In my younger days, I learned a simple technique for learning the constellations. Start with one that is easy to find, and work your way outward from it. Over the past few weeks, we have been doing just that, so let’s continue.
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 is topped with a bright star.
The constellation takes its name from those two bright stars, which are close together and farthest to the north – Pollux on the left and Castor on the right and farther to the north.
Castor is a famous binary, or double, star. Even a small department store telescope used at high power will show that Castor is really two stars, not one as it appears 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.
In 1804, a great astronomer named 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 is about the same color and temperature as the sun but it 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-orange star and expands into a dying red giant.
The ancients considered Castor and Pollux to be of equal brightness, but even the untrained eye can see that Pollux is a bit brighter. Stars measure their life spans in billions of years, so it’s very unusual to see such a big change in only a few thousand years. According to 365 Starry Nights by Chet Raymo, it is possible that Pollux may have increased in brightness in historical times, suggesting that it is near the point where it will expand to become a red giant and finally end its life cycle.
It is unusual to have two stars of such similar brightness so close together in the sky, so the ancients 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. 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. According to some accounts, Leda gave birth to a swan’s egg, and the twins popped out of it.
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 was a mortal man and Pollux an immortal god.
What made them special and got them their own hunk of sky was that they loved each other very deeply – but only in a clean, decent, manly sort of way, mind you. If you looked up the word “bromance” in a modern dictionary, you ought to see a picture of Castor and Pollux.
Castor and Pollux were great heroes to the Greeks and Romans. Castor was known for his ability to train and ride horses. Pollux was a great boxer.
Roman soldiers swore 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 Attica to rescue their half sister, the beautiful Helen, who was kidnapped by Attica’s king, Theseus.
They sailed with Jason and the Argonauts on their mission to steal the Golden Fleece. It is perhaps from that expedition that they became the patron gods of ancient sailors. Upon their death, Neptune, 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 never mind.)
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 underworld.
Zeus was so touched that he allowed Castor and Pollux to stay together. Pollux lost half of his immortality, which Zeus gave to Castor. For all eternity they spend part of their time in Hades and part in Olympus.
Their dual nature 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 become truly twins – both mortal yet both touched by the breath of immortality.
Tom Burns is director of the Perkins Observatory in Delaware.