Which description best matches the Sun's energy source?

• A. Hydrogen fusing into helium
• B. Helium fusing into hydrogen
• C. Oxygen fusing into carbon
• D. Carbon fusing into oxygen

Answer: (A)

Hydrogen fusing into helium is the process that powers the Sun. In the Sun’s center, extremely high temperature and pressure allow protons to overcome their electrical repulsion and fuse through the proton–proton chains, ultimately converting hydrogen into helium and, in the process, releasing a large amount of energy. This energy heats the Sun and provides the light and warmth we receive.

The other descriptions describe fusion of heavier elements or reverse/failure to release energy, which doesn’t happen in the Sun right now. Helium fusing into hydrogen would require energy input rather than output. Oxygen fusing into carbon or carbon fusing into oxygen occur only in later, much hotter stages of much more massive stars.