Which galaxy did Hubble prove was separate from the Milky Way in 1923?

Hubble proved that the Andromeda Galaxy was separate from the Milky Way in 1923. At the time, astronomers often called it the Andromeda Nebula and were still debating whether spiral nebulae were relatively small objects inside our galaxy or vast independent galaxies.

Using the 100-inch Hooker Telescope at Mount Wilson Observatory, Hubble found Cepheid variable stars in Andromeda. Because Cepheids have a period–luminosity relationship, their apparent brightness could be used to estimate their distance. The result placed Andromeda far beyond the known dimensions of the Milky Way, demonstrating that it was an external galaxy.

This discovery helped resolve the Great Debate between Harlow Shapley and Heber Curtis, dramatically expanding the known scale of the universe. Andromeda is now designated M31 and is the nearest major galaxy to the Milky Way, at roughly 2.5 million light-years away. It is a large spiral galaxy and, together with the Milky Way, the dominant member of the Local Group. The Triangulum Galaxy is also in that group, but it was not Hubble's landmark 1923 target.