Family background and early interest in astronomy (Elis Stromgren). Undergraduate and graduate studies at University of Copenhagen, late 1920s; studies at Niels Bohr Institute, 1927-1929; thesis work in classical astronomy (orbits of comets). Development of photoelectric photometry and observations; early electronics, 1925; conflicting results in calculations of opacities (Arthur Stanley Eddington, Gaunt, Thomas T. Sugihara, Svein Rosseland, J. R. Oppenheimer, Meghnad N. Saha, R. H. Fowler, E. Arthur Milne, Cecilia Payne-Gaposhkin); assistant at University of Copenhaagen, 1929; LaSilla Observatory. To University of Chicago and Yerkes Observatory (Otto Struve), 1936-1939; starts work on formation of H #II regions, 1939; work at Mt. Wilson Observatory on absorption lines (Walter S. Adams, Theodore Dunham); estimates of ages of stars (Hans Bethe); Hubble Constant; comparison of astronomy in Europe and U.S.; European astronomers in U.S. (Gerard Kuiper, Polydore Swings, Carl Osbourne, Ejnar Herzsprung); comments on history of Yerkes (Struve), teaching at Chicago; discussion of work on equation of ionization and calculations of opacities (Carl von Weizsacker, Struve, S. Chandrasekhar); comments on W. W. Morgan. Discussion of work on stellar evolution, ionization of interstellar hydrogen (Struve). Effects of World War II on astronomy; influence of European astronomers on Americans; Ludwig Biermann; European Southern Observatory; developments in astrophysics during the war; optical studies. Nazi occupation of Niels Bohr Institute (Werner Heisenberg); contacts with German astronomers; development of Brorfelde Observatory. Becomes director of Yerkes and McDonald Observatory (Robert Hutchins), 1950–1957; American astronomy after World War II; relation of scientific community and government (Office of Naval Research); stellar classification work. Institute for Advanced Study, Princeton (Oppenheimer), investigation of intermediate population II and extreme population II, 1957; establishment of Kitt Peak Observatory; return to Denmark, 1967. Also prominently mentioned are: Niels Bohr, Werner Bolton, George Ellery Hale, Jacobus C. Kapteyn, Lev Landau, and Harlow Shapley.