Jimmy Westlake: Spot the Twins this month
February 15, 2011
Steamboat Springs — High overhead in the early evening skies of late winter, you'll find a close pair of stars, nearly equal in brightness. Upon seeing these stars, one would not be surprised to learn that they always have been associated with the mythological Gemini twins.
These stars of winter represent the twin sons of Leda, from Greek mythology, and were known to the ancient Greeks as the Dioscuri, or Sons of Zeus. Leda's sons were not identical twins, but fraternal twins, because they each had a different father. Castor was fathered by Leda's husband, Tyndareus, while Pollux was fathered by none other than Zeus himself, the king of the Greek gods. Consequently, Castor was mortal but Pollux was immortal.
The two brothers were very close to one another and to their two sisters, Helen of Troy and Clytemnestra. The twins shared many adventures together, even sailing with Jason and the Argonauts in their quest for the Golden Fleece. When Castor was fatally wounded in battle, Pollux pleaded with Zeus to grant his brother immortality, too, so that he should not die. Zeus heard the plea and immortalized both brothers in the sky, side by side forever, as our constellation of Gemini, the Twins.
Gemini is one of the 12 constellations of the zodiac, which means that it lies along the path that the sun, moon and planets follow in their journeys across the sky. In fact, the northernmost position of the sun in our sky, marking our summer solstice June 21, lies right at the feet of the Gemini twins, near the famous star cluster M35. The stars of Gemini often serve as the backdrop for the moon and planets.
The stars Castor and Pollux represent the heads of the famous Gemini twins. The complete constellation is shaped like a long rectangle with Castor and Pollux at one end and the stars Propus and Alhena marking the feet of the twins at the other end. Gemini is positioned in the sky standing over the great hunter Orion as he fends off Taurus the Bull. Locate Orion and look for Gemini to his upper left.
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Close inspection shows that the star Pollux, representing the immortal twin, is slightly brighter than Castor, the mortal twin. That's how I remember which is which.
A medium-sized telescope aimed at Castor reveals not one, but two stars, very close together. Each of these stars again is double and this quartet is circled by yet a third pair of faint stars, making Castor a rare sextuple star system about 52 light years from Earth. Pollux is a little closer to us, at 35 light years, and has no known stellar companions, but recent studies show that it has at least one large planet in orbit around it, making it the brightest naked-eye star known to have a planet.
Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. He is an avid astronomer whose photographs and articles have been published all across the world. Check out Westlake's website at http://www.jwestlake.com.