Whereas the nuclear fission reaction creates energy by splitting a heavy nucleus into two nuclei, the nuclear fusion reaction creates energy by combining two light nuclei into a larger nucleus. Fusion is the most important phenomenon in nature: It powers the Sun and so is ultimately the energy source for all biological and physical processes on Earth. A representative fusion reaction combines deuterium and tritium and yields helium. The energy released by this reaction is about 18 MeV. Previously, we calculated that the energy released by a fission reaction is about (200 MeV)/(235 nucleons) = 0.85 MeV/nucleon. As shown above, the energy released by fusion is (18 MeV)/(5 nucleons) = 3.6 MeV/nucleon. Per unit mass, the energy released by a fusion reaction is about four times larger than that released by a fission reaction.
Fusion powers the Sun
Most of the visible matter in the universe, including the matter inside of stars, is in the form of the element hydrogen. In the core of the Sun and other stars, the temperature and the pressure are so high that nuclear fusion reactions convert abundant hydrogen into helium, releasing energy in the process. The Sun produces nuclear fusion energy in its core that then turns into thermal energy of the matter in the Sun. The thermal energy is eventually transported outward to the surface of the Sun and then radiated into space.
Elemental composition of the Sun
The overall reaction that fuels the Sun is called the proton–proton cycle. The p–p cycle converts four hydrogen atoms (containing four protons) into a helium atom (containing two protons and two neutrons). The Sun is composed mostly of hydrogen and helium (91.2% hydrogen and 8.7% helium). Larger nuclei up to carbon and oxygen are present at a very small fraction. The energy required to fuse nuclei of larger atomic mass is much greater and it is found in stars that are in a different stage of their evolution. Mass–energy equivalence is behind fusion, which powers the Sun!
Test your knowledge
Where does the p–p cycle of nuclear fusion take place?
in the Sun
in Earth’s oceans
in a tokamak reactor
in an inertial confinement reactor
The correct answer is a, the Sun. The p–p cycle needs high temperature and pressure to take place, so the core of the Sun is the perfect place for this. The reactors on Earth generally use a cycle starting with deuterium.
Whereas the nuclear fission reaction creates energy by splitting a heavy nucleus into two nuclei, the nuclear 
![Image of Sun and fusion reaction that powers the Sun [proton-proton chain]](Images/SB/61/LR/Fusion and the Sun.png "Image of Sun and fusion reaction that powers the Sun [proton-proton chain]")
The overall reaction that fuels the Sun is called the proton–proton cycle. The p–p cycle converts four hydrogen atoms (containing four protons) into a helium atom (containing two protons and two neutrons). The Sun is composed mostly of hydrogen and helium (91.2% hydrogen and 8.7% helium). Larger nuclei up to carbon and oxygen are present at a very small fraction. The energy required to fuse nuclei of larger atomic mass is much greater and it is found in stars that are in a different stage of their evolution. Mass–energy equivalence is behind fusion, which powers the Sun! 
