Which statement about stars is true?

• A. They produce energy by chemical reactions in their cores.
• B. They emit light but do not produce energy.
• C. They make energy through nuclear fusion of hydrogen into helium in their core.
• D. They are primarily composed of lithium and potassium.

Answer: (C)

Stars shine because fusion in their cores releases energy that powers them. In the hot, dense core, hydrogen nuclei fuse to form helium through processes like the proton–proton chain (and, in more massive stars, the CNO cycle). This fusion converts some of the mass into energy, which is emitted as photons that travel outward and eventually reach us as starlight. The conditions in the core—high temperature and pressure—allow these fusion reactions to occur steadily, balancing gravitational collapse and producing the star’s luminosity for long periods.

This statement is the best because it accurately describes how stars generate energy. The idea that energy comes from chemical reactions in the core isn’t correct; chemical reactions release far less energy than nuclear fusion and couldn’t power a star over millions to billions of years. Saying stars emit light but do not produce energy is also incorrect, since their light comes from the energy produced by fusion. Finally, stars aren’t primarily made of lithium and potassium; their composition is mostly hydrogen and helium, with trace heavier elements.