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M13: The Great Globular Cluster in Hercules
M13 is one of the most prominent and best known globular clusters. Visible with binoculars in the constellation of Hercules, M13 is frequently one of the first objects found by curious sky gazers seeking celestials wonders beyond normal human vision. M13 is a colossal home to over 100,000 stars, spans over 150 light years across, lies over 20,000 light years distant, and is over 12 billion years old. At the 1974 dedication of Arecibo Observatory, a radio message about Earth was sent in the direction of M13. The reason for the low abundance of unusual blue straggler stars in M13 is currently unknown.
Elliptical galaxies and globular clusters have a lot in common. There is no gas or dust in a globular cluster, and the stars are old. In fact, globular clusters may be the some of the oldest objects in the universe. The big difference is size! Globular clusters contain hundreds of thousands or millions of stars. Elliptical galaxies can contain hundreds of billions of stars! And while elliptical galaxies are sometimes round, globular clusters are never elliptical. Globular clusters are found both in spiral galaxies and elliptical galaxies. The Milky Way, for example, has over one hundred globular clusters throughout it's halo.