ItElement synthesis is quite remarkable that already in the first third of the twentieth century it was possible to develop a plausible theory of stellar structure that was already quite good in terms of the true conditions—without knowing anything concrete about the energy release mechanisms in stars (see Chap. 1 and (Eddington 1988)). It was not until 1925 that it was recognized that the sun consists mainly of hydrogen and that all other elements (with the exception of helium) are only minor admixtures of stellar matter (Payne 1925a). By this time, Albert Einstein’s special theory of relativity had already become common knowledge among physicists, and anyone could calculate how much energy was contained in a kilogram of “mass”, for example.