The sun is our nearest star and like other stars it is a giant gas ball made up mostly of hydrogen and helium. The sun generates energy in its core in a process called nuclear fusion. During nuclear fusion, the sun’s extremely high pressure and hot temperature cause hydrogen atoms to come apart and their nuclei to fuse or combine. Hydrogen nuclei (protons) fuse to create helium-4 atom. But the helium atom is less massive than the protons that combined to form it. Some matter is lost during fusion process. The difference in mass was converted to energy which is then emitted into space as radiant waves.