At their most basic, stars exploit E=mc^2 in order to generate energy, fusing each successive element with others already present in order to create heavier ones; the difference between the total masses at each step is given off as energy. Fusion of lighter elements is more efficient (yields greater net energy), and fusing iron or any of the elements past it requires more energy than it produces. The mass of the helium created from the proton-proton (p-p or pp) chain that represent the fusion process of stars less than twice the mass of the Sun is less than one percent lower than the mass of the individual protons involved in the process, yet that difference is enough to generate 26.73 MeV of energy (mega-electron volts, the common unit of particle mass/energy in physics). Compare this with a proton’s rest mass of 937 MeV. To give you a frame of reference, this is about eight times more energy per unit mass than is generated via fission of uranium, which is in turn about 880,000 times more energy per unit mass than is produced via combustion of gasoline. Larger stars undergo a different fusion process, one in which carbon, nitrogen, and oxygen serve as catalysts for the fusion process; however, the total amount of energy produced is almost the same, 26.8 MeV.
