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• Beyond the Milky Way

Star clusters are groups of stars close to each other in space that appear to have roughly similar characteristics and, therefore, a common origin. Some of the over 100 billion stars in the Milky Way, our home galaxy, are grouped together in either tight or loose star clusters. More than 100 tight groupings, called globular clusters, surround the galaxy's spiral arms in a great halo. Loose groupings, called open clusters or galactic clusters, are far more numerous and are found toward the center of the galaxy.

Globular clusters radiate with a continuous glow. These nearly spherical (ball-shaped) star systems contain anywhere from tens of thousands to one million stars. They are most heavily concentrated at the center of the cluster. While in reality there is a...