Hercules is a large constellation found in summer skies and containing two easy-to-observe globular clusters.
Hercules is known as the Strong Man in ancient mythology and has many stories associated with him. He was a son of Jupiter and had to perform the famous twelve labors.
How to Find Hercules
Hercules is located next to the bright star Vega in the constellation Lyra, which lies high in summer skies. Hercules lies west of Lyra and east of Bootes with its bright star Arcturus. The stars of Hercules are not particularly bright, which makes it hard to pick out a distinctive shape. It appears somewhat like a pinwheel, with arms of stars emanating outward from its central keystone shape.
Stars of Hercules
All the stars in the constellation Hercules are second magnitude and dimmer. Only one of the "keystone" stars of Hercules is 2nd magnitude. It is the magnitude 2.81 star Zeta Herculis lying 35 light-years away. At the opposite corner of the keystone (and the keystone star closest to Vega) is the magnitude 3.16 star Pi Herculis. Pi Herculis lies 367 light-years away. The northernmost keystone star is magnitude 3.48 Eta Herculis at 112 light-years. Opposite Eta Herculis and the dimmest of the four keystone stars is magnitude 3.92 Epsilon Herculis. It lies 163 light-years away.
The other two second magnitude stars in Hercules form an arm winding off of the 2nd-magnitude keystone star Zeta Herculis. Both of these stars are magnitude 2.78. The star closest to Zeta Herculis is Beta Herculis, or Komephoros. It lies 148 light-years away. The second magnitude 2.78 star lies close to the border with Ophiuchus. It is Alpha Herculis, lying 382 light-years away. This star is sometimes called Rasalgethi. Rasalgethi is actually three stars. The first component is a red giant and the other two are a double star system with a yellow giant and a yellow-white dwarf.
Globular Clusters in Hercules
The real attraction with the Hercules constellation is its two spectacular globular clusters. Both are Messier objects easy to find in binoculars and a treat through a telescope. The first , M13, lies right on the keystone. M13 two-thirds of the way on a line that stretches between the star Zeta Herculis and Eta Herculis. It lies just two and a half degrees from Eta. The Great Cluster in Hercules shines at magnitude 5.9, making it visible as a fuzzy patch with the naked eye from dark sites. At 25,100 light-years away, M13 is often considered the best globular cluster to view from the Northern Hemisphere. When looking at M13, you are looking at the combined light of hundreds of thousands of distant stars.
The second globular cluster is M92. M92 makes a triangle with the two northernmost stars in the keystone. It could be imagined as where Hercules' head would be. M92 lies about six and a half degrees north of Pi Herculis and nearly eight degrees from Eta Herculis. Shining at magnitude 6.5, M92 lies about 26,000 light-years away. It also can be seen without optical aid, but shows up easily in binoculars and a telescope. In 14,000 years from now, Earth will have wobbled on its axis so that M92 is less than one degree from the North Celestial Pole at that time. (Learn more about Earth's precession and the stars that will become the North Star over time.)
