US astronomer who discovered the existence of other galaxies outside our own, and classified them according to their shape. His theory that the universe is expanding is now generally accepted.
Hubble discovered Cepheid variable stars in the Andromeda galaxy 1923, proving it to lie far beyond our own Galaxy. In 1925 he introduced the classification of galaxies as spirals, barred spirals, and ellipticals. In 1929 he announced Hubble's law, which states that the galaxies are moving apart at a rate that increases with their distance.
Hubble was born in Marshfield, Missouri, and studied at Chicago and in the UK at Oxford. He briefly practised law before returning to Chicago to join Yerkes Observatory 1914. From 1919 he worked at Mount Wilson, near Pasadena, California.
His data on the speed at which galaxies were receding (based on their red shifts) were used to determine the portion of the universe that we can ever come to know, the radius of which is called the Hubble radius. Beyond this limit, any matter will be travelling at the speed of light, so communication with it will never be possible. The ratio of the velocity of galactic recession to distance has been named the Hubble constant.