Edwin Hubble showed that many so-called nebulae were actually galaxies. Before the 1920s, astronomers debated whether spiral nebulae were small clouds within the Milky Way or enormous, independent star systems far beyond it.
Working with the 100-inch Hooker Telescope at Mount Wilson Observatory, Hubble identified Cepheid variable stars in objects such as the Andromeda Nebula. Henrietta Swan Leavitt's period–luminosity relation made Cepheids useful standard candles: their pulsation periods revealed their intrinsic brightness, allowing astronomers to estimate their distances. The distances placed these objects well outside the Milky Way.
Hubble's observations in 1924 helped settle the Great Debate over the scale of the universe. The old word “nebula” survives in astronomy for clouds of gas and dust, such as the Orion Nebula, but galaxies are vast systems containing stars, gas, dust, and dark matter. Hubble also developed a widely used visual classification system for galaxies, later called the Hubble sequence.