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In the Sun's proton-proton chain, how many hydrogen nuclei fuse to make one helium nucleus?

Four hydrogen nuclei (protons) end up as one helium nucleus.

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  1. Four ✔
  2. Two
  3. Twelve

Why: The proton-proton chain is a three-step process. Protons fuse into deuterium, then helium-3, and two helium-3 nuclei finally make helium-4. Overall, four protons become one helium nucleus plus energy.

Huh, didn't know that: In the Sun, nearly all fusion occurs near the center, where temperatures exceed about 12 million K.

The dad joke
Two hydrogen atoms walk into a bar. One says, I lost my electron. The other asks, are you sure? Yes, I'm positive.
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References

Answer: Four. The proton-proton chain is a three-step process. Protons fuse into deuterium, then helium-3, and two helium-3 nuclei finally make helium-4. Overall, four protons become one helium nucleus plus energy.

  1. Astronomy 2e, 16.2 Mass, Energy, and the Theory of Relativity openstax.org/books/astronomy-2e/pages/16-2-mass-energy-and-the-theory-of-relativity
    a three-step process takes four hydrogen nuclei and fuses them together to form a single helium nucleus.
    Checked 2026-10-10.

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