An Ancient Universe: How Astronomers Know the Vast Scale of Cosmic Time


Coma Galaxy Cluster

This Hubble Space Telescope image shows a selection of galaxies great "islands" of billions of stars seen in every direction our telescopes can look. The area of sky seen here is very small about the size of President Roosevelt's eye on a US dime, when the dime is held at arm's length. The galaxy that appears so large at the upper left is 300 million lightyears away -- so far that light from it takes 300 million years to reach us. Some of the galaxies that look smaller are even further away. Everything on this image that is not a circular dot is in fact a galaxy of stars. (Courtesy of William Baum, the Space Telescope Science Institute, and NASA.)

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