Scorpions are creatures that attempt to avoid notice or attention, that lurk in the shadows and hide under rocks or within cracks.
These arachnids are chiefly active under the cover of darkness. Scorpions dispatch small prey by crushing them with their claws; larger food items are injected with venom. In his book "The Stars in Our Heaven" (Pantheon Books, 1948), Peter Lum wrote, "The scorpion is usually disliked, feared and avoided by anyone who has ever come in contact with him."
How ironic, then, that the scorpion we see in our summer nighttime sky is perhaps one of the most striking and magnificent of all the star patterns. Only Orion (the Hunter), the staple of the wintertime sky, has more bright stars. But Scorpius (the Scorpion), which now lies low over the southern horizon as darkness falls, is the most formidable of the constellations. [Best Night Sky Events of July 2015 (Stargazing Maps)]
It really does resemble a scorpion. The tail and body are shaped like a fishhook, and the body is formed by the upper stars of the pattern. The tail slants down toward the horizon, suddenly curving upward and to the left in a fine stream of stars ending in a wide pair of stars that marks the scorpion's stinger.
From around 40 degrees north latitude — for cities such as Philadelphia, Denver and Reno — the entire figure of Scorpius just manages to clear the southern horizon. Places near or north of latitude 50 degrees (such as Vancouver, Calgary and London) see little or nothing of the tail as it is mostly hidden below the horizon.
Supergiant Antares
The most noteworthy object in Scorpius is Antares, a distinctly reddish star of the 1st magnitude that marks the heart of the Scorpion. Antares, the 17th-brightest star in the sky, is a red supergiant and is one of the largest and most luminous observable stars. It's approximately 600 light-years away and also sports a faint green companion star.
Antares is a variable star that expands and contracts in size at irregular intervals. Recent estimates place its diameter at 883 times the diameter of our sun on average — so immense that, if Antares replaced the sun in our solar system, it would swallow up the orbits of Mercury, Venus, Earth and Mars.
Put another way, if the sun were shrunk down to the size of a baseball, Antares would be a globe measuring nearly 184 feet (56 meters) in diameter! And yet despite its tremendous size, the thin gases that compose Antares have an overall density of less than one-millionth that of the sun.
Antares appears red because it's relatively cool as stellar temperatures go. The star is red-hot with an estimated temperature of about 5,700 degrees Fahrenheit (3,150 degress Celsius). Our sun, in comparison, is yellow-hot at close to 10,000 degrees Fahrenheit (5,500 degrees C), while the star Rigel in Orion is blue-hot at 21,300-degrees Fahrenheit (11,800 degrees C).
The word "Antares" means "anti-Ares" or "Rival of Mars," likely because astronomers in ancient times thought the star's reddish color rivaled the color of Mars. Indeed, although Mars is known as the Red Planet, to the naked eye it shows more of a fiery orange-yellow tint and does not boast the distinctive ruddy coloration displayed by Antares. [Latest Photos by NASA's Mars Rover Curiosity]
In his book "Find the Constellations" (Houghton Mifflin, 1954), H.A. Rey advised readers to look for Antares when they're outdoors on the Fourth of July. On that date, the star lies almost due south in the sky at 10 p.m. local daylight time. Rey called Antares the "Fourth of July star."
He also coined a name for a close pair of stars in the Scorpion's tail — the "Cat's Eyes."
"You'll find the name quire fitting," Rey noted in his book.
Clipped claws
Scorpius used to occupy an even greater section of the night sky, for at one time the stars that we now know as Libra (the Scales) actually belonged to Scorpius. The brightest (Alpha) star in Libra still goes by the name Zubenelgenubi (the "Southern Claw"), and its Beta star is Zubeneschamali (the "Northern Claw").
These names have been derived through the Arabic from the Greek names current during the time of the astronomer and mathematician Ptolemy, who died in 168 A.D. The first star is yellow-white, while the other is pale green. It's worth checking out Zubenelgenubi with binoculars or a small telescope, as it's a wide double star.
But after a ruling made in 1930 by the International Astronomical Union (the same committee that demoted Pluto to a dwarf planet), astronomers divided up Scorpius and Libra. The boundaries accepted today give Libra possession of the two stars that once marked the tips of the Scorpion's claws. Some planetarium lecturers tell their audiences that Scorpius is reaching into Libra to steal two stars to which it no longer has any claim.
Orion's adversary
The legend of Scorpius connects the creature with Orion, the Hunter. According to Greek mythology, Orion boasted that no animal could overcome him. Then Zeus sent Scorpius to sting Orion in the heel, killing him.
When Orion was placed in the sky with his two faithful hunting dogs (Canis Major and Canis Minor) and the hare they were chasing (Lepus), Scorpius was also elevated, but on the opposite side of the heavens. So Orion cannot be seen when Scorpius is in the sky.
One final note: the Scorpion's astronomical name is Scorpius. If you see a reference to "Scorpio" that is a more ancient moniker primarily used by astrologers.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmer's Almanac and other publications, and he is also an on-camera meteorologist for News 12 Westchester, N.Y. Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com.
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