During nuclear fusion, hydrogen nuclei (protons) combine to form helium nuclei. The process of nuclear fusion occurs when the hydrogen nuclei are heated and placed under high pressure, causing them to collide with enough force for the strong nuclear force to overcome the electromagnetic repulsion between the positively charged protons. When two hydrogen nuclei collide, they fuse together to form a single helium nucleus, releasing a large amount of energy in the form of light and heat. This process is what powers the sun and other stars, and it is a promising source of clean energy for the future.