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

THE PLEIADES STAR CLUSTER

Stars are often found in groups. This relatively nearby grouping is about 400 lightyears away, and contains several hundred stars. The brightest of them are visible to the naked eye or in binoculars, and are labeled here with their names from classical mythology. The stars look fuzzy because there is a cloud of dust moving among them and the dust reflects the stars' light. (Courtesy of the Space Telescope Science Institute Digital Sky Survey.)

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Changes in Stars