From the Nobel Foundation:
After the early preparatory years, his scientific work has followed a certain pattern motivated, principally, by a quest after perspectives. In practise, this quest has consisted in his choosing (after some trials and tribulations) a certain area which appears amenable to cultivation and compatible with his taste, abilities, and temperament. And when after some years of study, he feels that he has accumulated a sufficient body of knowledge and achieved a view of his own, he has the urge to present his point of view, ab initio, in a coherent account with order, form, and structure.
There have been seven such periods in his life: stellar structure, including the theory of white dwarfs (1929-1939); stellar dynamics, including the theory of Brownian motion (1938-1943); the theory of radiative transfer, including the theory of stellar atmospheres and the quantum theory of the negative ion of hydrogen and the theory of planetary atmospheres, including the theory of the illumination and the polarization of the sunlit sky (1943-1950); hydrodynamic and hydromagnetic stability, including the theory of the Rayleigh-Bénard convection (1952-1961); the equilibrium and the stability of ellipsoidal figures of equilibrium, partly in collaboration with Norman R. Lebovitz (1961-1968); the general theory of relativity and relativistic astrophysics (1962-1971); and the mathematical theory of black holes (1974- 1983)…
Nobel Laureate, Physics Prize, 1983