Walter Baade

In 1944, Walter Baade resolved individual stars in the core of the Andromeda Galaxy for the first time, using the 100-inch Hooker Telescope at Mount Wilson Observatory during wartime blackout conditions. This observation led him to distinguish two stellar populations within the galaxy.

Walter Baade classified stars into Population I and Population II groups based on their age, composition, and location within galaxies. Population I stars are younger, metal-rich stars found in spiral arms, while Population II stars are older, metal-poor stars found in globular clusters and galactic halos.

Walter Baade worked primarily at the Mount Wilson Observatory in California after emigrating from Germany to the United States in the 1930s. He later also conducted research using the 200-inch Hale Telescope at Palomar Observatory when it came online.

Baade's distinction between Population I and Population II stars led him to revise the cosmic distance scale in the early 1950s, effectively doubling the estimated distance to galaxies. This correction roughly doubled the estimated age of the universe and resolved a paradox in which some stars appeared older than the universe itself.