As we saw in the preceding section, when the nuclei of heavy atoms split, energy is released. For light atoms, the opposite is true; when these nuclei combine (fuse together), energy is released. This is the process of nuclear fusion. Fusion of light elements, mostly hydrogen, is the force that powers energy release in the sun and in sun-like stars. Imagine the sun as a huge sphere of hydrogen. Because a star is so massive, the gravitation pull on the hydrogen atoms is sufficient to overcome the repulsion between the two nuclei to force them together to form an unstable ${\displaystyle {}_{2}^{2}{\text{He}}}$ nucleus. This immediately ejects a positron, leaving deuterium, ${\displaystyle {}_{1}^{2}{\text{H}}}$, and releasing a significant amount of energy. In the cascade of reactions deuterium fuses with another hydrogen to give ${\displaystyle {}_{2}^{3}{\text{He}}}$, and two of these combine to form helium, ejecting two high-energy protons in the process.