Introduction:As the main sequence fusion cycles (proton-proton and CNO) transform more and more hydrogen to helium, one of the most likely possibilities of fusion would involve two 4He nuclei fusing to create a nucleus with an atomic mass of 8. However, there are no stable isotopes of any element with an atomic mass of 8. 8Be in particular has a lifetime of only 10-17 seconds! At the temperatures in which the proton-proton and CNO cycle occurs, 8Be will break apart before it is involved in any further fusion reactions. This has become known as the beryllium bottleneck, because it is 8Be's instability that prevents the heavier elements from being formed relatively immediately as helium is created.
The Triple-Alpha ProcessWhen a large enough amount of hydrogen is converted to helium within the core (in our sun, about 10% of its mass), the core may begin to collapse on itself, increasing the density and temperature. When the temperature rises above 100 million K, helium nuclei may be converted to carbon (12C) through a very high, and extremely improbable, energy reaction called the triple-alpha process (remember an alpha particle is really just a helium nucleus). This is because the temperatures are high enough to fuse two 4He into the extremely unstable 8Be at a large enough rate so that there is always a small amount of 8Be. In the short amount of time that a 8Be nucleus exists, it may fuse w ith another 4He producing an "excited" carbon isotope with an atomic mass of 12. These carbon nuclei in their "excited" state are unstable, but they may release a gamma ray before breaking apart, thus becoming the stable 12C nucleus. This usually begins occuring during the red giant phase of a star (you will learn more about that later), at which point the hydrogen fuel in the core has been used up, and the temperature rises enough to trigger the triple-alpha process.
Further Element FormationAfter that, atoms of even higher mass may be created from the fusion of carbon with other nucleons. For example:
13C + 4He 16O + neutron (n)
This process of creating the heavier elements is called nucleosynthesis.
Elements up to iron may be created in this fashion as well as through a
variety of other fusion reactions. Elements heavier than iron are formed
through neutron capture, because the fusion of iron with other elements
must absorb energy, rather than release it. This situation of neutron
capture occurs during a supernova (more on this later), creating up to
the heaviest of natural elements.
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