The principal nuclear reactions inside the Sun convert hydrogen into helium in three stages. Because this chain of reactions starts with two hydrogen nuclei — that is, two single protons — it is called the proton -proton chain. In step 1, two protons collide and fuse, forming deuterium, which is designated 2 H or D.
The Sun's energy output derives from a sequence of nuclear reactions that converts hydrogen into helium, most of it from the fusion of two protons (the proton–proton or pp reaction) accompanied by the release of a low-energy neutrino. These neutrinos have proved elusive: only solar neutrinos from secondary reactions had been directly observed.
Figure 1. The proton-proton fusion process that is the source of energy from the Sun. The energy from the Sun - both heat and light energy - originates from a nuclear fusion process that is occurring inside the core of the Sun. The specific type of fusion that occurs inside of the Sun is known as proton-proton fusion.
Nature 512, 383–386 (2014) Cite this article In the core of the Sun, energy is released through sequences of nuclear reactions that convert hydrogen into helium. The primary reaction is thought to be the fusion of two protons with the emission of a low-energy neutrino.
Helmholtz realized that no ordinary chemical reaction can produce energy at this rate. Thus, the Sun is not "burning" in the normal sense of a chemical reaction. Helmholtz could not have known that the Sun is a powerful fusion reactor because nuclear physics was not yet understood.
Inside the Sun, this process begins with protons (which is simply a lone hydrogen nucleus) and through a series of steps, these protons fuse together and are turned into helium. This fusion process occurs inside the core of the Sun, and the transformation results in a release of energy that keeps the sun hot.