A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae
Hubble combined distances to roughly two dozen extra-galactic nebulae (galaxies), estimated largely from Cepheid variables and other stellar indicators, with their measured radial velocities to test for a systematic relationship. He found an approximately linear correlation in which the radial velocity of a nebula increases with its distance, with a proportionality constant of about 500 km/s per megaparsec. This velocity–distance relation, now known as Hubble's law, provided the first observational evidence that the universe is expanding and became a cornerstone of modern cosmology.