Thursday, July 24, 2008

Fishing for Scorpion

Scorpius the Scorpion is one of those rare constellations whose prominent star pattern actually resembles the figure it represents. The ancient Greeks, Arabs, Persians, and Turks all saw a starry scorpion in the pattern. It’s easy to see why. The long, lazy curl of stars that punctuates the constellation at its southern end looks like a scorpion’s toxic tail. Although this dramatic feature is easy to spot, it may not be wholly visible to skywatchers in the far northern latitudes, because Scorpius sits low in the south.

Image by Jim Kalisch

In Greek mythology, the scorpion was the infamous creature that stung Orion the Hunter to death. The prodigious huntsman had boasted that he could and would kill every wild animal in existence. This angered the gods, who dispatched the stealthy scorpion to do their dirty work. Afterwards, king of the gods Zeus placed both Orion and Scorpius in the sky, ensuring that the Hunter (winter constellation) was never in the sky at the same time as the Scorpion (summer constellation). They chase each other around the celestial sphere for eternity.

Scorpius in 19th century star atlas

Let’s see if we can spy the scorpion, while avoiding the business end of that sinister tail.

1) You’ll need to face south. If you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your right shoulder to the west, and you’ll be facing approximately south.

2) Wait at least one hour after sunset to begin observing, so that twilight’s over and your sky’s good and dark. On Thursday the 24th, you’ll have a couple dark hours to stargaze before the gibbous Moon rises. The Moon is considered gibbous whenever its face is more than half illuminated but not fully illuminated. On Friday the 25th, the Moon’s phase will be Last Quarter (commonly called a “half moon”), and it’ll rise around midnight. Each night for the rest of our observing week, the Moon will rise a bit later with its illuminated part continuing to shrink or wane.

3) Looking due south, you should see a tall curve of stars that hooks over to the left (east) after it skims the horizon. This is the asterism (recognizable star pattern) known as the Fishhook. It’s also the torso and tail of the mythological scorpion.

4) Let’s begin our survey of the Fishhook at the top, with the stunning red supergiant star Antares (pronounced ahn-TAHR-eez), brightest star in Scorpius. Due to its deep red color and its position in the imaginary scorpion’s body, Antares is commonly called the Scorpion’s Heart.

The name Antares is from the Greek for like Mars. Planet Mars periodically enters Scorpius and passes near Antares. The red planet and the red star can easily be mistaken for one another.

Luminous Antares is 10,000 times brighter than our Sun and much larger. If Antares were dropped into our solar system in place of the Sun, it would extend beyond the orbit of Mars, engulfing that planet along with Mercury, Venus, and Earth. Immense Antares is a candidate for death by supernova, a cataclysmic explosion that occurs when a massive star reaches the end of its life cycle.

5) Below Antares, nine bright stars wind around to form the Fishhook before it terminates in a glowing barb composed of a close-set pair of stars. This terminal pair is an asterism called the Cat’s Eyes. It’s also the stinger of the legendary scorpion. The eastern star in the pair is the second brightest star in Scorpius, Shaula (pronounced SHOWL-uh). Shaula is Arabic for stinger. The fainter western star is Lesath (pronounced LESS-utt), Arabic for sting. Although Shaula and Lesath appear cozy from our two-dimensional viewpoint, they’re more than 150 light years apart and don’t interact. We often forget to apply the third dimension of depth when we see visually pleasing star patterns.

6) The third brightest star in Scorpius is Girtab, riding low on the Fishhook where it turns up toward the Cat‘s Eyes. Girtab (pronounced GRRR-tahb) is from the Sumerian for scorpion. This yellow-white giant star can’t be seen from above 50 degrees north latitude, so folks in Spokane, Washington and Utica, New York will see an interrupted Fishhook dip below the horizon.

7) If you’re observing from a dark-sky location, try this naked-eye challenge. Can you spot the two star clusters just northeast of the Cat’s Eyes? They should look like fuzzy patches of light. They’re both open clusters. An open cluster is a family group of sorts, a loose collection of stars that formed around the same time in the same nebula, or cloud of gas and dust.

The cluster closest to Shaula, known as Messier 7 (pronounced MESS-ee-yay) or Ptolemy’s Cluster, is also the brighter of the two. It contains approximately 80 stars and was named for the ancient Greek astronomer Ptolemy (pronounced TAHL-uh-mee), who wrote about it in the year 130.

The naked-eye cluster slightly north and west of Messier 7 is called Messier 6 or the Butterfly Cluster. This whimsical cluster also contains around 80 stars whose arrangement, when viewed through binoculars, suggests a butterfly. The fainter Butterfly Cluster glimmers from nearly twice the distance of Ptolemy’s Cluster.

Ptolemy’s Cluster (lower left) and Butterfly Cluster (upper right)
Copyright T. Credner,

8) Our expedition wouldn’t be complete without a dip in the Summer Milky Way. Can you see the misty stream that rises from the southern horizon, winds behind the Fishhook and the two clusters, and then stretches northward across the sky? What we call the “Summer Milky Way” is only one arm of the pinwheel-like galaxy we inhabit. The hazy, mottled arm is a horde of stars too numerous and faint to be resolved into pinpoints with the naked eye. Because the southern part of that arm--the Sagittarius Arm--lies in the direction of the dense galactic core, Scorpius occupies an area of the sky teeming with deep-sky delicacies ready to be hooked with telescope and binoculars.

Gone fishing.