Final answer:
Deep inside stars, hydrogen atoms undergo nuclear fusion to form helium atoms, releasing heat and energy that we see as sunlight.
Step-by-step explanation:
Deep inside stars, the pressure and temperature are so high that hydrogen atoms undergo nuclear fusion to form helium atoms. This fusion process releases a tremendous amount of heat and energy, which is the source of sunlight. The fusion of hydrogen into helium powers the Sun and other stars, allowing them to maintain their high temperature and energy output.