Thursday, June 7, 2012

Carbon made in the stars - Fred Hoyle

Sun shines because of the nuclear fusion process in which hydrogen atoms fuse into helium. Learning to know about this proton-proton chain reaction is undoubtedly among the greatest achievements of the human race in studying God's creation.

Sunshine is a wonderful thing!  But even more amazing than the burning of the Sun giving us light and heat and energy is the production of carbon without which life as we know it is not possible at all.

Carbon is made in the stars in triple-alpha process. This is a rather complicated set of nuclear fusion reactions by which three helium-4 nuclei (alpha particles) are transformed into carbon.

The triple-alpha process is not self-evident so that simple logic could deduct the chain reaction by adding protons together. The making of carbon in stars is actually quite unlikely and requires an additional element in the picture in order to take place.

Sir Fred Hoyle
Introduced the term Big Bang
To figure this out required a true genius - the famous British astronomer Sir Fred Hoyle (1915-2001) based at the Institute of Astronomy at Cambridge. He had been given enough brain power by His creator so that he could just deduce just by thinking that so it must be or it does not work. It took quite some work later to prove that so it actually is in the nature.

Albert Einstein liked beautiful theories. Hoyle's general theory of stellar nuclear synthesis is a beautiful theory building every element in the Nature from the primordial single atom element hydrogen. Among other things, it explains, how carbon is made in the stars and gives the mathematics of it.

"The triple alpha process is highly dependent on carbon-12 and beryllium-8 having resonances with the same energy as helium-4, 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 an example of the application of the Anthropic Principle:

we are here, and we are 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." (wikipedia)

Beryllium, please!
Triple-alpha process making 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 produce lithium-5 and beryllium-8 respectively, both of which 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 begins to collapse until the central temperature rises to 1*108 K (8.6 keV). At this point helium nuclei are fusing together at a rate high enough to rival the rate at which their 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 nucleus to form carbon-12, which is stable.

Because the triple-alpha process is unlikely, it requires a long period of time to produce much carbon.

One consequence of this is that no significant amount of carbon was produced in the Big Bang because within minutes after the Big Bang, the temperature fell below that necessary for nuclear fusion.

Ordinarily, the probability of the triple alpha process 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 existence of this resonance was predicted by Fred Hoyle before its actual observation, based on the physical necessity for it to exist, in order for carbon to be formed in stars.

In turn, prediction and then discovery of this energy resonance and process gave very significant support to Hoyle's hypothesis of stellar nucleosynthesis, which posited that all chemical elements had originally been formed from hydrogen, the true primordial substance.

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

Supernova stuff
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 nickel (which decays later to iron); heavier elements (those beyond Ni) 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.

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