Walter Baade

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Baade, Walter

(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 populationsstellar populations,
two broadly contrasting distributions of star types that are characteristic of different parts of a galaxy. Population I stars are young, recently formed stars, whereas population II stars are old and highly evolved.
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, 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 variablesCepheid variables
, class of variable stars that brighten and dim in an extremely regular fashion. The periods of the fluctuations (the time to complete one cycle from bright to dim and back to bright) last several days, although they range from 1 to 50 days.
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 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 WayMilky Way,
the galaxy of which the sun and solar system are a part, seen as a broad band of light arching across the night sky from horizon to horizon; if not blocked by the horizon, it would be seen as a circle around the entire sky.
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. With Fritz Zwicky and Rudolf Minkowski he distinguished two types of supernovasupernova,
a massive star in the latter stages of stellar evolution that suddenly contracts and then explodes, increasing its energy output as much as a billionfold. Supernovas are the principal distributors of heavy elements throughout the universe; all elements heavier than
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 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).

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