Getting to Know Orion: the Belt

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‘Tis the season when the world’s most well-known constellation makes its reappearance in the night sky. I’m talking, of course, about the group of stars officially classified as the constellation Orion the Hunter, which these days is fully above the eastern horizon about an hour after sunset as it begins its reign over the winter sky. The brightness of its stars and the unusual regularity of its pattern make this, in my experience through all my years in the Planetarium, the constellation the average non-space nerd is most likely to know the name of and be able to spot in the wild.

This region of the sky has so many famous astronomical things found within it that what was originally going to be a single blog post focusing on this most famous of constellations will, instead be a sort of miniseries of a few posts, each one focusing on a different aspect of the area of the sky known as Orion. And we’re going to start off with what is arguably the most famous part: Orion’s Belt. So join me as we jump into the first part of our Orion deep dive miniseries!

Ducks…er, Stars…All in a Row

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Orion’s Belt is, of course, not a constellation itself, for all its fame. It’s the central piece of Orion, meant to represent the waistline of the hunter of Greco-Roman mythology. It’s what we call an asterism, a distinct and well-known star pattern that is not actually an official constellation (see also: the Big Dipper or the Summer Triangle).

The Belt’s fame comes largely from the ability of the human eye to easily pick it out in the sky. It’s three bright stars that appear to be almost exactly the same brightness almost perfectly evenly spaced in an almost perfectly straight line—none of which is actually true (more on that in a moment), but that’s what it looks like from the angle we’re looking at those stars from here on Earth.

The evenness of this pattern sticks out amongst the loose riot of stars one can see in a dark sky, and the brightness of the three stars means it’s a pattern that can be spotted (perhaps with some difficulty) even in the light-polluted skies of a city. As such it’s among the easiest star patterns to pick out. But things are a lot less nice and neat in the Belt than they look.

Not So in a Row

The thing with constellations is that they are 2D images—flat figures made of connect-the-dot line drawings. But the universe isn’t 2D and all those stars with lines connecting them are actually all in very different spots in space. Two stars that look like they’re near each other from our Earthly point of view may be many light years apart in reality. And there are few more obvious examples of how perspective can mess with your perception of space than the Belt.

First let’s meet the three stars of the Belt: up first, lowest and farthest east (so the left side of the belt) is Alnitak. Next is the middle star, Alnilam. And finally, the highest up and farthest west (the right side of the belt) is Mintaka.  Remember just a few paragraphs ago when I said these stars, which look so similar and evenly spaced, are not at all? Yeah, these three stars are very different.

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For one thing, while the end stars Alnitak and Mintaka are similar distances away from us (740 and 690 light years, respectively, and what’s 50 light years more or less?) Alnilam is much farther. Exactly how far is unknown, but could be as much as 2,000 light years away. The apparent evenness of their spacing is entirely an artifact of our Earthly viewing angle—they’re nowhere near each other. 

Then, of course, there’s the fact that they look so similar to each other in Earth’s sky. Indeed, their apparent magnitudes (a measurement of how bright they appear, as opposed to absolute magnitude which is a measurement of how bright they actually are) are very close to each other: going again left to right Alnitak’s apparent magnitude is 1.74, Alnilam’s is 1.69, and Mintaka’s is 2.25, making Alnilam the brightest of the three, but not by much.

But I just told you that Alnilam is three times farther away from us than the other two! Yet it still looks the brightest. That should give you an idea that Alnilam is a brighter beast than the other two. And that’s not the only major difference between them. Let’s get to know them as individuals.

Seeing Stars

Okay, when I said get to know them as “individuals” I was not being entirely honest, because Alnitak is not one star. It’s actually a trinary, three stars, and all three are big and bright. If you’ve read this blog’s past post about how stellar classification works, you know that the biggest, brightest stars are O-types, followed by B-types. Alnitak is one O-type (Alnitak Aa), which has one B-type close-in companion (Alnitak Ab), while another B-type orbits the two of them (Alnitak B).

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That might sound complicated, but it’s got nothing on Mintaka, which is a five-star system.  The complicated nesting nature of the orbits among these five stars also leads to an even more confusing naming system, so buckle up.

The biggest, brightest star in the Mintaka system is an O-type star named Mintaka Aa1. This star is very closely orbiting with a B-type companion, with the two dancing around each other once every 5.7 days. This companion is Mintaka Aa2. Going around these two at a much greater distance is another B-type star, Mintaka Ab. Together this triplet makes up the component of this system known as Mintaka A.

Then there’s Mintaka C, which is a binary pair, a B-type (Mintaka Ca) and A-type (next class down in size and brightness, known as Mintaka Cb) orbiting each other every 30 days. The Mintaka C pair is as far as a quarter of a light year from the Mintaka A triplet, but they are gravitationally bound together, with the C pair orbiting the A triplet. That makes them part of the same star system.

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What happened to Mintaka B, you ask? Well, there is a star that was given that designation, but it now looks like it’s probably not actually gravitationally part of this complex system, and just happens to appear to lie near it from our Earthly viewpoint (I’m telling you, perspective is everything).

And what about Alnilam? That appears to be simply one super-honkin’ bright B-type star. Notably, though the primaries in both Alnitak and Mintaka are O-types, which typically burn hotter and brighter than B-types, Alnilam is significantly brighter than either of the other Belt star systems.

This is because Alnilam is at a special stage in a large star’s life right before it reaches the dying stage. Shortly before beginning to swell into a red supergiant, large stars get even hotter and brighter, allowing the more distant Alnilam to (briefly, at least when talking about astronomical time scales) outshine the closer stars on either end of the Belt, despite being of a less powerful class.

When Alnilam finally does cross the threshold into its final red supergiant phase it will get more luminous still, and may rival or even surpass Orion’s current brightest star, Betelgeuse, for brilliance in the sky.

Other Belt Treats

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While it’s the arrangements of the stars that make Orion’s Belt such a famous asterism, there are some undeniable treats awaiting anyone who can peruse this region of the constellation with a telescope. In fact, two of the sky’s most famous nebulas can be found here!

No, I’m not talking about the Orion Nebula. That’s going to get its own post in my little Orion miniseries. I’m talking about the Horsehead and Flame Nebulas. These two cloudy regions bracket Alnitak, with the Flame Nebula above and to the east (left) of the star and the Horsehead below it.

These two regions are each stunning in very different ways. The Flame looks like a tree on fire and is well named, but of course the Horsehead is even more perfectly labeled. Consisting of a dark cloud silhouetted against brightly lit background clouds of hydrogen, it distinctly resembles the proudly arched head and neck of a horse.

Diamonds on a String

I’m willing to bet that you don’t need much help from me in finding Orion’s Belt in the sky. At this time of the year, January, just look eastward (roughly opposite where the Sun sets) after sunset and seek out the even line of three bright diamond lights. It is, perhaps, appropriate that the name Alnilam derives from an Arabic term referring to a string of pearls. It is an apt metaphor.

Of course, while the Belt is the most famous and easiest to spot part of Orion, there’s a great deal more of this constellation worth calling out. But that will have to wait for future posts. Keep your eyes open (and remember to always look up)!