The primary fusion process in the Sun involves protons fusing to form helium via which chain?

• A. CNO cycle
• B. Triple-alpha process
• C. Proton-Proton chain
• D. S-process

Answer: (C)

Hydrogen fusion in the Sun proceeds mainly through the proton-proton chain, a series of reactions that converts four protons into one helium-4 nucleus and releases energy. In the Sun’s core, temperatures around 15 million kelvin allow protons to get close enough to fuse despite their electric repulsion, aided by quantum tunneling. The first step fuses two protons into a deuterium nucleus, emitting a positron and a neutrino. A proton then joins this deuterium to form helium-3 and a gamma ray. Finally, two helium-3 nuclei collide to form helium-4, releasing two protons to keep the cycle going. In hotter, more massive stars, the CNO cycle contributes more, but in the Sun it’s a minor channel. The triple-alpha process, by contrast, fuses helium into carbon and occurs later in a star’s life during helium burning, not during the Sun’s hydrogen burning. The S-process involves slow neutron captures and happens in later stellar evolution, not as a chain that fuses protons into helium. So the main fusion path powering the Sun is the proton-proton chain.