By Tom Burns
March 2, 2014
Look straight south in the early evening and you’ll see the familiar constellation Orion high in the sky. Above Orion, to the northeast, the constellation Gemini, the Twins, will be easily visible.
Gemini consists of two long lines of stars, each of which culminates in two bright stars that are close together and similar in brightness. The constellation takes its name from those two stars, Castor and Pollux.
Castor is a famous binary, or double, star. Even a small department store telescope used at medium magnification will show that Castor is really two stars, not one as it seems to the naked eye.
The two stars of Castor are similar in brightness, but careful examination will show that one is slightly brighter that the other. Castor is a good test of the optics of a small telescope. If repeated attempts to split it into two stars fail, the optics of the ‘scope aren’t very good and you should return it for a refund. More than once, I’ve heard amateur telescopists complain, “This ‘scope is terrible. It won’t even split Castor.”
In 1804, the eagle-eyed British astronomer William Herschel noticed that the fainter of the two stars had shifted slightly in its position with respect to the brighter one. That discovery suggested for the first time that one star could revolve around another one in a true binary system. It takes about 350 years for the faint star to circle once around the brighter one.
Pollux is similar to our sun in some ways. It’s about the same color and temperature as the sun but is brighter and larger, suggesting that we might be seeing what our own sun will look like in a few billion years when it stops being a friendly yellow star and begins to die.
Two stars of such similar brightness so close together in the sky is unusual, to say the least. Thus, our ancient forebears named them after a pair of famous mythological twins. Castor and Pollux were the twin sons of Leda, the queen of Sparta.
Here’s the story. Zeus, the head man on Olympus, appeared in Leda’s bedchamber in the form of a swan. If that sounds odd, consider that Zeus had a “thing” for mortal women, and he often appeared as an animal for such trysts.
Later that night, the king of Sparta also visited Leda (Leda really got around). Nine months later, if you know what I mean and I think you do, Castor and Pollux were born. Castor was the son of the mortal king, and Pollux was the son of Zeus. Thus, the “twins” were about as different as they could be. Castor a mortal man and Pollux an immortal god.
What got them their own hunk of sky was that they loved each other very deeply as twins often do. They looked alike and even dressed alike. They were inseparable and thus shared many heroic adventures together.
Castor was known for his ability to train and ride horses. Pollux was a great boxer.
Roman soldiers would swear oaths “by Gemini,” and that phrase survives, more or less, as the oath “by jiminy.” Does anybody still say that anymore?
Castor and Pollux participated in the attack against Athens to rescue their half sister, the beautiful Helen, who was kidnapped by Theseus.
The twins sailed with Jason and the Argonauts on their mission to steal the Golden Fleece. They saved Jason’s crew several times and, as a result, became the patron saints of ancient sailors.
Upon their death, Poseidon, the god of the sea, gave them control over the winds and the waves. Many ships have borne their names, and sailors looked to them for protection from the dangers of the sea.
Their greatest battle was their last. Castor and Pollux got in a scrape with their cousins, Idas and Lynceus. Because he was a god and a great fighter, Pollux killed Lynceus. But the mortal Castor was slain by Idas. Zeus intervened and killed Idas with a lightening bolt. (You’d think he could have done it a few minutes earlier and saved Castor’s life, but that’s the way Zeus was. He liked to watch a good fight.)
Pollux was heartbroken. He told Zeus he could not walk the earth without the companionship of his brother. He offered to renounce his immortality to join Castor in the bleak, dark underworld, where all mortals went when they died.
Zeus was so touched that he allowed Castor and Pollux to stay together. For all eternity they spend part of their time in the underworld and part in Olympus.
Their dual status is symbolized by their presence in the sky as twin stars.
In the winter months they stand high in the heavens. As the summer months approach, they sink below the horizon into the underworld.
Thus, they finally have truly become twins – mortal yet both touched by the breath of immortality.
Tom Burns is the director of Perkins Observatory. He can be reached at firstname.lastname@example.org.