Triple-alpha process

The triple alpha process is the process by which three helium nuclei (alpha particles) are transformed into carbon.

This nuclear fusion reaction can only ocurs at temperature above 100,000,000 degrees and in stellar interior having a high helium abundance. As such, it occurs in older stars, where helium produced by the proton-proton chain and the carbon nitrogen oxygen cycle has accumulated in the center of the star. Because the helium initially does not produce energy, the star will collapse causing the central temperature to rise to the point where helium burning occurs.

⁴He + ⁴He ↔ ⁸Be + &gamma - 91.78 keV

⁸Be + ⁴He → ¹²C + γ + 7.367 MeV

The net energy release of the process is 7.275 MeV.

The ⁸Be produced in the first step is unstable and decays back into two helium nuclei in 2.6×10^-6 seconds; thus the production of a carbon atom requires the almost simultaneous collision of three alpha particles, hence the name triple-alpha process. Because the triple-alpha process is unlikely, it requires a long period of time to produce carbon. One consequence of this is that no carbon was produced in the big bang because the temperature rapid fell below the temperature necessary for nuclear fusion.

Ordinarily, the probability of this occurring would be extremely small, however beryllium-8 has an excited energy level which is almost exactly the energy of an alpha particle. This greatly increases the probablility that an incoming alpha particle will combine with beryllium-8 to form carbon. The fact that the existence of carbon depends on an energy level being exactly the right place, has been controversially cited by Fred Hoyle as evidence for the anthropic principle.

As a side effect of the process, some carbon nuclei can fuse with additional helium to produce a stable isotope of oxygen and release energy:

¹²C + ⁴He → ¹⁶O + γ

The next step of the change in which oxygen combines with an alpha particle to form neon turns out to be unlikely because of nuclear spin rules. This creates a situation in which stellar nucleosynthesis produces large amounts of carbon and oxygen but is prevented from converting most of these elements into neon and heavier elements. Hoyle has cited this as additional evidence for the anthropic principle. Both oxygen and carbon make up the ash of helium burning.