Triple-alpha process Totally Explained

Everything about Triple-alpha Process totally explained

The triple alpha process is a set of nuclear fusion reactions by which three helium nuclei (alpha particles) are transformed into carbon.
Older stars start to accumulate helium produced by the proton-proton chain reaction and the carbon-nitrogen-oxygen cycle in their cores. The products of further nuclear fusion reactions of helium with hydrogen or another helium nucleus (isotopes with mass number 5 or 8 respectively) are highly unstable and decay almost instantly back into smaller nuclei. When the star starts to run out of hydrogen to fuse, the core of the star starts to collapse until the central temperature rises to ~100 K. At this point helium nuclei are fusing together at a rate fast enough to rival the speed at which the product, Beryllium-8, decays back into two helium nuclei. This means that there are always a few Beryllium-8 nuclei in the core, which can fuse with yet another helium nuclei to form Carbon-12, which is stable:
alpha process for more details about this reaction and further steps in the chain of stellar nucleosynthesis. 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. The anthropic principle has been controversially cited to explain the fact that nuclear resonances are sensitively arranged to create large amounts of carbon and oxygen in the universe.
Fusion processes produce elements only up to iron; heavier elements (those beyond Fe) are created mainly by neutron capture. The slow capture of neutrons, the S-process, produces about half of these heavy elements. The other half are produced by rapid neutron capture, the R-process, which probably occurs in a core-collapse supernova.

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 30th 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 pressure. The pressure in the core is thus nearly independent of temperature. A consequence of this is that once a smaller star begins burning using the triple-alpha process, the core doesn't expand and cool in response; the temperature can only increase, which results in the reaction rate increasing further still and becoming a runaway reaction. This process, 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 isn't degenerate, the increased thermal pressure due to energy released by helium burning 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.

Discovery

The triple alpha process is highly dependent on carbon-12 having a resonance with the same energy as helium-4 and beryllium-8, and before 1952 no such energy level was known. Astrophysicist Fred Hoyle used the fact that carbon-12 is abundant in the universe as evidence for the existence of the carbon-12 resonance, in what is arguably the only case of success of the application of the Anthropic Principle: we're here, and we're made of carbon, so carbon must have originated somehow and the only physically conceivable way is through triple alpha processes that requires the existence of a resonance in a given very specific location in the spectra of carbon-12 nuclei. Hoyle suggested the idea to nuclear physicist William (Willy) A. Fowler, who conceded that it was possible that this energy level had been missed in previous work. After a brief undertaking by his research group at the Kellogg Radiation Laboratory at the California Institute of Technology, they discovered a carbon-12 resonance near 7.65 Mev.

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