M13

The Great Hercules Cluster is one of the sky's most precious jewels and the finest globular cluster 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 resolved members. 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, better known for predicting the return of the comet that still bears his name. M13 was selected as a target for one of the first radio messages broadcast to extraterrestrials from the Arecibo observatory in Puerto Rico. If alien civilizations exist in the Great Hercules Cluster, we won't receive their reply for 50 000 years, as M13 is 25 000 light years from Earth.