Walter Baade
Born	(1893-03-24)March 24, 1893 Schröttinghausen, German Empire
Died	June 25, 1960(1960-06-25) (aged 67) Göttingen, West Germany
Citizenship	German
Alma mater	University of Göttingen
Known for	Coined the term "supernova" and "neutron star" with Fritz Zwicky
Awards	Bruce Medal 1955
Scientific career
Fields	Astronomy
Institutions	Hamburg-Bergedorf Observatory, Mt. Wilson, Palomar Observatory
Doctoral students	Halton Arp Allan Sandage

Wilhelm Heinrich Walter Baade (March 24, 1893 – June 25, 1960) was a German astronomer who worked in the United States from 1931 to 1959.

Baade was born the son of a teacher in North Rhine-Westphalia, Germany. He finished school in 1912. He then studied maths, physics and astronomy at the universities of Münster and Göttingen. He received his PhD in 1919.^[1]

Baade worked at Hamburg Observatory at Bergedorf from 1919 to 1931.^[1] In 1920 he discovered 944 Hidalgo, the first of a class of minor planets now called Centaurs which cross the orbits of giant planets.

From 1931 to 1958, he worked at Mount Wilson Observatory^[2]

In 1937, the University of Hamburg wanted Baade as successor of Richard Schorr for the Hamburg Observatory, but he refused.^[3]

At Mount Wilson Observatory, during World War II, he 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.^[4][5][6] 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.^[7][8] Zwicky and he also proposed the existence of neutron stars, and suggested supernovae might create them.^[9]

Beginning in 1952, he and Rudolph Minkowski identified the optical counterparts of various radio sources,^[10] 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.

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He died in 1960 in Göttingen, West Germany.

Awards

• Foreign membership of the Royal Netherlands Academy of Arts and Sciences (1953)^[12]
• Elected Member of the American Philosophical Society (1953).^[13]
• Gold Medal of the Royal Astronomical Society (1954)
• Bruce Medal (1955)
• Henry Norris Russell Lectureship of the American Astronomical Society (1958)

Named after him

• Asteroid 1501 Baade
• The crater Baade on the Moon
• Vallis Baade, a vallis (valley) on the Moon
• One of the two Magellan telescopes
• The asteroid 966 Muschi, after his wife's nickname

• Baade's Window, an observational area he identified in the 1940s as being relatively free of dust that presents a view of the Galactic Center in Sagittarius
• Baade's Star, now known as the Crab Pulsar, was first identified as being directly associated with the Crab Nebula by him.

• Osterbrock, Donald E (2001-10-14). Walter Baade: A Life in Astrophysics. Princeton University Press. ISBN 0-691-04936-X.
• Dieke, Sally H. (1970). "Baade, Wilhelm Heinrich Walter". Dictionary of Scientific Biography. Vol. 1. New York: Charles Scribner's Sons. pp. 352–354. ISBN 0-684-10114-9.

• Bruce Medal page
• Awarding of Bruce Medal
• Awarding of RAS gold medal

• JRASC 55 (1961) 113
• MitAG 14 (1961) 5
• QJRAS 2 (1961) 118