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99% of the Sun's power is produced. Beyond 30% of the solar radius, where temperature is 7 million K and density has fallen to 10 g/cm³ the rate of fusion is almost nil.

There are two distinct reactions in which four hydrogen nuclei may eventually result in one helium nucleus: "proton-proton chain reaction" and the "CNO cycle".

Proton-proton chain reaction

The first reaction in which 4 H nuclei may eventually result in one He nucleus, known as the proton-proton chain reaction, is:

This reaction

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