Jimmy Westlake: The heart of the scorpion
July 8, 2007
There aren’t many constellations that resemble the objects for which they are named. Scorpius the Scorpion is a delightful example one that does. The celestial scorpion crawls across our southern horizon on summer evenings, so July is prime time for scorpion hunting.
As soon as it gets dark in the evening, step outside and face south. The brightest star you see there isn’t a star at all, but is the giant planet Jupiter. Just to the lower right of Jupiter you’ll see the flashy red heart of the Scorpion, a star named Antares. From Antares, trace a line of fainter stars toward the lower left that curls up on the end like a giant fishhook. This is the Scorpion’s tail, marked at the tip by the deadly stinger star, Shaula. To the right of Antares, you’ll spot a vertical trio of stars reminiscent of Orion’s Belt. The middle star, Dschubba, represents the Scorpion’s head, and the two stars on either side mark his pincers.
In mythology, Scorpius is notorious for stinging and killing Orion the Hunter. The legend tells us Orion once made the boast that he could kill every living creature on Earth, if he wanted to. The animals got together and decided they must make a pre-emptive strike just in case Orion was serious. They chose one of their smallest members, the scorpion, to teach Orion a deadly lesson. Stalking the hunter one day in the woods, the scorpion stung Orion on the heel. The great hunter wheeled around in pain and collapsed from the scorpion’s fatal poison. The scorpion and the hunter were both immortalized in the stars as our constellations of Scorpius and Orion, but they were placed on opposite sides of the sky so that the two mortal enemies could never be seen together. Orion rides high in the mid-winter sky, and Scorpius appears low in our mid-summer sky.
Of the 20 brightest stars in the sky, only two are classified as red supergiant stars. One is the star Betelgeuse, visible in the winter sky as the shoulder of Orion the Hunter. The other is Antares, the heart of the Scorpion. Antares is one of the largest stars known, measuring 700 times larger than our sun. If placed at the sun’s location in the center of our solar system, Antares would swallow the orbits of all of the inner planets – Mercury, Venus, Earth and Mars! The name Antares comes from the Greek words meaning “the rival of Mars.” Both Antares and Mars shine with an unmistakable reddish color, and novice skywatchers sometimes confuse Antares for Mars. One way to tell the difference is to recall the old rule of thumb that stars twinkle and planets don’t.
Antares is a cool, red supergiant star nearing the end of its life. Within its core, it is furiously fusing helium into carbon, a sure sign of old age in a supergiant star. Enjoy Antares now, because it probably doesn’t have much time left to live. One day, in the not-too-distant future, Antares will have fused all of its helium into carbon, all of its carbon into silicon and all of its silicon into iron. The iron core of the Scorpion’s heart will suddenly collapse inward under its own crushing weight and trigger a catastrophic explosion – a supernova! If anything remains of the Scorpion’s heart after this colossal explosion, it will either be a rapidly spinning neutron star, or a bottomless pit in the fabric of spacetime called a black hole. Either way, the heart of the Scorpion will be dead.
Scorpius has many treats for binocular observers. Try sweeping around the tail stars to find two beautiful star clusters named Messier 6 and Messier 7, or simply M6 and M7 for short. And don’t bypass Antares; it becomes a fiery red ember through binoculars! Just west of Antares, you should be able to spot a fuzzy-looking globular star cluster of half-a-million stars called M4. Look for Scorpius the Scorpion and its brightest star, Antares, almost due south in the sky around 10 p.m. on July evenings.