Triple-alpha process

The triple alpha process is the process by which three helium nuclei (alpha particles) are transformed\ninto carbon. This nuclear fusion reaction can only occur rapidly at temperatures above 100,000,000 degrees and in stellar interior having a high helium abundance.\nAs such, it occurs in older stars, where helium \nproduced 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 until the central temperature rises to the point where helium burning occurs.

⁴He + ⁴He ↔ ⁸Be \n: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. However, under the conditions of helium burning a small equilibrium abundance of ⁸Be is formed; capture of another alpha particle then leads to ¹²C. This conversion of three alpha particles to ¹²C is called the 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 rapidly fell below the temperature necessary for nuclear fusion. Ordinarily, the probability of this occurring would be extremely small. However, the beryllium-8 ground state has almost exactly the energy of two alpha particles. In the second step, ⁸Be + ⁴He has almost exactly the energy of an excited state of ¹²C. \nThese resonances greatly increase the probablility that an incoming alpha particle will combine with beryllium-8 to form carbon. \nThe 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 more difficult because of nuclear spin rules. \nThis creates a situation in which stellar nucleosynthesis produces large amounts of carbon and oxygen but only a small fraction of these elements is converted into neon and heavier elements. Fusion produces energy only up to Fe. Heavier elementes are created in supernova explosion with \nabsorption of energy. \nHoyle has cited this as additional evidence for the anthropic principle. \nBoth oxygen and carbon make up the ash of helium burning. \n\n