In 1944 he identified two different populations of stars in the Galaxy, and found that both contained Cepheid variables. Moreover, each population of Cepheids had its own period–luminosity relation. In 1952 he began to search for Cepheids in the Andromeda Galaxy, M31, but found none, even though they should have been luminous enough to be visible; he reasoned that M31 was over twice as distant as had been thought, which made the Universe both larger and older than had been believed. Baade collaborated with R. L. B. Minkowski on identifying radio sources, and with Minkowski and F. Zwicky on supernovae. He also discovered ten asteroids, including Hidalgo and Icarus.