Thursday, September 4, 2008

The Heart of Hercules

The constellation of Hercules is believed to be one of the oldest observed star groupings, as evidenced by the large number of ancient names attributed to it. Greek astronomers originally knew this star group by a name that meant Kneeling One. Its identification with the strongman of Greek legend came a bit later.

The origin of the name Hercules can be traced to the Phoenician word for traveler. Perhaps this was a reference to the famous Twelve Labors of Hercules, which kept him busy all over the ancient world. Hercules’s bags were permanently packed for parts unknown and high adventure.

Hercules in 17th century star atlas (Keystone asterism highlighted)
Courtesy of
Linda Hall Library of Science, Engineering and Technology

Let’s find Hercules by locating the well-known asterism (recognizable star pattern) that lies at the core of the constellation.

1) Facing west (the direction in which the sun sets), locate Arcturus, the brightest star in Bootes, and Alphecca, the brightest star in Corona Borealis. If you’re just tuning in to this blog and need assistance, read my previous two posts.

Charts created with Your Sky

2) Draw an imaginary line from Arcturus to Alphecca and keep going to the next fairly bright star. This star has no traditional name, so we call it Zeta (ZAY-tuh) for its star catalog designation. Zeta is situated at the lower right corner of the Keystone, a four-star asterism forming a quadrilateral that marks the torso of Hercules. Can you see all four stars? If not, try again at a darker location. The stars of the Keystone are not terribly bright, and they may be obscured in brightly lit urban or suburban areas.

A keystone is an architectural element: the wedge-shaped piece at the top of an arch. In fact, it’s the key element of the arch, locking the other pieces into place.

3) The Keystone contains one of the most spectacular globular clusters in the Milky Way: the Great Hercules Globular Cluster, also known as M13. M13 is shorthand for Messier 13 (MESS-ee-yay); it’s the 13th object in the catalog of the famed 18th century French astronomer Charles Messier. This catalog contains some of the finest objects in the night sky and is widely used by amateur astronomers as an observing list.

A globular cluster is a dense ball of gravitationally bound stars. There are at least 150 known globular clusters in our home galaxy, the Milky Way. M13 contains hundreds of thousands of stars.

M13 was discovered in 1714 by Edmund Halley (of Halley’s Comet fame). Because telescopes of his day were not powerful enough to resolve the cluster into stars, that is, separate it into distinct points of light, Halley saw only a fuzzy blob. Since it resembled a nebula, a cloud of gas and dust, for a time the object bore the name of “Halley’s Nebula.”

In 1974, the giant radio telescope at Arecibo Observatory in Puerto Rico targeted M13 to broadcast one of the first messages from Earth intended for intelligent extra-terrestrial life forms.

If you have binoculars, train them on the spot marked on the map above and look for the fuzzy blob that Edmund Halley saw. If you have a telescope, use it to resolve the showstopper cluster into individual stars. Then perhaps you’ll see why I refer to globular clusters as “galactic sugar piles.”