My daughter is in middle school and she was wondering about the Sun burning without oxygen. The conversation led to our visiting this page, where they say that rates of nuclear reactions are unaffected by catalysts. This seems to contradict what this place says though: " It should be noted that in the presence of carbon, nitrogen and oxygen, stars heavier than the Sun may burn hydrogen to helium by using the C, N and O as catalysts."
Depends upon what you mean by a catalyst.
Nuclear reactions are unaffected by chemical reactions, which is where we usually think of catalysts. But the pure definition of a catalyst - something that affects a reaction without itself being used up - is much broader than chemistry.
As AK84 notes above muons can catalyse fusion.
Nucleosynthesis is pretty complex, and the path to making heavier elements isn’t just a matter of adding protons and neutrons. Neutron capture makes a nucleus bigger, but it will often be unstable and decay, and that decay may involve a Helium nucleus. In particular, there is a circular path whereby carbon absorbs a neutron, turns to nitrogen, which absorbs another neutron, turns to oxygen which absorbs yet another neutron, and fissions back to carbon and helium. So the presence of carbon acts as a catalyst. Carbon plus a few neutrons gives you carbon and helium.
I followed the link and the text included this remark: “These two factors prevent muon-catalyzed fusion from becoming a practical power source, limiting it to a laboratory curiosity.” The site I originally quoted mentioned nuclear reactions being influenced by catalysts naturally in a star heavier than the Sun.
I see. So, I should conclude the statement in the lecture I cited in the OP is an oversimplification or something…
Perhaps an inadvertent oversimplification. It context it was clearly directed towards emphasising that chemical processes had no effect. But in doing so the breadth of the meaning of catalyst was overlooked.
One should probably mention cold fusion here. The basic (wrong) idea was that you could catalyse fusion by pulling the hydrogen atoms close together in a metal matrix. That counts as a form of chemical catalysis. Just. But given it doesn’t work, it acts to underline the point.
Ack. Just to add, the above description of the carbon, nitrogen, oxygen cycle is ever so slightly wrong. It is adsorption of protons, not neutrons. Not thinking straight when I wrote the above. Small problem in conservation needs observing.
Back to the internet page that caused me to initiate this thread. The author mentions the following as a major difference between nuclear and chemical reactions: “Rates of chemical reactions are influenced by temperature and catalysts. Rates of nuclear reactions are unaffected by such factors.” However, the presence of C, N, and O can catalyze fusion in stars. Then I guess Hoyle resonance and increases in temperature or density count as catalysts of nuclear reactions as well.
I’d think the major difference between chemical and nuclear reactions is that with the former you’re changing molecules to other molecules, while with the latter you’re changing atoms to other atoms.
but I guess it’s how you define catalyst. If you just stuff a bunch of fuel rods into a nuclear reactor, not much would happen. add water as a moderator to slow neutrons down and you get fission. Is water a “catalyst” in this case?
I feel perfectly comfortable describing both the carbon in the stellar CNO cycle, and the moderator in a nuclear reactor, as catalysts for nuclear reactions (they operate in very different ways, but then, chemical catalysts have analogous different modes of operation, too).