Historically, the source of energy generation on the Sun has been attributed to a number of causes, including gravitational collapse and radioactive decay. The possibility that nuclear fusion might be the source of solar energy could not be evaluated quantitatively until certain key pieces of information were in place. In particular, the masses of the relevant isotopes had to be measured to three or four significant digits before it became evident that atomic masses, although close to integer values, actually deviated from integers by small, but systematic, amounts. The deviations amount to only a few parts per thousand, but those small deviations are at the very heart of nuclear energy generation.