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Sagitta, the Arrow

Plus the planets for November.

 

There are many constellations representing hunters in the sky, and at least as many representing prey. But there are only a few weapons in the sky, and this month's constellation is one of them. Sagitta, the Arrow, is just a little southwest of the zenith as it gets dark this month. It lies, along with Vulpecula, between Cygnus and Aquila, two of the three constellations containing the first-magnitude stars of the Summer Triangle.

map

Click the image for a larger map.

 

Sagitta, the Arrow, is just past the zenith as it gets dark. This constellation is nestled between Aquila and Cygnus, which along with Lyra host the three bright stars of the Summer Triangle. The brightest star in this constellation is third magnitude, so it is not a conspicuous constellation. Even so, it is an old constellation whose mythology goes all the way back to the Persians.

Sagitta is an old constellation recognized by the Romans, Greeks, Hebrews and Persians. To the Greeks, Sagitta represented the arrows that Hercules shot at the Stymphalian birds. These nasty creatures had bronze beaks and could shoot sharp metallic feathers at their victims. Chased out of Arabia by wolves, they settled at Lake Stymphalia in Arcadia and commenced to breed and take over the countryside, destroying crops, trees and the local people.

Euryatheus sent Hercules to destroy these birds as one of his labours. The birds had been in the swamp near the lake, and Hercules could not go in there without sinking into the quicksand. Athena, seeing his plight, gave him Hephaestus' rattle. Shaking the rattle, Hercules scared the birds into flight and then shot his arrows at them. The birds that were not destroyed flew off, never to return. There are alternate Greek myths, and other groups had their own mythologies behind this constellation.

Among the third-magnitude stars that make up Sagitta is the star cluster M71. First observed by French astronomer Philippe Loys de Chéseaux in 1745 or 1746, M71 was catalogued by Charles Messier on Oct. 4, 1780, as a nebula without stars. This cluster was first resolved into individual stars by William Herschel in 1783 using his 18.7-inch telescope.

M71 appears to be an "open" cluster. It does not have the strongly condensed core of "globular" clusters, which have many more stars in their center, with the density of stars going up as you get closer to the center. M71 is 12,000 light-years from the Earth and spans a little less than a third of a Full Moon (7.2 minutes-of-arc across). At its computed distance, this would make the cluster about 27 light-years across. It is a rich cluster with many stars, whose total brightness is some 13,200 times that of our Sun.

So M71 was classed as an open cluster by astronomers until the 1970s. With improved instruments, astronomers were able to get the spectrum of many of the stars in M71 and discovered that instead of being a dense open cluster, this is a uncondensed globular cluster.

An analysis of M71's stars shows they are actually older than would be expect in an open cluster, some 9-10 billion years old. The stars have aged out of the main sequence (where our Sun is), but have not gotten so old that they have become RR Lyrae variable stars, the standard markers of globular clusters. But this is much younger than most globular clusters, which are closer to 11-12 billion years old.

So astronomers were fooled into thinking this cluster was an open cluster by the lack of RR Lyrae stars and because M71 is not as strongly compressed as most globular clusters. Nevertheless, this cluster has now been confirmed to be a very young globular cluster using the tools available to the modern astronomer.

 

 

The Planets for November

 

The Messenger of the Gods doesn't move too much this month among the stars, just slipping over the border from Scorpius into Libra. At the beginning of the month, Mercury is just barely above the southwestern horizon as it starts to get dark. This is a poor opportunity to view Mercury from the desert southwest due to the tilt of the ecliptic. By Nov. 10, Mercury will have slipped too close to the Sun to be seen. It passes the Sun on Nov. 17 and reappears in the morning sky in the last few days of the month. On Nov. 30, Mercury will be 12 degrees above the east-southeastern horizon as it gets light, having risen at 5:15 a.m. Since Mercury passed between the Sun and the Earth, it will still be a crescent, 45% illuminated and shining at magnitude -0.2. The disc is 7.5 seconds-of-arc across.

Mars is still in the evening sky, setting around 7:15 p.m. Mars' disc is a tiny 4.5 seconds-of-arc across, making it a poor telescopic target. The God of War starts the month in Ophiuchus, moving eastward into Sagittarius where it ends the month near the bright star Nunki. As it gets dark, Mars will be 16 degrees up in the southwest, shining at magnitude +1.2.

Jupiter is in opposition early next month, so this is a good time to take a telescopic look at the King of the Gods. With a disc 48.4 seconds-of-arc across, Jupiter's belts and zones will be most easily seen when the planet is near the zenith just after midnight. Jupiter is in Taurus shining at magnitude -2.8. It rises around 6 p.m. and is visible for the rest of the night.

Watch the Skies

(times MDT/MST)

 

Nov.4, 2 a.m. — Daylight Saving Time ends

Nov. 6, 5:36 p.m. — Last Quarter Moon

Nov. 13, 3:08 p.m. — New Moon, Total Solar Eclipse in South Pacific

Nov. 17, 1 a.m. — Leonid Meteor Shower

Nov. 20, 7:31 a.m. — First Quarter Moon

Nov. 26, 10 p.m. — Venus 0.6 degree south of Saturn

Nov. 28, 7:46 a.m. — Full Moon, Penumbral Lunar Eclipse 6 p.m. — Jupiter 0.6 degrees north of Moon

As morning approaches, Venus will appear in the eastern sky, rising around 4 a.m. Shining brightly at magnitude -4.0, Venus will be 24 degrees above the horizon as it starts to get light. The Goddess of Love's disc is 85% illuminated and 12.5 seconds-of-arc across, on the other side of the Sun from us. Moving eastward among the stars, Venus spends most of the month transiting Virgo, but moves into Libra as November ends. Venus will be close to Saturn on Nov. 26.

Saturn has come out from behind the Sun and is 12 degrees up in the east-southeast as it gets light. Moving slowly eastward in Virgo, Saturn is near the Virgo-Libra border. The Ringed Planet's disc is 15.4 seconds-of-arc across and the Rings are 35.0 seconds-of-arc across. They are tilted down 17.4 degrees with the northern face showing. Saturn shines at magnitude +0.6, rising just after 5 a.m.

There are two eclipses this month. A total solar eclipse on Nov. 13 will be visible starting over far northern Australia. The path then traverses most of the southern Pacific Ocean without further landfall, stopping just before it reaches South America. The second eclipse is a penumbral lunar eclipse on Nov. 28. The eclipse begins with the Moon 17 degrees up in the west, but the Moon will set around 6:48 a.m., well before mid-eclipse, making this eclipse virtually unnoticeable from the desert southwest. So as it gets cold, remember to bundle up and "keep watching the sky"!

 

 

An amateur astronomer for more than 40 years, Bert Stevens is
co-director of Desert Moon Observatory in Las Cruces.

 

 



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