As a star forms, at what point do nuclear reactions begin to occur?

The energy of the stars comes from nuclear fusion processes. For stars like the sun which have internal temperatures less than fifteen million Kelvin, the dominant fusion process is proton-proton fusion. For more massive stars which can achieve higher temperatures, the carbon cycle fusion becomes the dominant mechanism. For older stars which are collapsing at the center, the temperature can exceed one hundred million Kelvin and initiate the helium fusion process called the triple-alpha process.

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