The Constellation Hercules
Posted by Brian Ventrudo
Now let’s visit the celestial incarnation of the greatest of classical heroes, Hercules. With the famous “Keystone” of four stars and a pair of splendid globular clusters, the constellation Hercules is a must-see even for casual stargazers in June through August. Hercules lies high overhead in the north and well over the northern horizon in the southern hemisphere, so its sights are accessible to stargazers all over the world.
Dating back to the 2nd century, Hercules is one of Ptolemy’s original 48 constellations. But this star group was known earlier, much earlier than even classical Greek civilization. The Greeks first knew these stars as Engonasin, the “kneeling one”, and indeed these stars do suggest the form of a kneeling man bent by toil and fatigue.
Eratosthenes first linked this kneeling celestial figure to Hercules. And why not? Hercules, though blessed with immortality and unmatched strength, was burdened with the famous Twelve Labours he undertook as penance for slaying his own sons in a fit of delusional rage set upon him by Hera, who blighted most of the life of the mighty hero. Hera hated Hercules because he was the offspring of her husband, Zeus, and the wise and beautiful mortal Alcmene…
Hercules ranks as the 5th largest constellation in the sky, yet it has no bright stars. But that doesn’t mean its hard to find.
* * * Highly Recommended * * *
It lies between the semicircular shape of Corona Borealis and the small parallelogram shape of Lyra and its brilliant blue-white star Vega. Look for the four-sided Keystone asterism formed by pi (π) , eta (η) , zeta (ζ) , and epsilon (ε) Herculis.
Now trace out the rest of the constellation. The stars beta (β) and delta (δ) Herculis mark the shoulders of the hero, and his arms extend towards Lyra. His left knee and shin, which are pressed on the head of Draco, the Dragon, are marked by theta (θ) and iota (ι) Herculis.
The constellation’s brightest star is called Rasalgethi. It’s a 3rd-magnitude red-giant star found in the south end of the group, right next to 2nd-magnitude Rasalhague, the brightest star in the constellation Ophiuchus. Rasalgethi is a superb double star for a small telescope, as is the much fainter but prettier 95 Herculis. You can learn more about these lovely double stars in this article…
Even beginning stargazers know of Hercules’ most famous deep-sky object, the famous Hercules Globular Cluster, also known as M13. This is the brightest globular cluster visible from northerly locations. It is a splendid sight in a 4-inch or larger telescope, displaying a globe of pinpoint white stars that appear almost 3-dimensional when you use averted vision. The cluster is also easily seen in binoculars, and without optics at all in dark sky.
And don’t forget M92, the other bright globular cluster in Hercules. While not as large and bright as M13, it still ranks as one of the finest globulars in the night sky.
Hercules also lends its name to a galaxy cluster. The Hercules cluster lies some 500 million light years away, and is one component of the even larger Hercules Supercluster. You’ll need a big scope to glimpse any of these distant galaxies. But here’s an excellent image by Tony Hallas which shows an amazing cornucopia of galaxy spread over a patch of sky as wide as the full Moon.