are created via fusion
    • Theory must explain relative abundances observed
  • Hydrogen fusion
    • Proton-proton chain
    • Triple alpha process
  • Helium fusion
  • Carbon fusion
    • Carbon + Carbon -> Magnesium
    • Carbon + Helium -> Oxygen
  • Helium capture – alpha process
    • More likely to capture an alpha particle than another heavier nucleus
    • Heavy nuclei can be broken up by energetic photons
      • Create more available alpha particles
  • Neutron capture
    • Heavy nuclei capture neutrons and become unstable isotopes
      • Decay into other atomic nuclei

The relative abundances of elements are shown in the graph above. Notice that the graph is a log plot, the tick marks are in powers of ten.  Note that all of the abundances are given in terms of the hydrogen abundance - that is, hydrogen is shown as 1, and all other elements are shown as fractions of 1.

 

Our theory of nucleosynthesis must explain features shown in this diagram. For example, hydrogen is the most abundant, with helium second in abundance. Lithium is extremely rare, even though it has the third smallest number of nucleons, it must be relatively hard to produce. We see peaks for specific elements such as carbon, oxygen, neon, etc., with lower abundances for elements in-between.

Copyright 2005 Pearson Prentiss Hall, Inc.

The proton-proton chain accounts for much of the helium produced in the cores of stars. For details of this process, please refer to the Sun page.

Copyright 2005 Pearson Prentiss Hall, Inc.

The nuclei of helium atoms are also known as alpha particles. In the triple alpha process, three alpha particles fuse to form one carbon nucleus. This process takes place in two steps. First, two alpha particles fuse to form a beryllium nucleus. Beryllium is highly unstable and quickly decays back into two alpha particles. If a third alpha particle fuses with the beryllium nucleus before it can decay, the particle becomes a stable carbon nucleus. The process can be written like this:

 

4He + 4He -> 8Be + energy

 

8Be + 4He -> 12C + energy

 

The energy is released as kinetic energy of the nuclei and gamma ray photons.

Copyright 2005 Pearson Prentiss Hall, Inc.

The formation of heavier elements follows a similar pattern. The core of the star is plentiful with alpha particles, and the electromagnetic repulsion between a carbon nucleus and an alpha particle is lower than the repulsion between two carbon nuclei, so carbon fusion into oxygen is common.

 

12C + 4He -> 16O  + energy

 

The alpha capture process continues to be more probable than other kinds of fusion as the star fuses increasingly more massive nuclei in its core.

 

16O + 4He -> 20Ne  + energy

 

20Ne + 4He -> 24Mg  + energy

 

The process of alpha capture continues on to the heaviest elements fused in the core of the star. The high probability of the alpha capture process explains the peaks in abundance of the elements. Elements heavier than these can only be formed during the supernova process, where the shock heating increases the temperatures to the point where the speeds of the nuclei are great enough for the nuclei to get near enough to each other to overcome the high electromagnetic repulsion of the massive nuclei.

J&K logo
J&K logo