Annie Jump Cannon publishes the fundamental catalogue of star spectra. 1924 Hubble proves that the galaxies lie beyond the Milky Way. Oort proves that the center of the galaxy lies in direction of Sagittarius and rotates. Georges Lemaitre provides a model for an expanding universe. Edwin Powell Hubble discovers linear relationship between the galaxy distance and its radial velocity.
Galaxies recede from each other.
Jansky discovers cosmic radio waves. Dark matter discovered in galactic clusters. Baade and Zwicky propose supervova theory of dying stars. Chandrasekhar calculates mass limit of white dwarf star. Grote Reber constructs the first radio telescope.