Walter Baade

Wilhelm Heinrich Walter Baade (1893-1960) was a German-born astronomer who spent much of his career working at observatories in the United States, from 1931 to 1959. During the Second World War, as a German national exempted from internment, Baade had unusually extensive access to the Mount Wilson Observatory's 100-inch telescope while wartime blackout conditions darkened the skies over Los Angeles, and he used this opportunity to resolve individual stars in the central region of the Andromeda Galaxy for the first time. This work led him to identify two distinct populations of stars, which he termed Population I and Population II, differing in age, composition and location within galaxies, a distinction that became fundamental to modern astrophysics. Baade's recalibration of the period-luminosity relation for Cepheid variable stars in the early 1950s revealed that the universe was roughly twice as large, and twice as old, as previously calculated.