Final answer:
The sun is powered by nuclear energy, a result of nuclear fusion wherein hydrogen nuclei combine to form helium, releasing a significant amount of energy as light and warmth.
Step-by-step explanation:
The form of energy that powers the sun is nuclear energy. Inside the sun, nuclear fusion occurs when hydrogen nuclei (protons) combine under intense pressure and temperature to form helium nuclei. This process converts mass into energy according to the famous Einstein's equation E = mc², releasing a vast amount of energy that we experience as light and warmth. The nuclear fusion reaction in the sun primarily involves a sequence of steps known as the proton-proton chain, which ultimately transforms hydrogen into helium and is the principal source of the Sun's energy output.