 It remained located in Copenhagen until 1965, when it was moved to the Smithsonian Astrophysical Observatory, Cambridge, United States.In 1940 Ellis Strömgren was succeeded by his son Bengt Strömgren, who made the pioneering discovery that hydrogen is the most abundant element in the stellar interior.He was also the first to make the correct interpretation of the Hertzsprung–Russell diagram (the relation between the total energy output from a star and its surface temperature was independently found by the Danish chemist and astronomer Ejnar Hertzsprung and the American astronomer Henry Norris Russell).