The possibility of nuclear reactions in stars
We have seen in the previous chapter that many aspects of stellar structure can be understood without a detailed knowledge of stellar energy generation mechanisms. This is indeed fortunate because not much was known about energy generation mechanisms when Eddington was carrying out his pioneering investigations of stellar structure in the 1920s. Eddington (1920) correctly surmised that the Kelvin-Helmholtz hypothesis of energy generation by contraction (see§3.2.2) could not possibly be true and stellar energy must be produced by sub atomic processes. Nuclear physics, however, was still in its infancy and details of how the stellar energy is produced could not be worked out at that time. With the rapid advances in nuclear physics within the next few years, it became possible to work out the details of energy-producing nuclear reactions inside stars. To build sufficiently detailed and realistic models of stars and stellar evolution, a good understanding of energy generation mechanisms is essential. We turn to this subject now.
Let us consider a nucleus of atomic mass A and atomic number Z. It is made of Z protons and A Z neutrons. The mass mnuc of the nucleus is always found to be less than the combined mass of these protons and neutrons.