The source for the energy of the sun mystified scientists for centuries. They could easily compute that chemical reactions -- the usual burning of materials we use to make energy for our civilization -- was woefully inadequate. For a while, they hoped that gravitational energy could be released by contraction within the sun, but the sun was too large and moreover as it became clear from fossils that the output had been virtually constant for a tremendously long time, this explanation became untenable. Although this thought was resurrected briefly with the idea that a super-compact neutron star in the core released the energy (with the hope that this structure would fix the problem with the size), this idea was quickly shown not to work. In the 1930's, with the increasing understanding of nuclear physics, the correct explanation of thermonuclear burning of hydrogen finally emerged.