Describe the fusion process that forms helium.

• A. It fuses two hydrogen atoms to make a helium atom
• B. It splits helium into hydrogen
• C. It fuses carbon and nitrogen
• D. It fuses oxygen and neon

Answer: (A)

Hydrogen fusion into helium powers stars. In the proton-proton chain that dominates in Sun-like stars, hydrogen nuclei (protons) fuse through several steps to ultimately form helium-4 and release energy. First, two protons fuse to create deuterium while emitting a positron and a neutrino; deuterium then fuses with another proton to make helium-3; when two helium-3 nuclei meet, they form helium-4 and release two protons to keep the cycle going. Net result: four hydrogen nuclei become one helium nucleus, with energy carried away as light and heat. The idea described here is that hydrogen particles fuse to build helium, which is the essential concept. The other options describe splitting helium or fusing heavier elements, which are not how helium is formed in stars.