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home : columns : columns August 19, 2014

7/30/2013 12:55:00 PM
Stars over Sisters
The finest deep-sky object in Sagitta is M71, a loosely concentrated globular cluster located at a distance of some 13,000 light-years.
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The finest deep-sky object in Sagitta is M71, a loosely concentrated globular cluster located at a distance of some 13,000 light-years.
By Ron Thorkildson

Against the backdrop of the summer Milky Way lets fly a celestial arrow. This diminutive constellation is known as Sagitta and is surrounded by the little fox Vulpecula, the mythological hero Hercules, the eagle Aquila and the dolphin Delphinus. Sagitta is the third-smallest of all the constellations-only Equuleus and Crux are smaller.

In ancient Greek mythology, Sagitta was known as the weapon that Hercules used to kill the eagle (Aquila) that perpetually gnawed Prometheus' liver. Prometheus, a Titan and culture hero, is credited with the creation of man from clay. But he defied the gods and gave fire to humanity, an act that enabled progress and civilization. As punishment for his transgression, Zeus, father of gods and men, sentenced Prometheus to eternal torment. The immortal Titan was bound to a rock, where each day an eagle was sent to feed on his liver, which would then grow back to be eaten again the next day.

The observational highlight in Sagitta is a star cluster known as M71. This object is a very loose globular cluster and for many years was mistaken for a dense open cluster. Globular clusters are normally comprised of tightly packed balls of older stars located at distances of 10,000 light-years or more. Open clusters, on the other hand, are looser collections of much younger stars that lie only a few thousand light-years away. Discovered by French astronomer Philippe Loys de Cheseaux, M71 is about 13,000 light-years distant.

This year the annual Perseid meteor shower is expected to peak at around noon PDT on August 12. So the best viewing nights should occur between August 11 and August 13. Under dark skies the maximum meteor rate may reach 60 per hour. As with all meteor showers, the rate is greatest in the pre-dawn hours, since that side of the earth is facing toward the direction of earth's motion through space. The streaks of light are caused by debris shed by comet Swift-Tuttle entering Earth's atmosphere and being incinerated by the heat of friction. Fortunately, the first quarter moon will set at about 11 p.m., providing dark skies for early morning meteor observing.

Still well-placed for observing, the planet Saturn hangs about 30 degrees above the southwestern horizon 45 minutes after sunset. At the beginning of August the planet doesn't set until midnight, but it's gone from the sky at 10 p.m. by month's end. A prominent object in the western sky, Venus continues to brighten throughout the month, setting about an hour and a half after the sun.

Jupiter and Mars are both in the constellation of Gemini early in August. Jupiter rises at about 3:30 a.m. while Mars comes on the scene 20 minutes later. Throughout the month the solar system's largest planet lumbers across Gemini, while the Red Planet races on ahead and enters Cancer during the same interval of time.

For those who possess keen eyesight and some degree of patience, look for Mercury low in the east about 45 minutes before sunrise during the first half of the month. Thereafter, the planet falls back toward the sun and becomes lost in its glare.

The moon is new (dark) on August 6, first quarter on August 14, full on August 20, and last quarter by August 28. The next Stars over Sisters public starwatch is scheduled for Friday, September 6, at the Sisters Park & Recreation District building. A pre-star-watch presentation will begin at 8:30 p.m. Following the talk all are invited to view the night sky through telescopes that will be set up, courtesy of the Sisters Astronomy Club.

For information about Sisters Astronomy Club activities and events, visit

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