Energy in the Sun is created by nuclear fusion. Four hydrogen nuclei are fused to form one helium nucleus via a series of reactions called the proton-proton chain. The energy comes from the mass difference between the higher mass of the four hydrogen nuclei and the lower mass of the single helium nucleus. This difference is converted into energy, and the amount of energy created is given by Einstein's equation E=mc2. The energy is created only in the Sun's core. This is the only region within the Sun with both a high enough temperature (at least 10 million K is necessary), and a high enough density for the nuclear fusion reactions to occur at a self sustaining rate. High temperature is necessary to overcome the electrical repulsion between the positive charges in two protons. High density is necessary to ensure enough collisions occur to sustain the reaction rate. We can tell that the proton-proton chain is the Sun's source of energy because neutrinos are generated in the process, and we can count the number of neutrinos coming from the Sun, determine how many fusion reactions are occuring, and verify that the energy thus generated is equal to the Sun's luminosity.