Walter Baade

Wilhelm Heinrich Walter Baade was a German observational astronomer whose sharp eyes and ingenious exploitation of wartime blackout conditions recalibrated the entire cosmic distance scale. Emigrating to the United States in 1931 to work at Mount Wilson and Palomar Observatories, Baade was classified as an enemy alien during World War II. While other astronomers left for military ballistics work, Baade was granted near-exclusive access to the 100-inch telescope, capitalizing on the darkened skies of wartime Los Angeles.

Resolving stars in the central nucleus of the Andromeda Galaxy for the first time, Baade realized stars belong to two distinct generations: young, metal-rich Population I stars in spiral arms, and old, metal-poor Population II stars in galactic bulges and globular clusters. Recognizing that Edwin Hubble had conflated two types of Cepheids with different period-luminosity curves, Baade announced at the 1952 Rome IAU meeting that galaxies were twice as far away and the universe twice as old as previously thought. With Fritz Zwicky, he also coined the term 'supernova' and predicted neutron stars.