© AIP Emilio Segrè Visual Archives.
This photograph shows Edwin Hubble seated at the 100-inch Hooker telescope at Mt. Wilson Observatory. Hubble used this telescope to survey Cepheid variables in other galaxies. By taking many photographs of the stars over a long period of time, Hubble was able to measure the periodic change in brightness of these stars. He could then apply Henrietta Leavitt's relation between period and luminosity and the inverse square law to determine the distance to the stars, finding that they were far outside our galaxy. Hubble also measured the velocities of these distant galaxies, and realized that the universe was expanding rather than static as previously thought. (Unit: 11)