High School Why is the CNO Cycle considered catalytic in stellar nucleosynthesis?

[DrLich]

I'm trying to understand why the CNO cycle is considered catalytic in stellar nucleosynthesis. I know that carbon is regenerated in the process, but how does this make the reactions faster than the proton-proton chain?
Thanks in advance for your insights!

 

[phyzguy]

It's not the fact that the carbon is regenerated that makes the CNO reactions faster than the P-P reaction. The reactions are faster simply because the cross sections are larger. The cross section for the P-P reaction is extremely small - so small that it is not measurable in the lab and has to be calculated. The CNO reaction is considered catalytic because (as you said) the carbon is regenerated and not used up.

 

[WernerQH]

The first step in the p-p chain is the formation of a deuteron from two protons. This involves the weak interaction, leading to the production of a neutron. If I remember correctly, the chance for a proton to take part in such a reaction is of the order of one per a few billion years.

On the other hand, the weak interactions involving the $\rm^{15}O$ and $\rm^{13}N$ nuclei proceed on the time scale of minutes. If you didn't need neutrons to build up the heavier nuclei, fusion processes could proceedly rapidly with strong and electromagnetic interactions alone.