SESKStuff Everyone Should Know
SESK: Stuff Everyone Should Know · sesk.info/astro-fusion-mass-loss

When hydrogen fuses into helium, about what share of the mass turns into energy?

About 0.7%, which E = mc squared turns into a huge amount of energy.

Start playing free

  1. About 50%
  2. About 7%
  3. About 0.7% ✔

Why: A helium nucleus weighs slightly less than the four hydrogen nuclei that made it. The missing 0.71% of mass becomes energy. Since c squared is enormous, even that tiny fraction releases a lot.

Huh, didn't know that: Fusing just 1 kg of hydrogen releases about 6.4 x 10^14 joules.

The dad joke
Fusion saves every bit of mass it can. The rest goes into energy.
Like this card?
Play the game

References

Answer: About 0.7%. A helium nucleus weighs slightly less than the four hydrogen nuclei that made it. The missing 0.71% of mass becomes energy. Since c squared is enormous, even that tiny fraction releases a lot.

  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
    The mass lost, 0.02862 u , is 0.71% of the mass of the initial hydrogen.
    Checked 2026-10-10.

Think this answer is wrong? Tap "Challenge this answer" on the card in the game and tell us why.

Test yourself in the game

Astronomy & SpaceIn the Sun's proton-proton chain, how many hydrogen nuclei fuse to make one helium nucleus?Astronomy & SpaceWhich element was found in the Sun's spectrum before it was found on Earth?Astronomy & SpaceHow long does sunlight take to cross one astronomical unit, the average Earth to Sun distance?

That's stuff everyone should know.