Imagine drawing a line from Phecda, the star on the bottom of the bowl of the Big Dipper closest to the handle, and continuing through Mizar, the star at the bend of the handle. It will pass by the constellation Hercules. Using Mizar as your starting point, stretch your arm out at full length and measure about two spread hands from little finger to thumb.

Hercules is an ancient and faint constellation devoid of bright stars, such as those forming the Summer Triangle. Four of its main stars form a trapezoidal asterism called "the Keystone." On one side of the Keystone is a beautiful globular cluster—a stellar ball containing several hundred thousand stars.

The Hercules cluster—also named Messier 13 or simply M13—is one of the sky's most beautiful sights and the finest globular cluster in the Northern Hemisphere. It is visible with the unaided eye by observers far removed from a city's glow. In binoculars, M13 looks like a fuzzy star; it's one of the more spectacular objects in the sky when seen through a telescope.

Edmond Halley, better known for predicting the return of the comet that still bears his name, discovered the Hercules Cluster.

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, because M13 is 25,000 light-years from Earth.