THE USM SOUTHWORTH PLANETARIUM
207-780-4249 www.usm.maine.edu/planet
70 Falmouth Street Portland, Maine 04103
43.6667° N 70.2667° W
Altitude: 10 feet below sea level
Altitude: 10 feet below sea level
Founded January 1970
Julian date: 2458659.5
"The pessimist sees difficulty in every opportunity. The optimist sees opportunity in every difficulty."
-Winston Churchill
THE DAILY ASTRONOMER
Tuesday, June 25, 2019
Tuesday, June 25, 2019
The Keystone
There is never an inconvenient time to explain the difference between "constellation" and "asterism," because the latter is a part of the former even though the terms are often used synonymously. A constellation is one of the 88 star patterns desginated by the International Astronomical Union (IAU). These constellations include Sagittarius the Archer; Ursa Major the Great Bear and Hercules, also called the "kneeling one."
Within each of the three aforementioned constellations one would find "asterisms," well known patterns within these constellations. Sagittarius' asterism is the "teapot," Ursa Major's is "the Big Dipper," and Hercules' is the "Keystone."
The Hercules' Keystone: a four star quadrilateral marking the Hercules' mid-section. The "Keystone" is an asterism, or a star pattern within a larger constellation. One can locate the Keystone by drawing an imaginary line connecting Deneb and Vega in the Summer Triangle. Extend the line slightly westward and it will intersect the Keystone.
There is never an inconvenient time to explain the difference between "constellation" and "asterism," because the latter is a part of the former even though the terms are often used synonymously. A constellation is one of the 88 star patterns desginated by the International Astronomical Union (IAU). These constellations include Sagittarius the Archer; Ursa Major the Great Bear and Hercules, also called the "kneeling one."
Within each of the three aforementioned constellations one would find "asterisms," well known patterns within these constellations. Sagittarius' asterism is the "teapot," Ursa Major's is "the Big Dipper," and Hercules' is the "Keystone."
Today's focus is the "Keystone," a quadrilateral centered on Hercules. The image seen above shows Hercules "upside down," as he is portrayed as kneeling on Draco the Dragon's head. Draco is often identified mythologically as the dragon who guarded the golden apples within the Garden of the Hesperides. The eleventh of Hercules' twelve labours required him to steal these apples, a task rendered all the easier once he slew the dragon.*
Though it consists of no bright stars, this keystone is somewhat easy to find provided one first locates the Summer Triangle. Vega, the brightest Summer Triangle star, is just east of the Keystone. Extend a western line connecting Deneb, the Summer Triangle's eastern star, and Vega. This line slices neatly through the Hercules asterism. The Keystone stands out because,though not brilliant, is still brighter than the constellations's other features. Often, observers are only able to find the asterism, as the legs and arms are lost amongst the surrounding stars.
M13: The Hercules Star Cluster A globular star cluster "located" in the Hercules Keystone. Approximately 25,000 light years distant, this cluster contains more than 100,000 stars within a "globe-shaped" arrangement. Image: Sara Wager
Apart from being Hercules' sole prominent section, the Keystone also marks the location of M13,** a globular star cluster. The Hercules Cluster is a gorgeous globular cluster consisting of more than 100,000 stars located in the galactic halo. So named due to their resemblance to globes, a globular clusters are significantly older and larger than the open (galactic) clusters that swarm about the galaxy's spiral arms. Of course, we can easily see a couple such open clusters (Praesepe and the Pleiades), while the much more distant Hercules cluster is only visible through a telescope.
The Hercules Keystone is a splendid sight to begin our annual summer star tour now that we're about to start astronomical summer.
*In some versions, Hercules merely eluded the dragon instead of killing it. In another version, Hercules didn't know the Dragon's location and in desperation consulted Atlas, the Titan who had been condemned to hold up the world following the Titan's defeat in the Titanomachy (war against the gods.) Atlas offered to fetch the apples if Hercules agreed to take his place under the world. Hercules readily consented. Soon thereafter, Atlas returned with the apples, as promised. However, having been relived of the burden, Atlas told Hercules that he would take the apples, himself, and leave Hercules with the world on his shoulder. Apart from being angry, the shrewd Hercules expressed relief. "I am almost at the end of my labors," Hercules explained to the bemused Atlas. "Each labor has proven more difficult that the one before it and I knew that the final labor would be the hardest of them all and would likely have killed me. If I am condemned to hold the world now during my penultimate labor, I will have to forego the last one and can thereby live forever without ever having to face whatever peril awaits me. My friend, you may have the apples and with it you have my eternal gratitude. I was promised immortality if I completed the labors, but now I have it without having to finish them. One thing, though....my shoulder is a bit sore due to this burden. May I ask you to take the world for a moment so I can fasten a cushion to my shoulder?" Atlas promptly took the world back and watched helplessly as Hercules picked up the apples and fled.
**The M indicates "Messier Catalog." Charles Messier (1730-1817) compiled a catalog of celestial objects that resembled comets. His aim was to offer a list of nebulae and other structures that other astronomers might mistakenly classify as comets. Comet hunting was a real king's sport in the 18th and 19th century. Each object Messier included in his compilation is assigned an "M" designation.