Which process uses atoms to power stars?

• A. Fusion
• B. Fission
• C. Radioactive decay
• D. Combustion

Answer: (A)

Stars generate energy by fusing light nuclei into heavier ones. In the extreme temperatures and pressures of stellar cores, hydrogen nuclei (protons) fuse to form helium through processes like the proton–proton chain (and, in more massive stars, the CNO cycle). Each fusion reaction loses a small amount of mass, converting it to energy according to E=mc^2, and this energy powers the star’s heat and light for billions of years. The vast energy output and the need for very high temperatures and pressures point to fusion as the mechanism that drives stars.

Fission would split heavy nuclei into lighter ones, which isn’t how stars primarily generate energy since stars are mainly composed of hydrogen and helium and rely on creating more bound, lighter-to-heavier nuclei. Radioactive decay releases energy via unstable nuclei changing over time, but those rates are too slow to power a star continuously. Combustion is a chemical process involving electrons and molecular bonds, and it yields far less energy per reaction; it cannot sustain a star’s immense luminosity and long-term energy output.