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Sagittarius, the Archer

Plus a new supernova and the planets for August.

Low in our southeastern sky as it gets dark is the constellation Sagittarius, the Archer. Sagittarius represents a centaur, a half-man, half-horse from ancient mythology. The Greeks associated this constellation with the centaur Chiron. Chiron was different from the other more base and brutal centaurs — a great healer, astrologer and respected oracle. Such was his knowledge and gentleness that he was a teacher to Apollo, Artemis, Achilles and sometimes Heracles, among others.

starry dome

Click the image for bigger map


Looking south on these hot August nights, you can find Sagittarius, the Archer, off to the left (east). This constellation is marked by the Teapot asterism. While the Teapot appears to be the most solid part of Sagittarius, in the traditional constellation figure, the Teapot is mostly empty space in the Archer’s taut bow. The center of our Milky Way is off the spout of the Teapot, approximately the same distance from the bottom of the Teapot to the end of the spout along the same line.

Chiron was the offspring of the Titan Cronus and the minor goddess Philyra, whose union imbued him with great intelligence and honor. The other centaurs were brutal and stupid, offspring of the unholy rape of a minor cloud-goddess by the mortal King Ixion.

Chiron died during a visit to his fellow centaur Pholus, who lived in a cave on Mount Pelion. Chiron was accompanied by Heracles, who was performing his fourth labor of defeating the Erymanthian Boar. At supper, Heracles asked for wine to accompany his meal. Pholus, who ate his food raw, had only a vessel of sacred wine from Dionysus, to be kept for the rest of the centaurs. When Heracles opened it, the smell of the wine wafted out of the cave and intoxicated the wild centaurs who then attacked the cave. To drive them back, Heracles shot many arrows, poisoned with the Hydra's blood — one of which accidentally struck and killed Chiron. The centaur was honored with a place in the sky as the constellation Sagittarius and/or Centaurus (depending on the version of the myth).

Sagittarius is the 15th largest constellation in area and contains many astronomical objects that belong to our galaxy, including the Milky Way's center. Also in the southwest corner of Sagittarius is the galaxy PGC 64605. On July 8, Greg Bock of Queensland, Australia, discovered a supernova (dubbed 2012dn) in this galaxy. Astronomers immediately swung into action. Using the Gemini South telescope in Hawaii, J.T. Parrent and D.A. Howell spread light from the supernova out into its component colors to form a spectrum.

Hidden in the spectrum are bright and dark lines that appear against the continuous flow of the rainbow colors. Each line represents the signature of a particular element. A bright line means that the element is excited either by ultraviolet radiation or heat and is radiating energy in its signature colors. A dark line of the same color represents the same atom, but cold and absorbing energy in its signature colors. Using a computer, Parrent and Howell analyzed the lines in 2012dn and determined it is a Type Ia supernova, discovered approximately a week before maximum brightness.

Type Ia supernovae start as binary star systems where one star has reached the end of its hydrogen-burning life and has contracted to become a white dwarf. Over time, the two stars come closer together and the outer atmosphere of the still-active star is drawn into the white dwarf, increasing its mass. Eventually, the white dwarf reaches the maximum mass it can have without collapsing into a supernova. This mass was computed by Indian-American astrophysicist Subrahmanyan Chandrasekhar, who in 1930 predicted that a white dwarf heavier than 1.44 times the mass of our Sun would collapse to form a supernova. This is called Chandrasekhar's Limit.

But this limit assumes that the white dwarf is rotating slowly. If the white dwarf is rotating rapidly, centrifugal force counteracts the gravity trying to collapse the white dwarf. This allows it to get more massive than Chandrasekhar's limit without becoming a supernova. As material continues to accumulate on the white dwarf, the mass becomes large enough to overcome the rapid spin and it collapses into a supernova in what astronomers call a "super-Chandra event." This means the white dwarf was spinning rapidly and was heavier than the normal Type Ia supernova, giving an unusual spectrum that astronomers can study.

 

 

The Planets for August

 

Mars flies across most of Virgo during the month. As it progresses, it comes between Saturn and Spica on August 14. It will be only 40 minutes-of-arc from Spica and 2.8 degrees from Saturn, placing these three bright objects in a line only 3.5 degrees from end-to-end. On that date, the God of War is magnitude +1.1 with a disc 5.5 seconds-of-arc across. As it gets dark, Mars is 37 degrees up in the southwest; it sets around 10:45 p.m.

Saturn passed Spica last month and continues slowly eastward. The Ringed Planet appears in our sky on the same schedule as Mars this month. When Mars passes it, Saturn is magnitude +0.8 with a disc 16.2 seconds-of-arc across. Its rings are 36.8 seconds-of-arc across and tilted down 13.5 degrees with the northern face showing.

Jupiter is moving slowly eastward in Taurus. It rises a little after midnight and is visible the rest of the night. The King of the Gods has a disc 39.1 seconds-of-arc across, and shines at magnitude -2.4.

As the month begins, Taurus also hosts Venus, but Venus moves rapidly eastward and into the northeastern corner of Orion. It continues moving eastward, ending up in eastern Gemini by month's end. At midmonth, Venus is magnitude -4.5 with a disc 23.5 seconds-of-arc across and is 50% illuminated. It becomes fuller as the month progresses.

Watch the Skies
(times MDT)


August 1, 9:27 p.m. — Full Moon
August 7, 11 a.m. — Mercury stationary
August 9, 12:55 p.m. — Last Quarter Moon
August 12, 6 a.m. — Perseid Meteor Shower peak
     6 p.m. — Mars near Spica
August 16, 6 a.m. — Mercury greatest dist from  Sun (19°)
August 17, 2 a.m. — Mars 3° south of Saturn
     9:54 a.m. — New Moon
August 24, 7:54 a.m. — First Quarter Moon
August 31, 7:58 a.m. — Full Moon ("Blue Moon")

This month, Mercury is making an appearance in the morning sky. It can be found in the east-northeast shortly before the Sun comes up. It will be farthest from the Sun on August 16, a scant 12 degrees above the horizon as it starts to get light. On that day, Mercury rises at 5 a.m. with a disc 7.4 seconds-of-arc across and 41% illuminated. Mercury loops around in Cancer until August 25, when it travels eastward into Leo, where it passes Regulus as the month ends.

The annual Perseid meteor shower occurs this month as Earth passes through the debris trail of Comet 109P/Swift-Tuttle. The shower peaks in the early morning hours of August 12, when the meteors will appear to radiate from the constellation Perseus. The 25% illuminated Moon will rise around 2 a.m. but it will be in Taurus, not too far from Perseus. The Moon will make the sky a little brighter than would be desired, but is not a major obstruction. So put out the chaise lounge and watch these celestial visitors shoot through the starry dome 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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