Find the Keystone in Hercules, and the Hercules Cluster M13

Star chart of man-shaped constellation with bent arms and legs, and several labeled stars.
Hercules is a faint constellation. But its mid-section contains the easy-to-see Keystone asterism, that is, a star pattern. You can find Hercules between the bright stars Vega in Lyra the Harp, and Arcturus in Boötes the Herdsman. And once you find the Keystone, you can easily locate M13, the Hercules cluster. Chart via EarthSky.

Use Vega to locate the Keystone in Hercules

In late spring, from mid-northern latitudes, you can easily find the brilliant star Vega in the eastern sky at dusk and nightfall. The brilliant blue-white star Vega, in Lyra the Harp, acts as your guide star to the Keystone, a wedge-shaped pattern of four stars in the constellation Hercules.

Look for the Keystone asterism – star pattern – to the upper right of Vega. Or hold your fist at arm’s length, it’ll easily fit between Vega and the Keystone.

Also, you can locate the Keystone by using Vega in conjunction with the brilliant yellow-orange star Arcturus, in Boötes the Herdsman. The Keystone is found about 1/3 of the way from Vega to Arcturus, the two brightest stars to grace the Northern Hemisphere’s spring and summertime sky. From mid-northern latitudes this time of year, Arcturus is found quite high in the eastern sky at nightfall. Then, by late evening, Arcturus moves high overhead.

Star chart of man-shaped constellation with bent arms and legs, squarish in the middle with a labeled cluster.
Before you can find M13, you need to find the Keystone in Hercules, a pattern of 4 stars. So as darkness falls, look for the Keystone to the upper right of the brilliant star Vega. Chart via EarthSky.

Use the Keystone to find M13

Furthermore, the Keystone is your ticket to find a famous globular star cluster in Hercules, otherwise known as the Hercules cluster, aka Messier 13 or M13.

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. But through binoculars, this cluster looks like a dim smudge or a somewhat fuzzy star. However, a telescope begins to resolve this faint fuzzy object into what it really is, a great, big, globe-shaped stellar city populated with hundreds of thousands of stars!

Then, later in the evening, the Keystone and the Hercules cluster swing high overhead after midnight, and are found in the western sky before dawn.

White star chart with black dots and lines showing keystone shape and lines radiating outward.
Can you find the Keystone on this chart? See the compact grouping of 4 stars at the center of Hercules? That’s it. Note the whereabouts of Messier 13 within the Keystone pattern. Also, above the Keystone is another globular cluster, M92. It’s a bit smaller and dimmer than M13, but also easy to pick up in binoculars or a telescope. Image via International Astronomical Union/ Sky & Telescope/ Wikimedia Commons (CC BY 3.0).

Photos of M13 from EarthSky Community Photos

Spherical ball of stars in the center of a dark sky with multiple stars around it.
View at EarthSky Community Photos. | Jeremy Likness captured this image on April 28, 2023, in Florissant, Colorado. He wrote: “I was really just testing tracking and focus between shots, but this cluster (Messier 13) came out so clean with only 20 minutes total exposure time I decided it was a keeper!” It’s certainly a keeper. Thank you, Jeremy!
Tight cluster of thousands of dots of white light at center, diffusing out into blackness at the edges.
View at EarthSky Community Photos. | Randall Kayfes in Tucson, Arizona, shared this image on June 3, 2022. Randall wrote: “The Hercules star cluster M13 is one giant ball of stars and a favorite go-to star cluster in the summer.” Thank you, Randall!
Bright white cluster of thousands of stars at center with smattering surrounding in black sky.
View at EarthSky Community Photos. | Ron Haggett in Yuma, Arizona, took this image of a globular cluster on January 5, 2022. Ron wrote: “Messier 13 or the great globular cluster in Hercules. Fortunately for me it is viewable around 5 in the morning!” Thank you, Ron!

Bottom line: Let the bright star Vega guide you to a famous star pattern in Hercules – called the Keystone – and then to the Hercules cluster, aka M13, a famous globular star cluster.

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May 29, 2024

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