Caster, Pollux star as twins of the sky


By Tom Burns


People with even a casual interest in astronomy are often familiar with the unmistakable constellation Orion. Thus, it makes a good starting point to find other objects in the sky. Above Orion, to the northeast, look for the constellation Gemini, the Twins.

Gemini consists of two long lines of stars. Both lines are topped with bright stars. Those top stars have been of considerable interest to stargazers since ancient times because they are approximately equal in brightness.

The constellation takes its name from those bright stars. Look for them close together and farthest to the north at the top of the two lines. Castor is the star above, and Pollux is the star below. The stars form the heads of two heavenly twins, Caster and Pollux. The lines of stars represent their very slim bodies.

Castor is a well-known binary, or double, star. A tiny telescope used at high power will show that Castor is, in reality, two stars, not one as it seems to the naked eye.

The two stars of Castor are close to each other in brightness. However, careful examination will indicate that one is distinctly — if ever-so-slightly — brighter than the other. Castor is a good test of the optics of a small telescope.

I home-built many telescopes early in my stargazing career. Every grizzled telescopist gave me the same advice. If repeated attempts to split Castor into two stars failed, the telescope’s optics weren’t up to snuff. I needed to make or purchase a new mirror or lens for the ‘scope.

In 1804, eagle-eyed 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 orbit another in what astronomers later called a binary system.

Subsequent measurements indicate that it takes about 350 years for the faint star to orbit once around the brighter one.

Pollux is similar to our sun in some ways. Pollux is about the same color and temperature as the sun, but the sun is dimmer and smaller. Those characteristics suggest that we might be seeing what our own sun will look like in a few billion years.

At that time, old Sol will stop being a friendly yellow-dwarf star and begins to expand into a dying and deadly red giant.

The ancients described Castor and Pollux as equally bright. Even an untrained eye can see that Pollux is a bit brighter in modern times. Has our eyesight simply gotten better?

Stars measure their life spans in billions of years, so seeing such a significant change in only a few thousand years is unusual.

According to Chet Raymo’s “365 Starry Nights,” Pollux may have increased in brightness in historical times, again suggesting that the star is near the point where it will expand into a red giant and afterward end its life in a few tens of millions of years.

The presence of two stars of such similar brightness so close together in the sky is quite unusual.

Our ancient forebears were dutifully impressed, so much so that they named the stars — and the constellation — after Castor and Pollux, a pair of magnificent mythological heroes.

However, some Greek and Roman constellation commentators disagreed. Early references to the stars refer to the two stars as twins, but the early writer couldn’t decide what twins they were or even if they were true twins.

Aratus, a fourth-century BCE Greek poet, referred to the constellation only as the twins, or Didymoi, but he doesn’t say what twins he is writing about. Five centuries later, first-century CE writer Pseudo-Eratosthenes refers to them as the Dioskouroi, i.e., Castor and Pollux.

However, Hyginus, a first-century BCE scholar, reports an alternative set of brothers. They are Apollo, the god of wisdom and the arts, and the half-mortal, half-god Heracles, whom we know as Hercules.

Both were sons of Zeus, the king of the gods, but they were born of different mothers and unmistakably were not twins.

Apollo was born of Zeus’s relationship with the goddess Leto. He represented everything the Greeks loved: youthful beauty, creativity, intelligence, and charm.

Heracles resulted from an illicit relationship between Zeus and Alcmene, a mortal. They were both married, but not to each other.

As a result, Heracles is half-mortal with many mortal faults. To add insult to injury, Zeus’s spouse, the jealous Hera, persecuted poor Hercules mercilessly throughout his life.

Among his many misdeeds, he murdered his spouse and children in a fit of Hera-induced madness.

The two characters could not have been more different. The best we can call them is half-brothers.

In his seminal work, the “Almagest,” Ptolemy called the constellation the Twins, but he doesn’t say what twins.

In a later astrological (not astronomical) work, Tetrabiblos, Ptolemy appears to support Hyginus’s identification. He refers to the star we call Castor as “Apollo’s star” and Pollux as “Heracles’s star.” Given Ptolemy’s status, that’s a pretty strong endorsement.

The association of the stars with Castor and Pollux probably survived into modern times because it makes sense astronomically. The stars are twin-like in their brightness. More significantly, the death of one of the brothers helps to explain how they got into the sky.

Here’s the story, which includes the usual violence and marital infidelity found in most Greek myths and, come to think of it, frequently streams onto a screen near you. Hide your children.

Castor and Pollux were the twin sons of Leda, the queen of Sparta. And yet, in one of the weirdest stories in Greek mythology, they had different fathers.

Zeus, the head man on Olympus, appeared in Leda’s bedchamber disguised as a swan. Later that night, the king of Sparta also visited Leda (Leda got around).

Nine months later, Leda gave birth to Castor and Pollux. Castor was the son of the mortal king, and Pollux was the son of the immortal Zeus.

Thus, the “twins” were about as different as they could be. Castor was a mortal man, and Pollux was an immortal god.

They embodied the much-favored trait of brotherly love, and that what got them their own hunk of the sky was. Hyginus writes that the brothers “neither contended for the kingship” of their native land nor engaged “in any act without mutual consent.”

For that reason, the Greeks regarded the brotherly pair as great heroes. Castor was known for his ability to train and ride horses. Pollux was a proficient boxer.

Roman soldiers swore oaths “by Gemini.” That phrase survived into the early 20th century, more or less, as the oath “by Jiminy.”

Castor and Pollux were crew members with Jason and the Argonauts on their expedition to steal the golden fleece. Because of their bravery on that mission, Poseidon, the god of the sea, made them the patron saints of ancient sailors.

Upon their death, Poseidon gave them control over the winds and the waves. Many ships have borne their names as sailors curried their favor to gain protection from the sea’s often lethal dangers.

Mariners associated the twins with a strange atmospheric phenomenon called St. Elmo’s fire. During lightning storms at sea, glowing globes of electricity occasionally appeared in ships’ riggings.

Pliny, a Roman writer of the first century CE, writes that “stars alight on the yards and other parts of the ship.” If two such “stars” appear, they denote “safety and portend a successful voyage.” The twin appearances of St. Elmo’s fire are thus “called Castor and Pollux, and people pray to them as gods for aid at sea.”

Mostly, they were known for getting into fights, an unfortunate flaw that eventually led to their deaths.

Castor and Pollux got into a fatal fight with their cousins, Idas and Lynceus. The motive for the titan battle is unclear.

Hyginus writes that the brothers were nobly defending Sparta against an onslaught by Ida and Lynceus, also twins.

But other sources claim they were fighting for less noble reasons. Their fatal battle with Idas and Lynceus began because of a family feud over a couple of women.

In any case, Pollux killed Lynceus. He was a god and an excellent boxer, after all. In a rage of sorrow and sadness, Ida slew the mortal Castor.

At that moment, Zeus intervened and killed Idas with a lightning 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 so that he might join Castor in Hades, the dark and unpleasant underworld where the gods discarded mortals when they died. As Hyginus aptly describes it, “Pollux granted his brother half his own life.”

Their brotherly fidelity and dutifulness so touched Zeus that he allowed Castor and Pollux to stay together. For all eternity, they spend part of their time in Hades and part in Olympus.

Their yearly rising and setting symbolize their divided life and their undying affection. During the winter months, they stand high in the heavens. As the summer months approach, they sink below the horizon into the underworld.

Thus, they have finally become true twins — mortal, yet touched by the breath of immortality.

No posts to display