Keystone shape with arms and legs spiraling out from the corners and stars labeled for Hercules.

EarthSky | Hercules the Strong Man and a large globular cluster

The constellation Hercules lies among the bright stars vegan in Lyra and Arcturus in Bootes. a famous globular clusterknown as M13is located on the Cornerstone, and asterism in Hercules. Map via Chelynne Campion.

Hercules is the strongman of ancient mythology. He was a son of Jupiter who had to perform the famous Twelve Labors. Astronomers know Hercules as a constellation high in the northern sky on June evenings where a asterism known as the Keystone, where you can find what is possibly the best globular cluster for observers in the Northern Hemisphere: M13or the Great Cluster in Hercules.

Hercules is one of the largest of the 88 constellations and ranks 5th in size.

How do you find Hercules

Hercules is next to the bright star vegan in the constellation Lyra, which is high in the summer sky. In particular, Hercules lies west of Lyra and east of Boötes with its bright star Arcturus

Because the stars of Hercules are not particularly bright, it is difficult to distinguish the constellation. In general, the most distinctive form is the asterism of the Cornerstone near the center of the constellation. Overall, Hercules somewhat resembles a pinwheel, with arms of stars emerging from the central keystone shape.

Man on the roof of the city with Corona Borealis and Hercules outlined in the sky.
View on EarthSky Community Photos† † Talk Pandey in Bhopal, India, took this photo of Corona Borealis, Hercules and neighbors on April 3, 2021. He wrote, “Hercules and the neighboring constellations in the northeastern sky.” Thank you, Pratek!

Stars of the strong man

Although the stars of Hercules are not particularly bright, the Cornerstone is clearly in dark skies† The brightest star in Hercules’ keystone is size-2.81 Zeta Herculis, which is 35 . is light years away. In the opposite corner of the Keystone (and the Keystone star closest to Vega) is the magnitude-3.15 star Pi Herculis. Pi Herculis is 377 light-years away. The northernmost Keystone star is magnitude-3.48 Eta Herculis at 112 light-years. Opposite Eta Herculis and the faintest of the 4 Keystone stars is Epsilon Herculis of magnitude 3.92. It is located 155 light-years away.

In addition, the other 2 semi-bright stars in Hercules form an arm that unwinds from Zeta Herculis. Both stars are magnitude 2.78. The closest star to Zeta Herculis is Beta Herculis or Kornephoros. It is 148 light-years away. And the other magnitude 2.78 star is close to the border Ophiuchus† It’s Alpha Herculis, 360 light-years away. This star is also nicknamed Rasalgethi. In fact, Rasalgethi is actually 3 stars. The first component is a red giant and the other 2 are a binary star system with a yellow giant and a yellow-white dwarf.

White star chart with black dots and lines showing the keystone shape and lines radiating out.
The stars of Hercules the strong man. Image via IAU/ Sky and Telescope/ Wikimedia Commons

Globular Clusters in Hercules

The real draw of the constellation Hercules is first and foremost its 2 spectacular globular clusters. Both are Messier objects, easy to find in binoculars and a real treat through a telescope.

The first, M13, lies right on the Keystone (although in reality it is 25,000 light-years away, much further than the Keystone stars). M13 is two-thirds of the way down a line stretching between the star Zeta Herculis and Eta Herculis. It’s only 2 1/2″ degrees from Eta. The Large cluster in Hercules shines on size 5.9, which means it is possible to see it as a blurry spot with only your eye of dark places† When you look at M13, you are looking at the combined light of hundreds of thousands of distant stars.

Another globular cluster in Hercules is M92† M92 forms a triangle with the two northernmost stars in the Cornerstone. Imagine where Hercules’ head would be. M92 is about 6 1/2 degrees north of Pi Herculis and nearly 8 degrees from Eta Herculis. M92, at magnitude 6.5, is about 26,000 light-years away. Furthermore, you can see it without an optical aid, but with binoculars and a telescope it is easy to see.

Bright white cluster of dots in the center with a few words around it in black sky.
View on EarthSky Community Photos† † Ron Haggett in Yuma, Arizona, took this photo of a globular cluster on January 5, 2022. Ron wrote, “Messier 13 or the Great Globular Cluster in Hercules. Lucky for me, it’s visible around 5 a.m.!” Thank you, Ron!

M92 and the celestial pole

Another important point: in 14,000 years the earth will rock on its axis so that M 92 is less than 1 degree from north celestial pole in that time. (Read more about precession and which stars will become the North Star over time on The North Star: Does it ever move?

You can see in the simulation below that the north celestial pole passes through Hercules in the lower left corner of the visualization.

Bottom Line: Hercules the Strong Man is a great constellation to watch in June. With just a pair of binoculars, you can see the globular cluster M13 in the Keystone.

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