This week’s column is about an armpit. We’re not talking about some ordinary armpit here. We’re talking the underarm of a giant.

Go out after dark and take a look at the giant. His name is Orion. To the ancient Greeks and Romans, he represented a mighty hunter.

He consists of a large upright rectangle of stars with the three stars of his belt cutting him nearly in half. The upper left star of the rectangle is a red-orange star called Betelgeuse, commonly pronounced “beetle juice.” (Yes, that’s where the character in the old movie got its name.)

To be fair, the association of the star with an armpit is somewhat suspect. It dates back to a magnificent 1899 study of star names called Star Names and Their Meanings by American naturalist Richard Allen. He believed that the modern name is a corruption of a longer Arabic phrase that means “armpit of the giant one.”

The Arabic astronomers deserve the naming right. While western astronomy languished during the medieval period, the Arabs were doing some fine astronomical work. They named many of the naked-eye stars that western astronomers had never named.

However, Paul Kunitzsch, professor of Arabic Studies at the University of Munich, took a look at the original Arabic phrase and concluded that it referred to Orion’s hand and not his underarm.

However, Allen’s appellation is the one that stuck, and that makes sense. The star does indeed seem to mark Orion’s right shoulder. For a casual observer like me, it’s hard to imagine the star representing a hand. Where did the rest of Orion’s body go?

Betelgeuse is a type of star called a red supergiant. Our home star, the sun, is about 900,000 miles in diameter, below average size as stars go. At 770 million miles in diameter, Betelgeuse is so large that it would swallow up our Earth and extend out to the orbit of the planet Jupiter.

Betelgeuse contains 10-20 times the mass of our sun. However, as its large volume implies, those gases are extremely spread out. Out near its edge, its temperature hovers at about 6,000 degrees Fahrenheit. In effect, the star is a “red-hot vacuum,” as some astronomers have called it. Betelgeuse’s motto is “live fast, die young and leave a beautiful supernova.” It is only 9 or so million years old, yet it already has reached the end of its life cycle. By comparison, our sun may last up to 10 billion years by slowly and steadily converting hydrogen to helium in a thermonuclear reaction. The sun has been around for 5 billion years and it is just reaching a stable middle age.

How did Betelgeuse reach its sorry state? Like all stars, it started its life as a loose globe of hydrogen gas and dust. Gravity brought that “star stuff” closer together, with a subsequent increase in heat. When it reached 10 million degrees or so at its hellish center, it began to convert its hydrogen into helium in a hydrogen-bomb reaction that makes our attempts at such weapons look pathetic by comparison. At that stage, it was a massive blue-white giant, like Rigel, the bottom right star in Orion’s rectangle. It consumed its hydrogen 10,000 times faster than our more conservative sun.

Great size and brightness translate into a short life span for stars. After only a few million years, its center was composed almost entirely of helium. Over the next 10,000 years, it contracted even more and released incredible amounts of energy. The outer layers of gas surrounding the core became much larger, causing Betelgeuse to turn red, probably from embarrassment. The helium at its core fused into carbon, and heavier elements like calcium and iron were formed. But that’s not the worst of it. In less 100,000 years, Orion’s underarm will be destroyed in a spectacular stellar fireworks display that will light up the night.

Fairly soon, Betelgeuse’s center will be made up of almost entirely of gaseous iron, causing it to contract further. Its gravitational force will become so great that its outer envelope of gases will be forced down upon it, causing a massive explosion, called a supernova. When we see such explosions in other galaxies besides our Milky Way, the outburst is so great that a single star can briefly outshine an entire galaxy of 300 billion stars. Because Betelgeuse is so close to us (640 light-years, or a mere 3,600 trillion miles), such an explosion may turn night into day for weeks or days. During the day, it will be easily visible to the unaided eye.

Such naked-eye supernovas are exceedingly rare. The last one observed was in 1604 by Johannes Kepler, one of the greatest astronomers of any age. His careful study of the event led it to be called Kepler’s Star. After its bright outburst, it disappeared from the sky. It was only in 1941 that astronomers, using the powerful telescope on Mount Wilson, found the faint remnant of the explosion as a faint ball of expanding gas.

Betelgeuse is destined for the same fate. Afterward, it will probably disappear from the night sky, leaving Orion without a crucial part of his anatomy. Future generations may look at the night sky that humans have looked at since the inception of our species, but they will see a different sky.

Lately, Betelgeuse has been acting strangely, which gives some hope to astronomical thrill-seekers that Betelgeuse is about to pop.

Since October, observers have noticed that the star has dimmed considerably from its former brightness. Astronomers have been taking accurate measurements using an electronic device called a photometer for about 100 years.

Recently astronomers at Villanova University recorded a record low brightness. Remarkably, it has dimmed from the tenth brightest star in the sky to the 21st as of the middle of December. That dimness could be a sign that Betelgeuse has reached the end of its short life.

It’s supernova time! Or maybe not. Betelgeuse got almost as faint in 1926. Betelgeuse belongs to a class of astronomical objects called variable stars. The giant’s armpit goes through a regular cycle of brightening and dimming over a period of about six years.

Astronomers are unsure about the cause of Betelgeuse’s shorter-term variability. Some astronomers speculate that bubbles of gas rise to the surface from the interior of the star. Such stellar churning changes the mix of hotter and cooler gases and makes the star appear brighter or dimmer.

Red-supergiant stars measure their lives in millions of years, not billions of years like our sun.

In fact, the end may be near. Such red supergiants, as they are called, could pop at any time — in perhaps a few hundred or thousand years but probably not more than 100,000.

Astronerds like me are fond of saying that Betelgeuse may already be gone. Betelgeuse is relatively close to us at about 640 light-years away. That means the light from the star you see now has been in transit for 640 years. If Betelgeuse exploded any time in the interim, it is now gone, but we won’t find out until the information in the form of light reaches us.

Then again, probably not. Betelgeuse could explode tomorrow, but the chance of it happening during our much briefer human lives is still exceedingly small.

After all, we humans occupy a thin slice of cosmic history. The chance that anything really interesting will happen in our cosmic neighborhood during our particular lifetimes is, well, astronomically small.

Still, every clear night, I glance up at Orion. You never know.