The Great Hercules Cluster (M13) is one of the sky's most precious jewels and among the best globular clusters in the Northern Hemisphere. Visible to the naked eye under dark skies, M13 looks like a fuzzy ball of light in binoculars. A moderate-sized telescope and high magnification shows a blazing ball of stars with many individual members resolved.

The Great Hercules Cluster contains about 400,000 stars, spread across 140 light-years of space. The star density near the cluster's center is extremely high, with stars separated by only a few astronomical units.

M13 was originally discovered by Edmond Halley. Because it compacts a lot of stars in a small area, M13 was selected as a target for one of the first radio messages broadcast to extraterrestrials from the Arecibo Observatory in Puerto Rico. But don't stay on hold waiting for them to pick up the call. If alien civilizations exist in the Great Hercules Cluster, we won't receive their reply for at least 50,000 years; M13 is 25,000 light-years from Earth.

Click here to see if this object is visible in your sky tonight.