Jimmy Westlake: Meet the Gemini twins
February 4, 2013
High overhead on February evenings you'll find a close pair of bright stars, nearly equal in brightness. After seeing these stars, you probably would not be surprised to learn they always have been associated with the mythological Gemini twins.
Known to the ancient Greeks as the Dioscuri (dye-ohs-cure'-eye), or Sons of Zeus, these twin stars of winter represent the twin sons of Leda, from Greek mythology. Leda's sons were not identical twins, but fraternal twins, because they each had a different father. Castor's father was Leda's husband, Tyndareus, while Pollux's father was the king of the Greek gods, Zeus, so Castor was mortal but his brother Pollux was immortal. The two brothers were very close to one another and their two sisters, Helen (of Troy) and Clytemnestra.
The twins shared many adventures, even sailing with Jason and the Argonauts in their quest for the Golden Fleece. When Castor was mortally wounded in battle, Pollux pleaded with Zeus to grant his brother immortality, too, 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. Close inspection reveals that the star Pollux, representing the immortal twin, is slightly brighter than Castor, the mortal twin. That's how I remember which one is which.
Our constellation of Gemini is one of the 12 constellations of the Zodiac, which means it lies along the path that the sun, moon and planets follow in their journeys around the sky. In fact, the sun's northernmost position, marking our summer solstice on June 21, lies right at the feet of the Gemini twins, near the beautiful star cluster M35.
The stars Castor and Pollux represent the heads of the famous 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 fights off Taurus the Bull, so once you locate Orion, look for Gemini to his upper left.
A medium-sized telescope aimed at Castor reveals that it is a visual binary star. Each of these stars is again double and are 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. However, recent studies show it has at least one large planet orbiting around it.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. Check out his astrophotography website at http://www.jwestlake.com.
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