ROBERT GENDLER / SCIENCE PHOTO LIBRARY ROBERT GENDLER / SCIENCE PHOTO LIBRARY
Hercules globular cluster (M13), optical image. Globular clusters are groups of old stars that orbit a galaxy. They are tightly bound by gravity, giving them a spherical shape. The Hercules cluster, orbiting the Milky Way galaxy, consists of hundreds of thousands of stars and is 168 light years across. It lies around 25,100 light years from Earth in the constellation Hercules.
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