The concept of a galaxy as a vast organized system of stars originates with Galileo's Observation in 1610 that the Milky why cound be resolved into “innumerable” faint stars. Confirmation that many of the faint patches of light seen through small telescopes were actually galaxies like our own came in 1923, when Edwin Hubble, working with the just‐completed 2.4‐meter telescope at Mount Wilson in southern California, measured the brightnesses of individual stars in the Andromeda nebula. At about the same time, Vesto Slipher, working at the Lowell Observatory in Arizona, was making the first systematic measurements of the Doppler shifts of galaxies. Of several dozen galaxies measured, all but two were found to be receding. In 1929, Hubble showed that the recession velocities of Slipher's galaxies were directly proportional to their distances, thus introducing the concept of an expanding universe
Probing the Faintest Galaxies
Henry C. Ferguson, Robert E. Williams, Lennox L. Cowie; Probing the Faintest Galaxies. Physics Today 1 April 1997; 50 (4): 24–30. https://doi.org/10.1063/1.881724
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