Look southwest just after dark, 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 two stars, not one as it seems to the naked eye.

The two stars of Castor, called Castor A and Castor B, are similar in brightness, but careful examination will show that one is slightly brighter than the other.

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 in a gravitationally locked binary system. It takes about 350 years for the faint star to circle once around the brighter one.

As it turns out, Castor is composed of six stars and not two. A third star called Castor C is supposedly visible in large telescopes, but I’ve never seen it.

Each of the three stars is, in turn, a double star. The elements of Castors A and B orbit each other in a startlingly short period of nine and three days, respectively. But they are pikers compared to the two stars of Castor C, which travel around each other in less than a day.

Castor C travels around Castors A and B, but it takes thousands of years to make it once around.

Most stars in the sky are multiple-star systems like Castor. Our lonely, one-star sun is the exception, not the rule.

If the universe is beginning to sound like the Teacups ride at the state fair, that’s because it is.

Pollux is similar to our sun in some ways. Both are single stars. Pollux is a bit cooler and a bit larger than the sun. Those conditions suggest that we might be seeing what our sun will look like in a few billion years when it stops being a friendly yellow star and begins to die.

The ancients thought Castor and Pollux were equally bright, but even the untrained eye can see that Pollux is a bit brighter. Stars measure their lifespans in billions of years, so it’s unusual to see such a significant change in only a few thousand years.

According to Chet Raymo’s “365 Starry Nights,” 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 begin to kick the cosmic bucket.

To have two stars of such similar brightness so close together in the sky is somewhat unusual. Thus, the two stars in question have been significant to our forebears since early recorded history.

For evidence of their first historical appearance, we must look to the Babylonian boundary stones, or kudurru, which marked gifts of land by Babylonian kings between the 16th and 12th centuries BCE.

According to early-20th century astronomer E. W. Maunder, the first recorded appearance of the twin stars occurred on kudurru and Babylonian monuments. The same symbols appear repeatedly — a crescent moon lying on its back with two stars, presumably Castor and Pollux, above it.

Maunder speculates that the Kudurru record an old cultural and astronomical memory.

These days, we look for the full moon immediately following the spring equinox. We use that full moon to determine the dates of religious celebrations like Easter and Passover.

Since ancient times, humans have also looked for the first crescent moon — the “new” moon — after the spring equinox. In 6000 BCE, the first visible moon after the equinox appeared near the twin stars of Gemini.

According to calendar scholar E. M. Plunket, the ancients created twin constellation figures out of the twin stars. Of course, they recognized the similarity of brightness between the two stars. Most importantly, the twin figures symbolized the equinox’s equal day and night.

That recognition comes down to us as the famous mythological twins of the ancient Greeks and Romans. Both cultures worshiped the twins by offering sacrificial lambs at temples constructed in their honor.

Here’s the story. Castor and Pollux were the twin sons of Leda, the queen of Sparta.

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 liaisons.

Later that night, her husband, the king of Sparta, also visited Leda. 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 was a mortal man, but Pollux was an immortal god.

What got them their very own hunk of the sky was that they deeply loved each other, as twins frequently do. They looked alike and even dressed alike.

They were, in fact, inseparable and thus shared many heroic adventures. For example, they participated in the attack against Athens to rescue their half-sister, the beautiful Helen, who Theseus, the mythical king and founder of Athens, had abducted.

The twins sailed with Jason and the Argonauts to steal the golden fleece. They saved Jason’s crew several times and, as a result, became the patron saints of ancient sailors.

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?

Upon their death (more on that later), 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.

That nautical connection is reinforced by the Twins’ association with an odd electrical phenomenon called Saint Elmo’s fire. As first-century Roman historian Pliny wrote, “On a voyage, stars alight on the yards and other parts of the ship. If there are two of them, they denote safety and a successful voyage. For this reason, they are called Castor and Pollux, and people pray to them as gods for aid at sea.”

Their most significant battle was their last. Castor and Pollux got into 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 lightning 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 and 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. They spend part of their time in the underworld and part in Olympus for all eternity.

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.