Stellar Populations

Walter Baade discovered the stellar populations during World War II, which contributed to his discovery. During the war, most scientists worked on war-related projects, such as the Manhattan Project. Baade, who was on the staff of Mount Wilson Observatory near Los Angeles, was not allowed to work on war projects because he was German-born. He did, however, conduct research at Mount Wilson Observatory, which at the time had the world's largest telescope. The wartime blackouts in Los Angeles contributed to darker skies on Mount Wilson. There were also few astronomers wanting to use the telescope because they were working on war-related projects. So Baade had plenty of time to use the world's largest telescope under better than normal sky conditions.

During this time Baade was able to resolve, for the first time, the stars in the Andromeda Galaxy. He noticed that there were two distinct populations composed of predominantly red and blue stars. He labeled them Population I and Population II. Further study since that time has produced the additional subclassifications.