The Scorpion Crawls Over the Horizon

© 2002 by Robert C. Moler

Low in the south on July evenings can be found a constellation that represents a creature that’s better appreciated in the sky than in person. The stars portray a scorpion, which astrologers call Scorpio and astronomers call Scorpius. Luckily, around here, Scorpius is as close as we’ll get to the real thing.

The stars in this constellation really do exhibit the scorpion’s characteristic curved tail complete with the poisonous stinger at the end. The chart on this page shows the scorpion superimposed on the stars. It actually extends into Libra the scales to the right. As a matter of fact the northernmost star of Libra is called Zubeneschamali which means Northern Claw. The star in Libra to the far right is Zubenelgenubi (one of the great sounding star names), which means Southern Claw. In our drawing the scorpion’s southern claw doesn’t quite reach that star.

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In mythology Scorpius killed Orion with his stinger. The proof of this is that Orion flees the sky to the west in the late spring when the scorpion rises. In other myths Orion meets his end in other ways.

Scorpius’ brightest star is Antares, whose name means Rival of Mars. It is a red giant star whose color is nearly identical to the Red Planet. When Mars passes through northern Scorpius someone who didn’t know the constellations would be hard put to tell then apart. Mars stays close to the line on the chart that is roughly horizontal passing just south of Zubenelgenubi. Mars passes through here every two years. It will next be closest to Antares on February 1st, 2003, when they will be visible in the early morning sky.

Antares is a huge star which lies low in the south at its highest. Its brightness is magnitude 0.92 which varies a bit in an irregular way, as some red giant stars do. Its appearance to the unaided eye, binoculars, and telescope is greatly affected by the steadiness of the atmosphere. Lying low in the sky, due to its southerly position on the celestial sphere and our location on the northerly part of the earth means we are always looking at it through the thickest parts of the earth’s atmosphere. Antares twinkles merrily and changes colors to the unaided eye. In binoculars it appears to jump around and change colors. In telescopes it can appear like a multicolored sparkler spitting and sputtering all the colors of the rainbow. Consequently it results in more than a few UFO calls.

Antares also has a dim companion star that appears green probably in contrast to the red of it’s primary star. Compared to Antares 0.92 magnitude, the companion is 6.5 magnitude. It is difficult to spot. I’ve never seen it, though it would lie in the sparkler effect of the atmosphere on Antares itself.

Another remarkable star is Delta (d) Scorpii which lies in the center of the three star arc at the head of the Scorpius. In the past it has been magnitude 2.3, but for the past two years has brightened to magnitude 1.8 or brighter. A good check on brightness is to compare it with Beta (b) Scorpii, the top star of the arc, which is at magnitude 2.6. Delta is now definitely brighter than Beta. Another comparison star is the bottom star of the arc, Pi (p) Scorpii at magnitude 2.9, the dimmest of the three. How long Delta will stay at this brightness is unknown, but it will probably return to its former brightness. Unlike Antares, which is a red giant star Delta Scorpii, which has the name Dschubba meaning the forehead, is a blue-white giant star of spectral class B0.

Delta seems to behave like another variable star class called whose prototype is Gamma Cassiopeiae. It gained a half magnitude for a time, then its magnitude fell back and became erratic. So Delta Scorpii bears monitoring.

A note on magnitudes: The magnitude scale is roughly based on the 6 magnitudes of the Greek astronomer Hipparchus of the 2nd century BC. He invented a scale where the brightest stars are first magnitude and the faintest visible to the unaided eye are 6th

Modern astronomers have created a mathematical basis for the magnitude scale where each magnitude step is a factor of 2.512 times brighter of dimmer. Five magnitudes is a difference of 100 in brightness. In keeping with Hipparchus’ scale the brighter the star the lower the magnitude. The sun has a magnitude of about –26, while the faintest stars visible in telescopes is currently around +28.

The brightest star cluster in Scorpius is M4 near Antares. Its brightest stars are 10th magnitude, though most are 13th and dimmer. So a few of the stars of the star cluster may be visible in relatively small telescopes.

Another neat area to view in binoculars is around Zeta (z) Scorpii. On the chart above it’s near the top of the S for south symbol. It is actually a wide double star, as is the star on the chart above it. Here is a wonderful cluster of stars. I first noticed this on a trip to the Florida Keys in 1986 to view Halley’s Comet. There Scorpius rides a lot higher in the south than it does here. From here, on a crystal clear night we just get a hint of its glory.

The above objects are just a hint of the wonders that are located in Scorpius on a clear summer night.

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Updated: 09/02/02