The sun fits onto the relations derived from other systems, even though we determine its properties by quite different techniques. Colors and spectral types lead to consistent estimates of surface temperatures, even though these estimates are based on different physical principles. We develop this hypothesis into a theory of main sequence stars by applying known physics. The radius of the star is the one at which the emitted luminosity and produced luminosity match, indicating that fusion energy can supply the star's luminosity. If the star did not have this radius, it would expand or contract until it did.
Experiments detect neutrinos from the sun, direct evidence for fusion.
Excellent agreement with solar oscillation measurements. Explains mass-luminosity relation. Explains mass-radius relation. Explains existence and location of main sequence in HR diagram. Given the other successes, it seems unlikely that the solar neutrino problem will lead us to reject our basic theory of stars.