In 1946, research in solar spectrophotometry began (D. Chalonge, J.C. Pecker, R.Michard), as well as research on light diffusion (G. de Vaucouleurs). Geophysics started being used in studies on atmospheric ozone, auroras, zodiacal light and nocturnal sky (D. Barbier, E. Vigroux). A “calculations office” was created, where new methods of numerical computation were worked out. Mechanical and optical workshops, as well as a mirror aluminizing facility were also created to design new generations of observing tools and to improve analysis instruments.

Simultaneously, theoretical research was also being developed: theories of stellar and solar atmospheres (V. Kourganoff starting in 1945, J.C. Pecker starting in 1946), theories on stars’interiors and evolution (E. Schatzman and his students). Theoretical astrophysics gradually gained a more important role over the years, and its main objective was to understand the nature and the evolution of celestial bodies. After Henri Mineur died in 1954, the IAP was run by André Danjon (from 1954 to 1960), then by André Lallemand (from 1960 to 1971), whose research activities were still based in the Observatoire de Paris. Many subjects of study were added to the ones previously mentioned: plasma physics, radiation transfer, interstellar matter and HII regions, laboratory astrophysics (plasma arcs, atomic physics), stochastic problems, physics of planets and terrestrial atmospheres. Astrophysics began to be taughts in university curricula, alongside fundamental astronomy and celestial mechanics, attracting many fans of this new science. The IAP’s international renown grew in the early sixties, with the significant scientific contributions of Evry Schatzman and his students, who made up a particularly dynamic and productive group of theoretical astrophysicists.