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

EarthSky | Hercules the strongman and a large globular cluster

The constellation of Hercules lies between bright stars Vega in Lyra and Arcturus at Boots. A famous globular clusterknown as M13sits on the Keystone, a asterism in Hercules. Graphic via Chelynne Campion.

Hercules is the strong man of ancient mythology. He was a son of Jupiter who was to perform the famous twelve labours. Astronomers know Hercules as a constellation high in the northern sky on June evenings that is home to a asterism known as Keystone, where you can find what may be the best globular cluster for northern hemisphere observers: M13or the Great Cluster of Hercules.

Hercules is one of the largest of the 88 constellations, ranking 5th in size.

How to find Hercules

Hercules stands next to the shining star Vega in the constellation Lyra, which sits high in the summer sky. Specifically, Hercules is west of Lyra and east of Boötes with its shining star Arcturus.

Since the stars of Hercules are not particularly bright, it is difficult to distinguish the constellation. Overall, its most distinctive form is the asterism of the keystone near the center of the constellation. Generally, Hercules looks a bit like a pinwheel, with star arms emanating from its central Keystone form.

Man on top of the city with Corona Borealis and Hercules outlined in the sky.
View EarthSky Community Photos. | Prateek Pandey in Bhopal, India, captured this photo of Corona Borealis, Hercules and his neighbors on April 3, 2021. He wrote: “Hercules and neighboring constellations in the northeast sky. Thank you Pratek!

The strongman stars

Even though the stars of Hercules are not particularly bright, the keystone is evident in dark skies. The brightest star in the Keystone of Hercules is magnitude-2.81 Zeta Herculis, which sits 35 Light years a way. At the opposite corner of the Keystone (and the closest Keystone star 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 112 light years away. Opposite Eta Herculis and the darkest of the 4 Keystone stars is Epsilon Herculis of magnitude 3.92. It is 155 light years away.

In addition, the other 2 semi-bright stars of Hercules form an arm extending from Zeta Herculis. These two stars have a magnitude of 2.78. The closest star to Zeta Herculis is Beta Herculis, or Kornephoros. It is 148 light years away. And the other 2.78 magnitude star is near the border with Ophiuchus. This is Alpha Herculis, 360 light years away. This star also bears the nickname of Rasalgethi. In fact, Rasalgethi is actually 3 stars. The first component is a red giant and the other 2 are a dual star system with a yellow giant and a yellow-white dwarf.

White star map with black dots and lines showing the trapezoidal shape and outward radiating lines.
The stars of Hercules the Strongman. Image via IAU/Sky and Telescope/ Wikimedia Commons.

Globular clusters at Hercules

Mainly, the real attraction with the constellation of Hercules is its 2 spectacular globular clusters. Both are Messier objects, easy to find with binoculars and a real treat through a telescope.

The first one, 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 on a line that stretches between the star Zeta Herculis and Eta Herculis. It is only 2 1/2 degrees of your. The Great Cluster of Hercules shines at magnitude 5.9, which means it’s possible to see it as a blur with your eye alone from dark websites. When you look at M13, you are looking at the combined light of hundreds of thousands of distant stars.

Another globular cluster of Hercules is M92. M92 forms a triangle with the two northernmost stars in the keystone. Imagine it as where the head of Hercules would be. M92 is about 6 1/2 degrees north of Pi Herculis and about 8 degrees from Eta Herculis. Shining at magnitude 6.5, M92 lies about 26,000 light-years away. Also, you can see it without optical aid, but it is easily seen in binoculars and a telescope.

Cluster of bright white dots in the center with a bit of surrounding in the black sky.
View 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 of Hercules. Luckily for me it is visible around 5am!” Thanks Ron!

M92 and the celestial pole

Another key point: in 14,000 years, the Earth will have oscillated on its axis so that M92 is less than 1 degree from north celestial pole at this moment. (Read more about precession and which stars will become North Star over time at The North Star: does it ever move?)

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

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

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