Jimmy Westlake: The Gemini twins
February 24, 2010
High overhead on early evenings of late winter you'll find a close pair of bright 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.
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, since they each had a different father. Castor was fathered by Leda's husband, Tyndareus, and Pollux was fathered by none other than Zeus himself, the king of the Greek gods. Consequently, Castor was mortal, but his brother Pollux was immortal. The two brothers were very close to each other 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 fatally 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.
Our constellation of 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 around the sky. In fact, the northernmost position of the sun in our sky, marking our summer solstice on 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. For example, on the evening of today, the fat gibbous moon will sit right in the middle of Gemini, forming a striking triangle with the twin stars and the blazing planet Mars nearby.
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 fights off Taurus the Bull, so locate Orion and look for Gemini to his upper left.
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 one is which.
A medium-sized telescope aimed at Castor reveals not one, but two stars, very close together. 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, 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.
Professor Jimmy 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 around the world. His "Celestial News" column appears weekly in the Steamboat Today. His "Cosmic Moment" radio spots can be heard on local radio station KFMU. Also, check out Jimmy's Web site at http://www.jwestlake.com.