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SESK: Stuff Everyone Should Know · sesk.info/astro-triple-alpha-process

About what core temperature is needed for helium to fuse into carbon?

About 100 million K, for the triple-alpha process.

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  1. About 15 million K
  2. About 1 million K
  3. About 100 million K ✔

Why: Fusing helium needs far more heat than hydrogen because helium nuclei carry twice the charge. At about 100 million K, three helium nuclei (alpha particles) fuse into one carbon nucleus.

Huh, didn't know that: In stars of about 0.8 to 2 solar masses, helium ignites all at once in a burst called the helium flash.

The dad joke
Helium walks into a bar. The bartender says we don't serve noble gases. Helium doesn't react.
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References

Answer: About 100 million K. Fusing helium needs far more heat than hydrogen because helium nuclei carry twice the charge. At about 100 million K, three helium nuclei (alpha particles) fuse into one carbon nucleus.

  1. Astronomy 2e, 22.4 Further Evolution of Stars openstax.org/books/astronomy-2e/pages/22-4-further-evolution-of-stars
    It takes a temperature of 100 million K to slam three helium nuclei (six protons) together and make them stick.
    Checked 2026-10-10.

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