Walter Baade

(1893—1960) German-born American astronomer

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German-born US astronomer, who made valuable contributions to knowledge of stellar and galactic evolution.

The son of a schoolteacher, Baade was educated at the universities of Munster and Göttingen, where he obtained his PhD in 1919. He began his career at the Hamburg Observatory, but moved to the USA in 1931 in search of bigger telescopes. The rest of his career, until his retirement in 1958, was spent at the Mount Wilson and Palomar Observatories in California.

Baade's most significant work was carried out in the 1940s, when the Los Angeles blackout imposed during World War II provided an unusually dark sky. As an enemy alien he was permitted to use the 100-inch telescope while other astronomers were doing their war work. In these favourable conditions he began a careful survey of the Andromeda galaxy, finding that, for the first time, stars in the central region could be resolved. He went on to distinguish two types of stars: population I stars – young, hot, and blue but found only in the arms of the galaxy; and the older population II stars – found in the central galactic area. Baade was able to use the distinction between the two stellar types to more than double the estimated age of the universe. He is also remembered for his discovery of two minor planets, Hidalgo in 1920 and Icarus in 1949.

Subjects: Science and Mathematics.

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