In 1924, Edwin Hubble discovered that galaxies are immense collections of gravitationally bound stars. Astronomers have since catalogued thousands of galaxies. Hubble also found a close relationship between a galaxy's distance and its "red shift," a change in light wavelengths caused by rapid movement away from us. As telescopes have improved, the estimated number of galaxies has grown to tens of billions.
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Ready to Watch The Joy of Science, Season 1, Episode 31?