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 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 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 argument makes good sense. 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.

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 640 million miles in diameter, Betelgeuse is so large that it would swallow up our Earth and extend out to the orbit of the next planet, Mars.

Betelgeuse’s motto is “live fast, die young and leave a beautiful supernova.” It is only 100 million years old, yet it already has reached the end of its life cycle.

By comparison, our own 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 this “star stuff” closer together, with a subsequent increase in heat. When it reached 10 million degrees or so, it began to convert its hydrogen into helium in a hydrogen-bomb reaction that makes our own 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 2 million years or so, 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 was burned into carbon, and heavier elements like calcium and iron were formed.

But that’s not the worst of it. In less than one million years, Orion’s underarm will be destroyed in a spectacular way.

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, for a period of weeks or days, turn night into day.

Afterward, Betelgeuse 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.

In fact, Betelgeuse may already be gone. Such red supergiants, as they are called, could pop at any time. 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. We humans occupy a thin slice of cosmic history. The chance that anything really interesting will happen in our cosmic neighborhood during our individual lifetimes is, well, astronomically small.

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