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Baade, Wilhelm Heinrich Walter

population stars relationship galaxy

[ bah duh] (1893–1960) German–US astronomer: classified stars into different population types; his work gave larger estimates for the size and the age of the universe.

Educated in Germany at Göttingen, Baade was on the staff of the University of Hamburg for 11 years before moving to the USA in 1931. He spent the Second World War at the Mount Wilson and Palomar Observatories studying the Andromeda galaxy (as a German immigrant he was excluded from military service). He used the 100 in telescope and had the advantage of the wartime blackout of Los Angeles, which cleared the night sky. He identified two fundamentally distinct classes of star in the galaxy – hot young blue stars in the spiral arms of the galaxy, which he called Population I stars, and older redder stars in the central region, which he called Population II (see HR diagram, p. 311). This distinction was to prove fundamental to theories of galactic evolution.

He showed that cepheid variable stars found in Andromeda, whose period/luminosity relationship had been discovered 30 years earlier by and quantified by as a means of calculating their distance, could also be divided into the two categories. In 1952 he demonstrated that Leavitt and Shapley’s period/luminosity relationship was only valid for Population I cepheids, and calculated a new relationship for Population II cepheids. , in the 1920s, had used the cepheid variable technique to calculate the distance of the Andromeda galaxy as 800 000 light years, from which he estimated the age of the universe to be 2000 million years. However, Hubble’s estimate proved to have depended upon Population II cepheids, for which the original period/luminosity relationship was invalid; using his new relationship Baade showed that Andromeda was more than 2 million light years away and that the universe was therefore at least 5000 million years old. (This revised time scale came as a relief to geologists, who had estimated the age of the Earth as 3000–4000 million years or more.)

Baade also discovered two asteroids, Hidalgo and Icarus, which strangely are those with (respectively) orbits which take them farthest and nearest to the Sun of all known asteroids. He also worked on supernovae and the optical identification of radio sources.

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