Fusion joining two light atomic nuclei together to make a heavier one releases more energy per kilogram of fuel than fission splitting a large atomic nucleus into smaller pieces . Stars are powered by fusion joining two light atomic nuclei together to make a heavier one reactions in the cause. In our Sun, over 500 million tonnes of hydrogen a chemical element with the symbol H and atomic number 1 fuses together per second, releasing sunlight and warmth into the Solar System. Most of the life on Earth ultimately depends on nuclear fusion joining two light atomic nuclei together to make a heavier one in the centre of the Sun.