He 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.
At Mount Wilson observatory in 1923 he discovered Cepheid variable stars in the Andromede galaxy, 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.
