Baade worked at
Hamburg Observatory at Bergedorf from 1919 to 1931. In 1937, the University of Hamburg wanted Baade as successor of
Richard Schorr for the Hamburg Observatory, but he refused. During World War II, while working at
Mount Wilson Observatory, Baade took advantage of wartime blackout conditions (which reduced
light pollution), to resolve stars in the center of the
Andromeda Galaxy for the first time. These observations led him to define distinct "
populations" for stars (
Population I and
Population II). The same observations led him to discover that there are two types of
Cepheid variable stars. Using this discovery he recalculated the size of the known universe, doubling the
previous calculation made by
Edwin Hubble in 1929. He announced this finding to considerable astonishment at the 1952 meeting of the
International Astronomical Union in Rome. Together with
Fritz Zwicky, he identified
supernovae as a new category of astronomical objects. Zwicky and he also proposed the existence of
neutron stars, and suggested supernovae might create them. Beginning in 1952, he and
Rudolph Minkowski identified the optical counterparts of various
radio sources, including
Cygnus A. He discovered 10
asteroids, including
944 Hidalgo, which has a long orbital period (it is actually the first
centaur ever discovered, although they were not recognized as a distinct dynamical class until 1977); the
Apollo-class
1566 Icarus, the perihelion of which is closer than that of
Mercury; and the
Amor-type
1036 Ganymed. ==Personal life==