Constellation of the Month: Serpens, the Serpent

Serpens, the Serpent is a unique constellation. It is the only one that is divided into two parts, Serpens Caput ("the Head of the Serpent") and Serpens Cauda ("the Tail of the Serpent"). What's in between? Ophiuchus, the Serpent Holder. He is holding the body of the Serpent, which is portrayed as a long snake. Even though Serpens is broken into two parts, it still counts as one constellation.

Click Image to Enlarge Serpens Caput and Serpens Cauda bracket Ophiuchus high in our southern sky as it gets dark these hot July evenings. Jupiter is just below them this month, in southern Ophiuchus. Above them, almost overhead, is Hercules, which hosts another bright globular cluster, M13. Below Jupiter is Scorpius, the Scorpion, and left of that is Sagittarius. This is where the center of our galaxy is located.

The tale of Ophiuchus and the snake was told in these pages last August. In short, Ophiuchus was a half-son of Zeus and a great medicine man. He once strangled a serpent. But as he dropped the body on the ground, another serpent appeared with an herb that was placed in the dead snake's mouth, reviving it. Ophiuchus grabbed some of the herb and thereafter was able to revive the dead himself. This irritated Pluto, king of the Underworld, who was not getting his quota of new arrivals. He complained to Zeus, who sent an eagle (Aquila) down to Earth with one of his thunderbolts. The eagle hit Ophiuchus head-on, killing him. Zeus placed Ophiuchus in the sky so that his knowledge would not be forgotten.

Serpens is home to the Eagle Nebula (M16), some 7,000 light-years distant. The Hubble Space Telescope took a high-resolution image of part of this nebula in 1995, which was released to the public as the "Pillars of Creation," a region of intense star formation. It was originally believed that the "pillars" were formed by protostars, aggregates of dust and gas that are traveling through the dust- and gas-rich environment of the Eagle Nebula, and their gravity was sweeping up all the material near them as they pass through an area. This would leave a void where the proto-star has been, forming the "pillars."

Since then, the Chandra X-Ray Observatory has also observed the Eagle, looking for the X-rays that come from young stars. Chandra found that the X-ray sources are scattered randomly throughout the Eagle. They do not correspond to the heads of the pillars, leading many astronomers to believe that star formation there peaked a million years ago. If the pillars do contain protostars, they are not yet hot enough to emit X-rays.

Another object in Serpens is the globular cluster M5. This 13-billion-year-old cluster is one of the older ones orbiting the Milky Way Galaxy. It is 24,500 light years away and contains up to half a million stars. At magnitude 6.65, M5 can just be glimpsed by the naked eye at a very dark site. Telescopically, M5 is 23 minutes-of-arc across, just a little smaller than the full moon. The brightest stars in this cluster are magnitude 12.2. This is one of the better globular clusters in our sky.

The Planets for July 2007

Venus and Saturn are together in the western sky as it gets dark, which is pretty late these summer evenings. These two planets will be only 0.8 degrees apart on July 1. A telescopic view of the two will highlight the brightness difference between them. The magnitude -4.5 Venus is 31.8 seconds-of-arc across its 35-percent illuminated crescent. It has been swinging toward the Earth for the last few months, thereby getting larger and brighter. At the same time, it has been changing from almost full to being a crescent. This combination of factors will make Venus the brightest it will be this apparition on July 12. After this date, the effect of the shrinking crescent will overpower the effect of the expanding disc, making Venus fainter, but still brilliant. Venus spends most of the month in Leo, but creeps over the border into Sextans as the month draws to a close.

Saturn is much fainter, being some 13 times farther from the Sun, making the sunlight 176 times weaker at Saturn than at Venus. Through the telescope, Saturn looks much fainter and washed-out by comparison. At midmonth, Saturn is magnitude -0.4 with the disc of the planet 16.4 seconds-of-arc across. The beautiful ring system is 37.2 seconds-of-arc across, and is still tipped up at an angle of 12.9 degrees with the southern face showing. Saturn remains in Leo all month. Both planets will set around 10 p.m.

Jupiter is riding along the southernmost part of the ecliptic in Ophiuchus, putting it low in our southern sky as darkness falls. Because it is so relatively low, Jupiter does not provide as clear an image as it will be when it is in the summer (northern) part of the ecliptic six years from now. Jupiter is magnitude -2.5 and its disc is 43.6 seconds-of-arc across. The King of Planets sets around 3 a.m.

Just an hour before Jupiter sets in the west, Mars comes up in the east. The God of War starts the month in Aries, moving eastward among the stars until it slips into Taurus on July 27. Mars is still only 6.9 seconds-of-arc across at magnitude 0.6, but it is starting to get bigger as the Earth catches up to it. Mars will reach opposition in December. Look for NASA to launch a few Mars probes in August and September to take advantage of December's close approach.

The Messenger of the Gods makes its appearance this month in the morning sky beginning around July 10. Mercury will be in northern Orion, low on the east-northeastern horizon. Having just emerged from between the Earth and Sun, Mercury is a 15-percent illuminated crescent 9.9 seconds-of-arc across. Mercury will be farthest from the Sun on July 20, only 20 degrees away along the ecliptic, when it will be in Gemini at magnitude 0.4 with the crescent widened to 37 percent illuminated, but the disc shrunk to 7.8 seconds of arc. By the end of the month, Mercury will be back near the Sun, now 73 percent illuminated and only 6.0 seconds-of-arc across at magnitude -0.9. So don't miss it for the few days it will be visible in the last half of the month, and "keep watching the sky"!

An amateur astronomer for more than 35 years, Bert Stevens is co-director of Desert Moon Observatory in Las Cruces.

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