In main-sequence stars like the Sun, which fusion process converts hydrogen into helium?

• A. Electron capture
• B. Triple-alpha process
• C. Carbon-nitrogen-oxygen cycle
• D. Proton-proton chain

Answer: (D)

In Sun‑like stars, hydrogen is fused into helium mainly through the proton‑proton chain. In the hot, dense core, four protons ultimately come together through a series of steps: two protons fuse to form deuterium, releasing a positron and a neutrino; the deuterium then captures another proton to make helium‑3; finally, two helium‑3 nuclei collide to form helium‑4 and release two protons. The net result is four protons becoming one helium‑4 nucleus, with energy carried away as photons and neutrinos that power the star. This process is slow because it relies on the weak interaction and quantum tunneling through the proton repulsion, which is why stars burn hydrogen steadily for billions of years.

The other options describe different processes: the triple‑alpha process fuses helium into carbon (not hydrogen into helium) and occurs later in a star’s life; the carbon‑nitrogen‑oxygen cycle is another hydrogen‑to‑helium pathway that uses C, N, and O as catalysts and becomes more prominent at higher core temperatures; electron capture is a separate process not responsible for converting hydrogen into helium in main‑sequence stars.