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

Keystone shape with arms and legs spiraling out of the corners and stars marked for Hercules.
The constellation Hercules is located between the bright stars Vega in Lyra and Arcturus in Boötes. A famous klothopknown as M13located on Keystone, a asterism and Hercules. Map via Chelynne Campion.

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

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

How to find Hercules

Hercules is next to the bright star Vega in the constellation Lyra, which is high in the summer sky. More specifically, Hercules is located west of Lyra and east of Boötes with its bright star Arcturus.

Because Hercules’ stars are not very bright, it is difficult to select the constellation. Overall, its its most distinctive form asterism of Keystone near the center of the constellation. In general, Hercules looks like a pinwheel, with arms of stars emanating outward from its central Keystone shape.

Man on the roof of the city with Corona Borealis and Hercules outlined in the sky.
View on EarthSky Community Photos. | Prateek Pandey in Bhopal, India, took this photo of Corona Borealis, Hercules and neighbors on April 3, 2021. He wrote: “Hercules and the nearby constellations in the northeastern sky.” Thanks, Prateek!

Stars of the Strong Man

Although the stars in Hercules are not very bright, Keystone is obvious in dark clouds. The brightest star in Keystone of Hercules is magnitude-2.81 Zeta Herculis, located 35 light years away. At the opposite corner of Keystone (and the Keystone star closest to Vega) is the star Pi Herculis with a magnitude of 3.15. Pi Herculis is 377 light-years away. The northernmost Keystone star has a magnitude of 3.48 Eta Herculis at 112 light-years. Opposite Eta Herculis and the darkest of the four Keystone stars, the magnitude is 3.92 Epsilon Herculis. It is 155 light-years away.

In addition, the other two dimming stars in Hercules form an arm that winds its way from the Zeta Herculis. Both of these stars have a magnitude of 2.78. The star closest to Zeta Herculis is Beta Herculis, or Kornephoros. It is 148 light-years away. And the other star with a magnitude of 2.78 is close to the limit Ophiuchus. It is Alpha Herculis, located 360 light-years away. This star is also nicknamed Rasalgethi. Rasalgethi is actually three stars. The first component is a red giant and the other 2 are a double star system with a yellow giant and a yellow-white dwarf.

White star diagram with black dots and lines showing keystone shape and lines radiating outwards.
The stars of Hercules the Strong Man. Image via IAU / Sky and Telescope / Wikimedia Commons.

Globular clusters in Hercules

In the first place, the real attraction of the Hercules constellation is its two spectacular globular clusters. Both are Messier objects, easy to find in binoculars and a real pleasure through a telescope.

The first, M13, is located right next to Keystone (although in fact 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’s only 2 1/2 degrees from Eta. The Large cluster in Hercules lights on magnitude 5.9, which means that it is possible to see it as a fuzzy patch with the eye only from dark places. When you look at the M13, you are looking at the combined light from hundreds of thousands of distant stars.

Another globular cluster in Hercules is M92. M92 makes a triangle with the two northernmost stars in Keystone. Imagine what Hercules’ head would be like. The M92 is about 6 1/2 degrees north of Pi Herculis and almost 8 degrees from Eta Herculis. M92 shines with a magnitude of 6.5 and is approximately 26,000 light-years away. In addition, you can see it without optical assistance, but it is easily visible in binoculars and telescopes.

Light white cluster of dots in the middle with small surroundings in black sky.
View on EarthSky Community Photos. | Ron Haggett in Yuma, Arizona, took this picture of a globe heap on January 5, 2022. Ron wrote: “Messier 13 or the great globe heap in Hercules. Fortunately for me, it is visible around 5 in the morning!” Thanks, Ron!

M92 and the celestial pole

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

You can see in the simulation below that the North Pole goes through Hercules in the lower left corner of the visualization.

Summary: Hercules the Strong Man is a fantastic constellation to see in June. With just a pair of binoculars, you can see the globe heap M13 in Keystone.


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