Mount Wilson

As a student at Chicago, Hubble had attracted the attention of the well-known astronomer G. E. Hale, and after the war Hale offered him a staff position at Mount Wilson Observatory near Pasadena, Calif. Except for the period 1942-1946, when Hubble was with the Ordnance Department in Aberdeen, Md., he was connected with the Mount Wilson Observatory for the rest of his life.

Hubble's early observations at Mount Wilson were made with its 60-inch reflecting telescope and concentrated on objects within our own galaxy, for example, novae, nebulous stars, and variable stars. Gradually he began to observe more distant objects. To determine the distances of the spiral nebulae (galaxies), he used Cepheid variable stars. This method derived from Henrietta S. Leavitt's 1912 discovery that the period of variationin the intensity of these stars is directly related to their absolute magnitude, so that by measuring the former, one may easily determine the latter. By knowing the star's absolute magnitude and measuring its apparent magnitude, its (relative) distance may be readily calculated from the inverse-square law.

In 1923 Hubble definitely recognized a Cepheid variable in the Andromeda Nebula, known to astronomers as M31. Others were soon found in M31 and its companion nebula M33. To obtain his photographs, Hubble used Mount Wilson's 100-inch telescope. Once he had located the variables and determined their periods and apparent magnitudes, he used Leavitt's period-luminosity relationship to determine their distances. He concluded that the great spiral Andromeda Nebula is roughly 900,000 light-years away, a fantastically large distance, placing it clearly outside our