As the sky grows dark on very clear evenings this week, an ancient star pattern can be found almost directly overhead: Hercules, the mighty strong man and hero of many legends.
The stars that compose this pattern are, for the most part, rather dim, and it seems rather incongruent that for such a powerful figure, the representative constellation is rather weak. Indeed, there is really no striking pattern to be found here. It is supposed to represent a kneeling man and was known as the "Kneeling One" in pre-Greek cultures.
A check of a number of various stargazing guidebooks might suggest you look for a large crooked letter H, or even a butterfly that can be traced out in several different ways. In "The Stars/A New Way to See Them," author H.A. Rey (1898-1977) managed to picture Hercules as a kneeling man swinging a club.
Within the center of this constellation is a pattern that is often labeled the "Keystone," and is composed of Eta, Pi, Epsilon, and Zeta Herculis. It is an asterism, which is defined as a distinctive grouping of stars forming part of the recognized constellation outlines, or lying within its boundaries. Ranging in size from sprawling naked-eye figures to minute stellar settings, they are found in every quarter of the sky and at all seasons of the year. The larger asterisms — such as the Big Dipper in Ursa Major and the Great Square of Pegasus — are often better known than their host constellations.
Some might not be acquainted with what a keystone is. Many arches are built with a large, central stone that helps to make the arch self supporting. The keystone was one of the most significant ancient architectural discoveries, along with the vault. Pennsylvania acquired its nickname "The Keystone State" because of its central location among the 13 original American colonies. If you have ever traveled on "America's First Superhighway," the Pennsylvania Turnpike, you might recall the keystone symbol found on its traffic signs. And the keystone symbol can also be found on the logo for the Little League (whose World Series is played in Williamsport, Pennsylvania).
Admittedly, the Hercules Keystone is slightly lopsided, but then few star pictures are perfect (even the famous Great Square of Pegasus looks more rectangular that square-like). A third of the way down the Keystone's western edge is the famous star cluster M 13, which may be glimpsed with the unaided eye. It is a celebrated object; the brightest and most dramatic example of a globular cluster north of the celestial equator, often referred simply as "The Great Hercules Cluster." It's a concentration of over 100,000 stars, located at a distance of roughly 23,000 light years from us.
Globular clusters are masses of stars that typically lie on the outskirts of our galaxy.
If you have good binoculars, you should have no difficulty in locating M 13 for it will appear as a hazy-looking "star" of about sixth magnitude (which is about the threshold of naked-eye visibility). In a small telescope it will appear as a bright, round nebula about one-third the apparent size of the Moon. With a 4 to 6-inch telescope, the round glow begins to become resolved into hundreds of tiny star-points. In telescopes of 12-inches or larger a truly memorable view is obtained. Astronomer Robert H. Baker (1880-1962) called it a "wonderful chrysanthemum of stars."
For those who regularly attend the annual Stellafane convention, just outside of Springfield, Vermont (which in 2007 takes place Aug. 9-12), the Great Hercules Cluster is the object that Stellafane's 12-inch Porter turret telescope is directed toward most often.
Many years ago, the legendary deep sky observer, Walter Scott Houston (1912-1993) — known as "Scotty" — noticed a long line of people patiently waiting their turn to get a view through the eyepiece. "What are you folks looking at?" he asked as he poked his head through the observatory door. From out of the darkness, came the response: M 13. "M 13?" replied Scotty, with a tinge of incredulity. "So many people have looked at it you would think it'd be all worn out by now!"