Two stars responsible for Gemini constellation


In my younger days, I used a simple technique for learning the constellations. Start with an easy one to find and work your way outward from it.

Look to the southeast 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.

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 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 finally end its life cycle.

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. Those facts suggest that we might be seeing what our sun will look like in a few billion years when it stops being a friendly yellowish star and begins to expand into a dying red giant.

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 than 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, British astronomer William Herschel noticed that the fainter of the two stars had shifted slightly in its position with respect to the brighter one. His discovery suggested for the first time that one star could revolve around another one in a true binary system. The faint star takes about 350 years to revolve once around the brighter one.

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.

They didn’t have the same father, however. Both Zeus, the king of the gods, and Leda’s husband, the king of Sparta, visited Leda’s bedchamber on the same night, if you get my drift.

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 the 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?

Among their many dubious accomplishments, 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.

Neptune, the sea god, gave them control over the winds and the waves upon their death. Many ships have borne their names, and ancient 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 lightning bolt. (You’d think he could have done so a few minutes earlier and saved Castor’s life, but nevermind.)

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.

Hidden among the stars of Gemini is a glowing fuzzball of green gas that represents the next stage in the evolution of the star Pollux and our sun as well.

I hardly know what to call it. It was formerly known as the Eskimo Nebula, but now its name is shrouded in controversy.

Humans like to translate unfamiliar experiences into familiar ones. That’s why we see bunnies in puffy clouds and why astronomical objects have names like the Horsehead Nebula and the object formerly known as the Eskimo.

Astronomers are becoming just as sensitive, belatedly, as sports teams about the nicknames they have given to astronomical objects. Just as the Cleveland Indians are changing their name, astronomers are changing the potentially offensive names of astronomical objects.

As a NASA press release puts it, “Eskimo” was a “name forced on indigenous people in the Arctic region by colonial explorers and settlers; it was given to the planetary nebula for the dying star’s passing resemblance to a face within a fuzzy parka.”

An alternative like the “Clown Face Nebula” is considered just as offensive. Referring to it by its catalog designation is, frankly, boring, but that’s what NASA wants us to do.

Thus, I will go with noted telescopist Stephen J O’Meara when he calls it the Lion Nebula. It does indeed resemble a lion’s head seen straight on, shaggy mane and all.

To add to the confusion, the Lion is one of the best examples of a class of astronomical objects called planetary nebulas. Such hazy balls of light don’t have anything to do with planets. They merely look vaguely like planets in small telescopes. (If they looked like bunnies, we would call them rabbitary nebulas.)

They are dying stars that are giving back some of the substance they borrowed from the universe billions of years ago.

The Lion is one of the easiest planetaries to see in a small telescope. A decent set of star maps will have it marked with its catalog number, NGC 2392.

When you look through your telescope, you’ll see that the star is fuzzy and very green. Switch to a higher magnification, and the fuzzy star will have a bluish central star surrounded by a haze.

Look carefully. You will see the soon-to-be history of Pollux and, come to think of it, your own star, the sun, in five or six billion years.

Billions of years ago, the Eskimo Nebula was a fragment of a much larger cloud of hydrogen gas. The gas formed into a spinning ball. When the ball had gathered enough hydrogen, it erupted into the hydrogen-bomb reaction we call a star.

A medium-sized star begins to run out of fuel as large quantities of its hydrogen are fused, deep in its core, into helium and other, heavier elements. The hydrogen-bomb reaction starts to falter. Pressure and temperature at the core become unstable.

As the star cools, the reaction at its center weakens, and the star begins to collapse. That heats the interior, and the star expands to thousands of times its original volume to become a red giant star.

A red giant is inherently unstable. It sheds its outer layer, or “shell,” back out into space. The filaments that form the Lion’s mane are caused by strong stellar winds erupting from the tremulous red giant.

The gassy outer layer continues to expand relatively slowly by stellar standards at tens of miles per second.

The gas in the shell is exceedingly thin — more like a heated vacuum than a cloud. We see it because the dwarf star is still producing energy that irradiates the gas in the shell and causes it to glow. In a million years, it will have dissipated so much that it won’t be visible.

For now, it’s a glorious sight.

By Tom Burns


Tom Burns is the former director of the Perkins Observatory in Delaware.

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