What are stars made of and how do they produce energy?

• A. Mainly hydrogen and helium; they release energy through nuclear fusion (H -> He)
• B. They are made of heavier elements and energy via fission
• C. Made primarily of carbon and oxygen; energy via nuclear fusion
• D. Energy from chemical reactions with oxygen

Answer: (A)

Stars are powered by nuclear fusion in their cores, where the immense temperatures and pressures force hydrogen nuclei to combine into helium. This fusion releases energy because some mass is converted into energy, which pushes outward and balances gravity, keeping the star in a stable state. The material in most stars is mainly hydrogen and helium, with only a small fraction of heavier elements—this is why the primary energy source is hydrogen fusion.

Other options don’t fit because fission involves splitting heavy nuclei, which is not how stars generate energy; the fusion of hydrogen to helium is the dominant process during most stars’ lifetimes. While stars do contain carbon and oxygen, they are not made primarily of those elements, and energy from chemical reactions (which involve electrons and binding energies of molecules) is far too weak to power a star.