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 occur rapidly at temperatures above 100,000,000 kelvins 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 until the central temperature rises to the point where helium burning occurs.

4He + 4He ↔ 8Be
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. These resonances greatly increase the probability 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 chain in which oxygen combines with an alpha particle to form neon turns out to be more difficult because of nuclear spin rules. This 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. Both oxygen and carbon make up the ash of helium burning.

Fusion produces energy only up to iron. Heavier elements are created in a supernova with the absorption of energy. Hoyle has cited this as additional evidence for the anthropic principle.

Reaction Rate and Stellar Evolution

The triple-alpha process is strongly dependent on the temperature and density of the stellar material. The energy released by the reaction is approximately proportional to the temperature to the 40th power, and the density squared. Contrast this to the PP chain which produces energy at a rate proportional to the fourth power of temperature and directly with density.

This strong temperature dependence has consequences for the late stage of stellar evolution, the red giant stage.

For lower mass stars, the helium accumulating in the core is prevented from further collapse only by electron degeneracy. The volume of the core is thus dependent only on density and not on pressure. A consequence of this is that once a smaller star begins burning using the triple-alpha process, the core temperature can only increase, which results in the reaction rate increasing further still and becoming a run-away reaction. This run-away reaction, known as the helium flash, lasts only for minutes but burns 60-80% of the helium in the core and produces prodigious quantities of energy.

For higher mass stars, the helium burning occurs in a shell surrounding a degenerate carbon core. Since the helium shell is not degenerate, the energy released by helium burning increases temperature and causes the star to expand. The expansion cools the helium layer and shuts off the reaction, and the star contracts again. This cyclical process causes the star to become strongly variable, and results in it blowing off material from its outer layers.

See also

de:Drei-Alpha-Prozess fr:Réaction triple alpha it:processo tre alfa


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