 During a sabbatical year at Princeton University in 1960 , Christy began an investigation of Cepheid variables and the smaller RR Lyrae variables , classes of luminous variable stars . At the time it was a mystery as to why they varied . He used the knowledge of the hydrodynamics of implosion gained at Los Alamos during the war to explain this phenomenon . This earned him the Royal Astronomical Society 's Eddington Medal for contributions to theoretical astrophysics in 1967 . 
