Stargazing: The stars of Orion’s belt


By Tom Burns - Stargazing



I have a problem writing this week’s column. I want to write about the belt stars of the constellation Orion, but I’m not sure how to word their geometric relationship with each other.

The issue has to do with a peculiar difference in the way we refer to queuing in America. Say you’re waiting to get onto a ride at the Ohio State Fair. The group of people you are waiting with is commonly called a “line.” I might call the queuing situation standing “in line,” but I frequently hear it called standing “on line.”

The latter description makes absolutely no sense to me. It assumes that there is some sort of line on the ground and people are standing on it. I have looked on several occasions and the line in question appears to be completely imaginary. In fact, the humans standing there actually constitute the line. They are therefore in it and not on it.

If all this sounds silly (and it is, of course), consider the way NASA refers to satellites they have launched into orbit around Earth. Their spokespeople always say that the satellite is “on orbit” as it there is some tangible line the object is traveling on. However, the satellite forms the orbit as it goes. Thus, it is in orbit, thank you very much.

Those of you who are reading this piece online (ha!) should feel free to comment on this issue.

In the meantime, allow me to say that it’s hard to miss the three belt stars of Orion. They are of about equal brightness in an almost equally spaced line.

Look for them — and the rest of Orion — low in the southeast just after dark. The Hunter is tilted on his side so that his belt is more vertical than horizontal.

The three belt stars are among the most famous in the nighttime sky. Many cultures throughout history and around the globe have their own special and distinct names for what we call Orion’s belt.

Among the most notable literary references appears in the Bible’s Book of Job.

Job is upset with God because he has tried mightily to follow God’s law, and all he’s gotten from it is pain and poverty. There seems to be no justice in the world. In response, God asks Job, “Canst thou bind the sweet influence of the Pleiades, or loose the bands of Orion’s Belt?”

Of course, Job can’t, but that doesn’t mean that the “bands” aren’t coming loose already, as we shall see.

The top star of the belt is called Mintaka, an Arabic word that means “belt.” It is the closest of the three stars at 1,500 light years away. At six trillion miles per light year, that’s pretty distant as far as bright, naked-eye stars go. Its brightness in the sky is caused by its phenomenal energy output. Mintaka is a hot, young, blue star producing 20,000 times more energy than our daystar, the sun.

But Mintaka is a piker compared to the middle star of the belt, a ferocious energy-producer called Alnilam, the “String of Pearls.” Alnilam emits 40,000 times more energy than the sun and is one of the hottest stars known at about 80,000 degrees Fahrenheit. (Compare that to the sun’s rather chilly 10,000 degrees.) It is also the belt’s farthest star at 1,630 light years away.

The belt’s bottom star is named Alnitak, “The Girdle.” Its energy production is about midway between the other two stars at 35,000 suns. It is a bit closer to us than Alnilam at 1,600 light years.

In fact, the belt stars are not in a line at all ­ — or on one either. The three stars look bound together like the holes in a belt, but that appearance is an illusion based on our Earthly point of view, which is no more important than the point of view from any place in our Milky Way galaxy of 300 billion stars.

Note the distances to the stars from Earth I listed above. The two stars closest to each other, Alnilam and Alnitak, are at least 30 light years away from each other. Compare that with the distance of our own sun with Sirius, the closest star to it. Sirius is a scant 8.6 light years away from the sun, and the two stars can hardly be called “bound,” gravitationally or otherwise.

Orion’s girdle is made up of stars that are placed at incredible distances from each other. If you stood on a planet orbiting Alnilam, you would see the other two stars on opposite sides of the sky.

And the stars are “unbinding” even further. As you read these words, they are moving away from each other at ungodly velocities. In the far distant future, if humans survive to look at the heavens, Orion’s belt will have disappeared, its stars scattered to the far ends of the great deep.

As for the on and in thing, it turns out to be a function of regional dialects. New Yorkers and, apparently, NASA spokespeople tend to use on instead of the more logical in. But nobody every claimed that language was logical in the first place. If you don’t believe me, check it out in-, er, on-line.

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By Tom Burns

Stargazing

Tom Burns is director of the Perkins Observatory in Delaware.

Tom Burns is director of the Perkins Observatory in Delaware.

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