Edwin Hubble

Edwin Powell Hubble was born in Marshfield, Missouri, in 1889. He was an unusual recruit to astronomy: a strong athlete, a Rhodes Scholar at Oxford where he studied law, and briefly a practicing lawyer in Kentucky before abandoning the profession entirely. 'I chucked the law for astronomy,' he later said, 'and I knew that even if I were second-rate or third-rate, it was astronomy that mattered.' He earned his doctorate at the Yerkes Observatory in 1917, served in the U.S. Army in France during the First World War, and in 1919 took a post at the Mount Wilson Observatory in California, home to the new 100-inch Hooker telescope — the largest in the world.

Hubble's first epochal result came in 1923-1925. By identifying Cepheid variable stars in the Andromeda 'nebula' and applying Henrietta Leavitt's period-luminosity relation, he measured a distance of roughly 900,000 light-years, placing Andromeda far outside the Milky Way. The spiral nebulae were independent galaxies — 'island universes' — and the cosmos was vastly larger than the single galaxy most astronomers had assumed. The debate over the scale of the universe, dramatized in the 1920 Shapley-Curtis 'Great Debate,' was settled by Hubble's photographic plates.

In 1929 Hubble combined his Cepheid-based distances with the recession velocities Vesto Slipher had measured spectroscopically and published a plot of velocity against distance for 24 galaxies. The points scattered, but the trend was clear and linear: the more distant a galaxy, the faster it recedes. This velocity-distance relation, v = H_0 d, is now called Hubble's law, and it is the foundational observation of physical cosmology. Hubble's slope was about seven times too steep because his distance ladder was miscalibrated, but the linear relation itself was correct and became the empirical anchor of the expanding-universe picture.

Hubble spent the rest of his career classifying galaxies (the 'Hubble tuning-fork' morphological sequence remains in use) and extending the velocity-distance survey to fainter, more distant systems with his collaborator Milton Humason. He campaigned, ultimately successfully, to have astronomy recognized by the Nobel Prize committee — though he died of a cerebral thrombosis in 1953 before the rules changed, and the prize is not awarded posthumously. The Hubble Space Telescope, launched in 1990, carries his name, and the central constant of cosmology, H_0, is his initial.