From mid-northern latitudes, you can easily find the brilliant star Vega in the eastern sky at dusk and nightfall on these late May evenings. Vega acts as your guide star the Keystone – a pattern of four stars in the constellation Hercules. The Keystone, in turn, is your ticket to finding a famous globular star cluster in Hercules, otherwise known as the Great Cluster in Hercules, aka Messier 13 or M13.
As darkness falls, look for the Keystone asterism – star pattern – to the upper right of the brilliant blue-white star Vega. Hold your fist at an arm length. There is easily enough room between Vega and the Keystone for your fist to fit beneath the two.
You can also locate the Keystone by using Vega in conjunction with the brilliant yellow-orange star Arcturus. From mid-northern latitudes, Arcturus is found way high in the southern sky at nightfall and evening. The Keystone is found about one-third the way from Vega to Arcturus, the two brightest stars to grace the Northern Hemisphere’s spring and summertime sky.
Most likely, you’ll need binoculars to see the Hercules cluster, although sharp-eyed people can see it with the unaided eye in a dark, transparent sky. Through binoculars, this cluster looks like a dim and somewhat hazy star. But the telescope begins to resolve this faint fuzzy into what it really is – a great big, globe-shaped stellar city populated with hundreds of thousands of stars!
The Keystone and the Hercules cluster swing high overhead after midnight, and are found in the western sky before dawn. Before seeking out the Keystone this evening, don’t forget to look in the west at evening dusk for the planets Mercury and Jupiter.
Sky chart of the constellation Hercules, the Keystone and M13
Bottom line: Let the bright star Vega guide you to a famous star pattern in Hercules – called the Keystone – and then to the Great Cluster in Hercules, aka M13, a famous globular star cluster.