The primary source of energy for the sun is nuclear fusion. This process occurs in the sun's core, where hydrogen atoms are fused together under immense pressure and temperature to form helium. During this reaction, a tremendous amount of energy is released in the form of light and heat. This energy is what makes the sun shine and provides the necessary warmth and light for life on Earth. The fusion process in the sun is similar to that which occurs in hydrogen bombs, but in the sun, it occurs in a controlled manner over billions of years.