In the twenties the new atomic physics opened the way toward a quantitative interpretation of stellar spectra. It was one of the aims in this new branch of astrophysics to determine in a quantitative way the chemical composition of stellar atmospheres, i. e. of those parts of the stars which are accessible to spectro scopic observation. It was only in the forties that the first reasonably accurate and complete analyses became available. In the meantime nuclear physics and the theory of the internal constitution of the stars had reached the important conclusion that the main sequence of the Hertzsprung-Russell-diagram is taken up by stars which in their central parts produce energy by the conversion of hydrogen into helium. It was BAADE who in the early fifties stimulated observational and theoretical work concerning the colour-magnitude-diagrams of star clusters and their connection with the nuclear evolution of stars. Obviously, this type of research opened also the way towards much more difficult and far reaching fields of research: the formation of galaxies and the origin of the chemical elements and their abundance distributions.