ABSTRACT
The developments in nuclear physics in the mid-1930s provided a clue to one of the greatest puzzles in astronomy, one that had baffled astronomers and physicists for well over a century – the source of the Sun’s energy. The Sun was known to be composed mainly of hydrogen, it was very hot at its surface and it was known that this meant that it was extremely hot in its interior. The actual fusion mechanism was not even remotely understood at that time, but it was known that any initial step would require protons to have velocities high enough to overcome the strong electrical repulsive force of another proton before it could combine with it. The next important step was to determine the luminosities or total energy flux being emitted by the stars and this required a measurement of their distance, a critical parameter in astronomy but a difficult one to determine.
