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Ophiuchus, the Serpent Bearer

Plus the planets for August.

 

Straddling the celestial equator high in our southern sky is the large constellation Ophiuchus, the Serpent Bearer. This is an irregularly shaped constellation that is higher (north-south) than it is wide (east-west). Area-wise, it is the 11th largest constellation in the sky, covering 948 square degrees. Ophiuchus has no first-magnitude stars, with its brightest being Rasalhague, a magnitude-2.1 star near the northern edge of the constellation. The ecliptic, where the planets travel, runs through the southern part of this constellation even though it is not one of the signs of the zodiac.

Ophiuchus, the Serpent Bearer, is two-thirds of the way up in our southern skies. This is the 11th largest constellation in the sky. To its left (east) is the Caput, the Serpent’s Head, and to its right (west) is Serpens Cauda, the Serpent’s Tail. These two form the constellation Serpens, the Serpent, that Ophiuchus is holding.

Ophiuchus has no hard mythology behind it, but it was first mentioned in the fourth century BC by the Greek didactic poet Aratus in his poem "Phaenomena." There he mentions Ophiuchus trampling on Scorpius, the Scorpion, the constellation just to his south. Ophiuchus is often depicted as a man holding a long and winding serpent in his hands. Coils of the serpent are wrapped around his wrists and encircle his waist. The ancient Greeks believed this constellation represented the god Apollo fighting the huge snake that guarded the Oracle of Delphi.

Being a large constellation, Ophiuchus has a number of star clusters in it such as Messier 9, Messier 10, Messier 12, Messier 14, Messier 19, Messier 62 and Messier 107. But there are also a couple of galaxies, with NGC 6240 being one of the more interesting ones.

NGC 6240 is an irregular galaxy in northwest Ophiuchus. It looks like a crab or a butterfly and shines at magnitude 12.8 in visible light. But it is very much brighter in the infrared part of the spectrum that our eyes cannot see. Astronomers call this an ultraluminous infrared galaxy. As galaxies go, NGC 6240 is relatively nearby, only 400 million light-years away.

This galaxy also has many new stars forming in it, qualifying it as a starburst galaxy. The stars in starburst galaxies are formed from turbulent gas swirling through the galaxy. This gas runs into slower-moving gas, causing the gas to be compressed to form stars. Some of the new stars are so massive that they have already run through their life cycle and exploded as supernovas. This star formation is not continuous, but happens in short bursts. The last burst occurred about 20 million years ago and lasted for about 5 million years.

At first glance, this irregular galaxy appears as a chaotic blob. But looking closer, wisps of gas, dust and stars extend far outward from the galaxy. There are also faint trails of similar matter moving away from the galaxy, as if something had pulled it out of NGC 6240 sometime in the past, but there are no other galaxies near enough to be interacting with it. The galaxy is surrounded by a gigantic cloud of hot gas some 300,000 light-years across.

Looking deep into the center of NGC 6240, astronomers found not one nucleus, but two. So this is not just a single galaxy, but two spiral galaxies that are in the process of merging into a single elliptical galaxy. The merger began about 30 million years ago when the two galaxies were pulled together by their mutual gravity.

The two nuclei each have a supermassive black hole at the center. They began orbiting around each other as the two galaxies started to merge. They have been coming closer and closer together and are now only 3,000 light-years apart. Eventually, they will probably merge to form a single nucleus with a single, even larger, black hole at its center. The process of merging these two spiral galaxies to form a single elliptical galaxy will finally be complete some tens or hundreds of millions of years in the future.

 

 

The Planets for August

 

Mercury makes an appearance in our evening sky during the last week of the month. For us this will be a very poor apparition, with Mercury getting only five degrees up from the western horizon as it gets dark at the end of the month. The Messenger of the Gods starts the month in Cancer, traveling eastward all the way across Leo and into western Virgo by the end of the month. At that time, it will have an 82% illuminated disc, 5.4 seconds-of-arc across.

Mars is in eastern Virgo as the month starts, moving eastward into central Libra by the end of the month. The God of War is 31 degrees up in the southwest as it gets dark and sets around 11:30 p.m. At midmonth it shines at magnitude +0.5 with a disc that is 7.3 seconds-of-arc across.

Saturn is 36 degrees up in the south-southwest as it gets dark, setting by midnight. It is moving very slowly eastward in central Libra during the month. The Ringed Planet's disc is 16.6 seconds-of-arc across at midmonth while its rings are 37.6 seconds-of-arc across and tilted down 21.3 degrees with the northern face showing.

Watch the Skies
(times MDT)

 

August 3, 6:50 p.m. — First Quarter Moon

August 10, 12:09 p.m. — Full Moon

August 12, 6 p.m. — Perseid meteor shower peaks

August 17, 6:26 a.m. — Last Quarter Moon

                    9;10 p.m. — Venus 12 minutes-of-arc N of Jupiter

August 25, 8:13 a.m. — New Moon

August 27, 7 a.m. — Mars 4 degrees S of Saturn

In the morning sky, Venus starts the month in central Gemini, moving quickly eastward through Cancer and into western Leo by the end of the month. On the 17th, Venus will pass 12 minutes-of-arc north of Jupiter, but by the time it rises here, Venus will be 22 minutes-of-arc northeast of Jupiter. The Goddess of Love rises around 5 a.m. in the east-northeast. At midmonth its disc is 10.4 seconds-of-arc across and 95% illuminated. Next month is Venus's last month in the morning sky.

Jupiter slips out of the Sun's glare into the morning sky around the middle of the month. Moving eastward in central Cancer, the King of the Planets rises around 5 a.m. By the end of the month, it will be 20 degrees up in the east as it gets light, shining at magnitude -1.8. Jupiter's disc will be 32.0 seconds-of-arc across.

The Perseid meteor shower peaks this month just two days after Full Moon. This makes it a very poor year for observing this reliable meteor shower. If you still want to try it, put out your chaise lounge looking to the northeast in the morning before it starts getting light. Even through it is mid-summer, you will probably need to cover yourself with a blanket as you "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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