Baade, Walter definition of Baade, Walter in the Free Online Encyclopedia.

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 populations stellar 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.
..... Click the link for more information. , 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 variables Cepheid variables (sē`fēĭd), class of variable stars that brighten and dim in an extremely regular fashion.
..... Click the link for more information. 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 Way Milky 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.
..... Click the link for more information. . With Fritz Zwicky and Rudolf Minkowski he distinguished two types of supernova supernova, 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.
..... Click the link for more information. based on their spectra and on their maximum absolute magnitudes. In 1949 he discovered Icarus, an asteroid whose orbit takes it close to Earth.

Bibliography

See W. Baade, Evolution of Stars and Galaxies (1963).