November 23 – Edwin Hubble announces his discovery that Andromeda, previously believed to be a nebula, is actually another galaxy, and that the Milky Way is only one of many such galaxies in the universe.[1]
^L. Vaillard; Agathe Floderer; Alexandre Wauthier. "Vallin Émile Arthur". cths.fr (in French). Comité des travaux historiques et scientifiques. Retrieved 11 February 2021.