Nuclear fusion is the process that combines gas to form helium and produces most of the sun's energy. In nuclear fusion, two or more atomic nuclei combine to form a single heavier nucleus, releasing a great amount of energy in the process. This process is what powers the sun and other stars.

In the sun, the most common type of nuclear fusion is proton-proton fusion. In this process, two protons, which are the positively charged particles in an atomic nucleus, combine to form a deuteron, which is a nucleus of heavy hydrogen. The deuteron then combines with another proton to form a helium-3 nucleus. Two helium-3 nuclei then combine to form a helium-4 nucleus, which is the most stable isotope of helium.

The energy released by nuclear fusion in the sun is what provides the sun with its light and heat. The sun's core is extremely hot and dense, which allows the protons to overcome their mutual repulsion and fuse together. This process is constantly occurring in the sun, and it is what has been powering the sun for billions of years.