Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an American astronomer. He played a crucial role in establishing the fields of extragalactic astronomy and observational cosmology.
Hubble proved that many objects previously thought to be clouds of dust and gas and classified as "nebulae" were actually galaxies beyond the Milky Way. He used the strong direct relationship between a classical Cepheid variable's luminosity and pulsation period (discovered in 1908 by Henrietta Swan Leavitt) for scaling galactic and extragalactic distances.
Hubble confirmed in 1929 that the recessional velocity of a galaxy increases with its distance from Earth, a behavior that became known as Hubble's law, although it had been proposed two years earlier by Georges Lemaître. The Hubble law implies that the universe is expanding.