Hubble Sees Andromeda: Universe Expands Beyond the Milky Way

A single photograph changed humanity's understanding of the universe. Edwin Hubble, working at the Mount Wilson Observatory above Los Angeles, announced on November 23, 1924, that what astronomers had called the Andromeda Nebula was actually an entirely separate galaxy, lying roughly 900,000 light-years beyond the Milky Way. In one stroke, the known universe expanded from a single galaxy to a cosmos of staggering, perhaps infinite, scale. The prevailing scientific consensus held that the Milky Way was the entire universe. The fuzzy patches visible through telescopes, called nebulae, were assumed to be gas clouds within our galaxy. A 1920 debate between astronomers Harlow Shapley and Heber Curtis, known as the Great Debate, had failed to resolve the question. Shapley argued the nebulae were local. Curtis believed at least some were separate "island universes" at enormous distances. Neither had conclusive proof. Hubble found it. Using Mount Wilson's 100-inch Hooker Telescope, then the world's most powerful, he identified Cepheid variable stars within Andromeda. These stars pulsate at a rate directly related to their true brightness, allowing astronomers to calculate distance by comparing actual brightness to apparent brightness. Hubble's measurements placed Andromeda far beyond the boundaries of the Milky Way. His original estimate of 900,000 light-years was later revised to approximately 2.5 million, but the fundamental conclusion was unassailable. The discovery demolished the small-universe model overnight. Hubble went on to show that distant galaxies are receding from us at speeds proportional to their distance, establishing the expansion of the universe and laying the observational foundation for the Big Bang theory. A shy, meticulous man who had once practiced law before turning to astronomy, Hubble reshaped cosmology more profoundly than anyone since Copernicus.