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From what I understand, main sequence stars only fuse Hydrogen into Helium, and this is mainly due to the proton-proton chain or the CNO cycle.

However, the CNO cycle requires a carbon catalyst. If these main sequence stars only fuse hydrogen into helium, how can the CNO cycle even take place if there isn't any carbon present?

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You are correct in that CNO needs a catalyst. Fortunately we can find the carbon from outside sources.

Since a dying star will end up containing large amounts of heavy elements like carbon, beryllium etc. Once it dies, those elements are mixed into the next generation of stars, altering their metallicity.

Current stars are part of the second or third generation, allowing them to contain small amounts of heavier elements like carbon. This can then be used by stars that are massive enough for the CNO cycle.

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