At the start of the twentieth century, astronomers knew that the Milky Way was an enormous stellar system, but the status of the faint spiral nebulae remained unresolved. Were they objects inside our Galaxy, or distant “island universes” comparable to the Milky Way itself?
The dispute was serious enough to produce the 1920 Great Debate between Harlow Shapley and Heber Curtis. Shapley’s enlarged estimate of the Milky Way made it easier to place the spirals inside it, while Curtis argued for an extragalactic interpretation.
The decisive evidence was distance. Edwin Hubble identified Cepheid variable stars in the Andromeda nebula and used their period–luminosity relationship to estimate a distance far beyond the known bounds of the Milky Way. Andromeda was not a nearby cloud inside our Galaxy; it was another galaxy.
Similar measurements rapidly expanded the universe from one stellar system to a cosmos filled with galaxies.
The pre-1920s view was not unanimous, and the change did not come merely from sharper photographs. It came from turning visible objects into measured distances. The Milky Way went from being plausibly almost everything to being one galaxy among an immense population.
