D e s e r t E x p o s u r e
August 2009
Constellation of the Month: Scorpius, The Scorpion
One of the constellations that actually looks like what it is supposed to represent is Scorpius, the Scorpion. Here in the desert Southwest we are all too familiar with the scorpion. The celestial scorpion appears to be climbing the wall of our southern sky as it gets dark on August evenings. The claws and head are at the northwest end of the constellation. The body starts with the brilliant red star Antares and trails along to the southwest until it turns into the tail around Mu2 Scorpii. From there, it curves off to the left and then turns upward again, ending in two adjacent stars that are the scorpion's stinger.
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Click the image for a larger map
Scorpius, the Scorpion, winds its way through our southern skies on warm August evenings. The Scorpion's heart is the red giant star Antares, which glows with a ruddy color and stands out even in the Milky Way. Scorpius has many interesting objects, including dust clouds, star clusters and bright nebulae.
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Scorpius is one of the oldest constellations. Many ancient cultures identified this star group as a scorpion. It is one of the 12 original constellations of the Zodiac. As with all zodiacal constellations, the annual path of the Sun goes through Scorpius, but only for a short distance before entering Ophiuchus. The Sun enters Scorpius on Nov. 21 and exits on Nov. 29. Scorpius is also in the Milky Way and has many deep-sky objects.
Mythologically, Scorpius is usually associated with Orion, the Hunter. Orion was a renowned hero of Greek mythology, whose gigantic stature and good looks were widely known. There are a number of versions of this story, with different characters interacting with Orion, but always ending in a battle with a scorpion. Many of the stories relate to Orion's boastfulness and skill with the ladies.
One version involves Eos, goddess of the dawn, who opened the gates of heaven so Apollo could daily ride his Sun-chariot across the sky. Eos invited Orion to spend the night with her, which he eagerly accepted. Afterward, Orion bragged of his conquest and hunting prowess, boasting he would exterminate all the wild beasts. This angered Apollo, who was also the guardian of herds. He persuaded Gaia to send a giant scorpion with impenetrable armor to slay this braggart. After a battle, the scorpion succeeded in stinging and killing Orion. A slightly different ending has the scorpion chasing Orion out to sea, where Artemis did him in with an arrow.
In either case, after Orion's death, Apollo and Artemis hung his image in the sky. The scorpion also got a place in the sky, but on the opposite side of the starry dome. So every time Scorpius rises, Orion sets, still appearing to be running from his killer.
The brightest star in Scorpius is the brilliant class M supergiant star, Antares. It is 800 times larger than our Sun, but only 18 times heavier and 10,000 times brighter than the Sun. Antares' surface area is 640,000 times that of the Sun, yet its surface temperature is 1/64th that of the Sun. Antares' relatively cool surface puts its peak light output not in the yellow part of the spectrum like our Sun, but in the red part. Indeed, its name means "against Mars," because it has the same reddish hue as the God of War.
This supergiant is so large that if it were our sun, the outer surface of Antares would be in the asteroid belt between Mars and Jupiter. As an old red giant star, it pulsates slowly with an irregular period. Antares can be as bright as magnitude +0.9 and as faint as magnitude +1.8. Antares has a companion, spectral type B2.5, 2.9 seconds-of-arc distant. This blue star (called Antares B) is 13 times farther from Antares than Pluto is from the Sun and takes 878 years to orbit the main star. It shines with the light of 170 Suns, but its light is swamped by Antares' baleful glow. The color contrast between the two stars makes the 5.5-magnitude Antares B appear more greenish than its true blue.
The Planets for August 2009
Saturn is low in our western sky shortly after sunset at the beginning of August, but as the month progresses, it will head for the Sun, becoming quickly lost from our view. The Rings are continuing to close, but the Earth will pass through the plane of the rings on Sept. 4, when Saturn will be too close to the Sun to observe it. Saturn is still in eastern Leo, moving slowly eastward. On August 5, the Rings will still have their southern face showing and be tilted up only 1.6 degrees. The ball of the planet is 16.2 seconds-of-arc while the rings are 36.7 seconds-of-arc across. The Ringed Planet is magnitude 1.1 and sets by 10 p.m. If you can still find it, Saturn will be passed by Mercury on August 17, at a distance of 3 degrees.
Watch the Skies (all times MDT) August 5, 6:55 p.m. — Full Moon (penumbral lunar eclipse) August 12, noon — Perseid Meteor Shower Peaks August 13, 12:55 p.m. — Last Quarter Moon August 14, noon — Jupiter at opposition August 20, 4:02 a.m. — New Moon August 24, 10 a.m. — Mercury greatest elongation east (27°) August 27, 5:42 a.m. — First Quarter Moon |
Mercury makes an appearance in the evening sky late in August. It reaches its greatest elongation (27 degrees) from the Sun on August 24, at magnitude 0.3. The Messenger of the Gods will be 52% illuminated and becoming more of a crescent every day as it gets larger. Mercury will be 7.3 seconds-of-arc across at greatest elongation.
Jupiter will be rising as it gets dark. It will be on the opposite side of the Earth from the Sun on August 14, allowing Jupiter to be visible all night. This is the best time of this year to observe Jupiter with a telescope, though Jupiter is very far south, keeping it from getting very high in our sky. The King of the Planets is 48.9 seconds-of-arc across at midmonth at magnitude -2.9, moving slowly westward among the stars of eastern Capricornus.
Mars starts the month in eastern Taurus, moving quickly eastward. Six days before the end of the month, it will slip into Gemini, where it ends the month. At midmonth, Mars is 5.5 seconds-of-arc across and glows with a reddish tint at magnitude 1.0. The God of War rises at 2 a.m., visible the rest of the night.
Shortly before 4 a.m., Venus comes over the east-northeastern horizon. Shining at magnitude -4.0, its disc is 78% illuminated, becoming more full and growing smaller; it shrinks from 13.6 seconds-of-arc at mid-month to 12.6 by month's end. The Goddess of Love starts the month in western Gemini, crosses the entire constellation and ends up in mid-Cancer.
The Perseid Meteor Shower peaks this month on the morning of August 12 in the northeastern sky. But this year, the Moon is still 66% illuminated, hiding the fainter meteors. The normal peak is actually well after sunrise this year, but there is the possibility that the trail of meteors left by Comet Swift-Tuttle in 1610 may cause a secondary peak around 3 a.m.
The last eclipse of the current eclipse season is a very penumbral eclipse on August 6. The long total solar eclipse of July 21 is flanked by penumbral lunar eclipses at the Full Moons before and after it. This month's is so slight you probably won't even notice. So enjoy the hot summer observing weather when the monsoon clouds aren't blocking the view and "keep watching the sky"!