Baade, (Wilhelm Heinrich) Walter (1893–1960)

German-born US astronomer who made observations that doubled the distance, scale, and age of the universe. He discovered that stars are in two distinct populations according to their age, known as Population I (the younger) and Population II (the older). Later, he found that Cepheid variable stars of Population I are brighter than had been supposed and that distances calculated from them were wrong.

Baade, born in Shröttinghausen, studied at the universities of Münster and Göttingen. He was awarded a PhD by Göttingen in 1919. He emigrated to the USA in 1931, working at Mount Wilson Observatory, Pasadena, California, until 1948 and nearby Mount Palomar Observatory until 1958, when he returned to Germany.

During World War II, in 1943, Baade made use of the blackout darkness to study the Andromeda galaxy and was able to observe, for the first time, some of the stars in the inner regions of that galaxy. He found that the most luminous stars towards the centre, which he called Population II stars, are not blue-white but reddish, and proposed that these have differing structures and origins from the stars in the spiral arms of the galaxy, which he called Population I stars. Population I stars, which are bluish, are young and formed from the dusty material of the spiral arms – hydrogen, helium, and heavier elements; Population II stars, reddish, are old, were created near the nucleus and contain fewer heavy elements.