Is Oxygen More Abundant Than Carbon in the Universe Due to Nuclear Fusion?

[iced199]

From my rather weak understanding of nuclear fusion in stars, four hydrogen atoms are combined into one helium atom. Then, three helium atoms are combined into one carbon atom. What I don't understand is why there is more oxygen in the universe (from the wikipedia article) than carbon, if carbon is produced first in the fusion process. Why is there more oxygen?

 

[Simon Bridge]

Have you considered that some of the carbon gets synthesized into oxygen?
i.e. there is more oxygen than carbon because the carbon gets made first?

 

[twofish-quant]

It turns out that the reactions are

helium -> carbon (a little difficult)
carbon -> oxygen (not too hard)
oxygen -> neon (really, really hard)

So what ends up happening is that more of the stuff "bunches" up in oxygen

A lot of this depends on some detailed physics. There just happens to be a nuclear spectral line that makes it not too difficult to go from helium to carbon, and there also happens to be a spectral line that blocks the conversion from oxygen to neon.

Fred Hoyle, who discovered this, believed that this was too much to be conincidence and he took this as evidence of God. On the other hand, he also believed that life couldn't have started on earth, and he also was a very strong opponent of the big bang.

 

[Drakkith]

What exactly is a "Nuclear Spectral Line" twofish? And how does it apply to the nuclear reactions in nucleosynthesis?

 

[twofish-quant]

What exactly is a "Nuclear Spectral Line" twofish? And how does it apply to the nuclear reactions in nucleosynthesis?

Here is one of thinking about it.

You have an atom with electrons at different energy levels. If that atom happens to hit a photon at the exact frequency as the energy level, the atom is much more likely to absorb the photon, and you get a spectral line. Change the photon so that it is a slightly different energy then it misses the energy level and it's not absorbed.

The same thing happens with nuclei. Nuclei have different energy levels. If you throw one nuclei at another and there happens to be an energy level at exactly that energy, then the reaction rate increases a lot. This is critical for the 2 He4 -> Be8 + He4 -> C12 reaction. If you calculate the "ordinary" reaction rate, it's quite low, and it wouldn't happen. However C12 happens to have a spectral line at exactly the right energy to make the Be8 + He4 go much, much faster than it ordinarily would. The math of nuclei absorbing helium nuclei is exactly the same as atoms absorbing photons. It's also called a resonance.

This is one reason that people sort of take the anthropic principle seriously. Fred Hoyle was able to deduce that C12 *must* have an spectral line at the energy that speeds up the Be8 + He4 reaction.

Now it turns out that the reaction O16 + He4 -> Ne20 is something that you'd normally expect to happen at a high rate, which would be bad because that would mean that you wouldn't get a build up of O16 and C12. But there is some nuclear thing (I think it has to do with spin) that just happens to work out to block that reaction.

 

[Drakkith]

Ah ok, that makes sense. Thanks.

 

[mfb]

It is not just the carbon atom: The 8Be nucleus has approximately the energy of 2 alpha-particles, which increases this reaction rate, too.