Walter Baade

Walter Baade was a German-American astronomer who, taking advantage of the darker night skies produced by wartime blackout conditions around the Mount Wilson Observatory during the Second World War, was able to resolve individual stars in the central region of the Andromeda Galaxy for the first time. This let him identify two distinct populations of stars, which he named Population I and Population II, differing in age, chemical composition and the region of a galaxy where they are typically found, a distinction that became fundamental to understanding how galaxies form and evolve. Baade further showed that the Cepheid variable stars used as standard distance markers came in two similarly distinct types with different luminosities, and correcting for this previously unrecognized difference in 1952 roughly doubled the accepted scale, and inferred age, of the observable universe. Earlier, with Fritz Zwicky in 1934, Baade had proposed that supernovae, extremely energetic stellar explosions, could produce extraordinarily dense remnant objects called neutron stars, a prediction confirmed decades later.