A Universe of Galaxies

Using the Hooker telescope at Mount Wilson, Hubble identified Cepheid variable stars in Andromeda. Their inferred distances placed the object far beyond the Milky Way, supporting the view that spiral nebulae were separate galaxies.

The measurement depended on Leavitt’s period–luminosity relationship for Cepheids. Hubble’s observational reach and that prior calibration together expanded the known scale of the universe.