It is reasonably certain that the Sun (as do all other Main Sequence stars) generates energy via the fusion of hydrogen into helium in its core. By so doing, it has been able to maintain thermal equilibrium for the lifetime of the Solar System and will continue to do so for the next 5.4 billion years or so. Here, we will discuss the manner in which the Sun fuses hydrogen into helium and some other points about nuclear fusion in general.

• Nuclear fusion, as its name implies, is the merging together of low mass nuclei into more massive nuclei. In the process, a little bit of mass may be converted into energy (Einstein's famous relation stating the equivalence of mass and energy, E=mc²). We also know, however, that there are such things as fission reactions which also generate energy. Fission is the breaking apart of a massive nucleus into smaller nuclei and so is the inverse process of fusion.
  How can a process and its inverse both generate energy?

• Now, under what sorts of general conditions will nuclear reactions take place?

• The Sun generates energy via the Proton-Proton (pp) chain More massive stars convert hydrogen into helium through another process, the Carbon-Nitrogen-Oxygen (CNO) cycle.