Walter Baade (väl´tər bä´də), 1893–1960, German-born American astronomer. From 1919 to 1931 he was on the staff of the Hamburg observatory; from 1931 to 1958, at the Mt. Wilson observatory. Baade studied the Andromeda Galaxy, M31, and other spiral galaxies and presented evidence for the existence of two different stellar populations, the younger Population I, and the older Population II. From these data he inferred that similar spiral patterns could be found in the Milky Way. Perhaps his most important contribution came in 1952 from observations of Cepheid variables in nearby galaxies through the 200-in. reflecting telescope at the Palomar Observatory; he calculated that it was necessary to double the cosmic-distance scale, i.e., the distances between external galaxies and the Milky Way. With Fritz Zwicky and Rudolf Minkowski he distinguished two types of supernova based on their spectra and on their maximum absolute magnitudes. In 1949 he discovered Icarus, an asteroid whose orbit takes it close to Earth.
See W. Baade, Evolution of Stars and Galaxies (1963).