8th The Sun-Earth-Moon System (C20) Practice Test

Nuclear fusion is the Sun’s power source. In the hot, dense core, hydrogen nuclei fuse together to form helium, and a little mass is converted into energy that escapes as light and heat. The main pathway for a Sun-like star is the proton–proton chain, where four protons ultimately become a helium nucleus, with energy carried away by photons and neutrinos. This fusion process explains how the Sun shines for billions of years without needing heavy elements or radioactive decay.

Fission and radioactive decay aren’t what powers the Sun. Fission splits heavy nuclei and requires conditions and materials not present in the Sun’s interior, while decay relies on unstable isotopes releasing energy too slowly to account for the Sun’s sustained output. Ignition isn’t a process by itself; it’s the start of a reaction, not a mechanism that releases energy in stars.