School of Physics

Hi,
The CNO cycle to fuse hydrogen to helium only occurs at over 1.6x10^7 K. At this stage the only fusion product is helium... so where is the carbon, nitrogen and oxygen coming from? Also, some texts call C, N and O all catalysts, while others say that only C is a catalyst! I seem to think the latter, as N is modified and O is consumed! Is this judgement right?

Thanks

So where is the carbon, nitrogen and oxygen coming from? Good question! The process can only occur in stars that contain material from a previous generation of stars; stars that produced some of these heavier elements and then seeded the interstellar medium with them - either through mass loss or because they blew up in a supernova or whatever. The complete CNO process has two cycles, the second of which includes 16O and 14N but not 12C. (For that reason it's sometimes called the CNO bi-cycle...) C, N and O can therefore all act as catalysts. However, the single cycle involving 12C is by far the more dominant.