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Walter Baade
Wilhelm Heinrich Walter Baade (March 24, 1893 – June 25, 1960) was a German astronomer who worked in the USA from 1931 to 1959.
== Biography ==
He took advantage of wartime blackout conditions during World War II, which reduced light pollution at Mount Wilson Observatory, to resolve stars in the center of the Andromeda galaxy for the first time, which 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 Hubble in 1929.〔Baade W (1944) The resolution of Messier 32, NGC 205, and the central region of the Andromeda nebula. ApJ 100 137-146〕〔Baade W (1956) The period-luminosity relation of the Cepheids. PASP 68 5-16〕 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.〔W. Baade, F. Zwicky, 1934, "(On Super-Novae )". Proceedings of the National Academy of Sciences of the United States of America, 254-259.〕〔Donald E. Osterbrock, ''Walter Baade – A Life in Astrophysics'', Princeton und Oxford: Princeton University Press 2001. ISBN 0-691-04936-X. In his biography Osterbrock states, p. 32, that Baade in his inaugural lecture 1929 in Hamburg already used the German phrase "Hauptnova", "chief nova, Baades early word for a supernova" (Osterbrock).〕 Zwicky and he also proposed the existence of neutron stars, and proposed that supernovae could create neutron stars.
Beginning in 1952 he and Rudolph Minkowski identified the optical counterparts of various radio sources,〔Baade, W. and Minkowski, R., 1954. Identification of the Radio Sources in Cassiopeia, Cygnus A, and Puppis A. Astrophysical Journal, Vol. 119, p. 206-214 (January 1954) (ADS: 1954ApJ...119..206B )〕 including Cygnus A. He discovered 10 asteroids, including notably 944 Hidalgo (long orbital period) and the Apollo-class asteroid 1566 Icarus (the perihelion of which is closer than that of Mercury) and the Amor asteroid 1036 Ganymed.
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