The constellation of Hercules is between the bright stars Vega in Lyra and Arcturus in Boötes. A famous globular cluster, known as M13, is located in the Keystone, an asterism of Hercules. Graphic by Chelynne Campion.
Hercules is the strongman of ancient mythology. He was a son of Jupiter who was to perform the famous twelve works. Astronomers know Hercules as a high constellation in the northern sky on the eve of June that hosts an asterism known as Keystone, where you can find what may be the best globular cluster for observers in the northern hemisphere: M13 or Great Cluster of Hercules.
Hercules is one of the largest of the 88 constellations, ranking fifth in size.
How to find Hercules
Hercules is next to the bright star Vega in the constellation Lyra, which is at the height of the summer sky. Specifically, Hercules is located 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 choose the constellation. In general, its most distinctive shape is the asterism of the Keystone near the center of the constellation. In general, Hercules looks a bit like a windmill, with arms of stars emanating outward from its central stone shape.
See photos from the EarthSky community. | Prateek Pandey in Bhopal, India, captured this photo of Corona Borealis, Hercules, and their neighbors on April 3, 2021. He wrote, “Hercules and the neighboring constellations in the northeast sky.” Thanks, Prateek!
Strong man stars
Although the stars of Hercules are not particularly bright, the Keystone is evident in dark skies. The brightest star in the Keystone of Hercules is Zeta Herculis of magnitude 2.81, which is 35 light-years away. On the opposite corner of the Keystone (and the nearest Keystone star to Vega) is the star Pi Herculis of magnitude 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. In front of Eta Herculis and the faintest of the 4 Keystone stars is the magnitude of 3.92 Epsilon Herculis. It is 155 light-years away.
In addition, the other 2 half-shining stars of Hercules form an arm that moves away 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 star of magnitude 2.78 is near the border with Ofiuc. It’s Alpha Herculis, it’s 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 double star system with a yellow giant and a yellow-white dwarf.
The stars of Hercules the strong man. Image via IAU / Sky and Telescope / Wikimedia Commons.
Globular clusters in Hercules
Mainly, the real attraction of the constellation of Hercules is its 2 spectacular globular clusters. Both are Messier objects, easy to find with binoculars and a real delight through a telescope.
The first, M13, is right on the Keystone (though it’s actually 25,000 light-years away, much farther away than the Keystone stars). M13 is two-thirds of the way along a line between Zeta Herculis and Eta Herculis. It is only 2 1/2 degrees from Eta. The great cluster of Hercules shines at a magnitude of 5.9, which means that it is possible to see it as a blurred patch with your eye alone from 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 stars further north of Keystone. Imagine where Hercules’ head would be. M92 is about 6 1/2 degrees north of Pi Herculis and almost 8 degrees from Eta Herculis. Shining at a magnitude of 6.5, M92 is about 26,000 light-years away. In addition, it can be seen without optical aid, but is easily shown with binoculars and a telescope.
See photos from the EarthSky community. | Ron Haggett in Yuma, Arizona, took this picture of a globular cluster on January 5, 2022. Ron wrote, “Messier 13 or the great globular cluster in Hercules. Luckily for me, it can be seen around the globe. 5 in the morning! ” Thanks, Ron!
M92 and the celestial pole
Another key point: 14,000 years from now, the Earth will have oscillated on its axis so that M92 is less than 1 degree from the celestial north pole at that time. (Read more about precession and which stars will become the North Star over time in The North Star: Does it ever move?)
You can see in the simulation below that the celestial North Pole borders Hercules in the lower left corner of the view.
Due to the precession of the Earth, Polaris, Alderamin, Vega and Thuban become the North Star every 26,000 years. Due to proper movement, even Arcturus made a turn 58,000 years ago.
Simulation: higher resolution, more years, including the brief reign of Arcturus. pic.twitter.com/5diNCQsqL6
– Tony Dunn (@ tony873004) June 8, 2022
Conclusion: Hercules the strong man is a big constellation to see in June. With just a few binoculars, you can see the M13 globular cluster in the Keystone.
Kelly Kizer Whitt
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Kelly Kizer Whitt has been a science writer specializing in astronomy for over two decades. He began his career in Astronomy Magazine, and has made regular contributions to AstronomyToday and the Sierra Club, among other media. His children’s picture book, Solar System Forecast, was published in 2012. He has also written a dystopian novel for young people entitled A Different Sky. When he’s not reading or writing about astronomy and looking at the stars, he enjoys traveling to national parks, creating crossword puzzles, running, tennis, and paddleboarding. Kelly lives with her family in Wisconsin.