The Winter Circle

Hey, sky fans. Thanks for tuning in.

With December behind us, and the long, cold nights ahead until the crocuses (croci?) start to pop up, it might seem like there’s nothing but bleak bleakness between now and the start of spring. Well, ok, if you’re are a fan of thick, dark beers, like I am, then we have that, too.

What would you say if I told you that just by taking out the trash tonight, you can perk yourself up by taking a look at an easy-to-find and enormous group of stars? A group sixty degrees high, and fifty degrees across. That’s six fists-at-arm’s-length by five! An asterism that covers six constellations, an open cluster, double stars, pulsating variables stars, and six of the 18 brightest stars in the sky other than our own sun? Great, right? But, wait, there’s more. If you include stuff within and nearby, you get another open cluster, the closest star-forming area to the earth, and one of the most famous constellations of them all! Where do I sign?!

This is a good time to explain the difference between an asterism and a constellation. An asterism is group of stars that forms some kind of recognizable pattern in the sky. These are the patterns that ancient people saw, which eventually helped to define the official constellations. Asterims can be parts of single constellations, like the Big Dipper or Orion’s belt, or they can span more than one constellation, as the the Summer Triangle and Winter Circle do. A constellation, on the other hand, is one of 88 areas of the sky that are defined and used by astronomers to identify where things are. They border each other like states or countries do. These 88 official constellations cover the entire sky. The Big Dipper is an asterism within the constellation Ursa Major, and Orion’s belt is part of the constellation Orion, for example.

This time of year, the earth faces a part of the galaxy with more bright objects than it faces at other times of the year. Have a look at this screenshot, which I whipped up in Stellarium. It shows the sky around 9:00 tonight to the northeast, over the houses across the street from mine. I had to jog it a little to get it to fit.

winter circle
The Winter Circle as seen around 9pm from the northeast of the US (image from Stellarium)

Just over to the right of center are the three stars of Orion’s belt, oriented more or less vertically. The orange giant Betelgeuse, which is Orion’s left shoulder, is almost smack in the center of the scene. Running around the outside, you’ll see six highlighted stars. Those six stars: Capella (in the constellation Auriga); Pollux (Gemini); Procyon (Canis Minor); Sirius (Canis Major); Rigel (Orion); and Aldebaran (Taurus) form the asterism called the Winter Circle. I like to include Castor, which is unlabeled on the very left-most edge of the screen, above Pollux. See if you can try to remember them all. I’ll quiz you in July. Capella, Castor, Pollux, Procyon, Sirius, Rigel, Aldebaran — two Cs, two Ps, an S, an R, and an A.

These stars are all pretty close to us as these things go. Other than Rigel, the furthest of the bunch is Aldebaran, which I’ve written about before and will again later this month. It’s only 65 light years away (390 trillion miles at 6 trillion miles to the light year). Sirius, which is the brightest star in the entire night sky, is the nearest of the bunch; just over eight light years away.

And, what of Rigel? With the constellation Orion more or less enveloped entirely by the circle, Rigel seems like a far-flung outlier. Its light has spent more than 800 years trying to get to your eye! What you’re looking at is actually three stars, with the biggest of them all kicking out over 100,000 suns’ worth of light. While it’s by far the most distant of the Winter Circle’s stars, it’s one of the nearer of Orion’s main stars, which all sit close to the circle. This shows you what kind of depth we’re talking about when we look up. For the most part, these stars are completely unrelated and extremely far apart, but just appear to be together thanks to our view from Earth and years upon years of evolving to have great pattern-recognition skills.

Did I say something about clusters? Back to Aldebaran for a minute. It’s at the top of a V-shaped asterism called the Hyades, which is actually the nearest open star cluster to earth, only 153 light years away. Aldebaran isn’t actually part of the cluster. It’s less than half as far away, but happens to be right in the light of sight between us and them. Don’t worry, I didn’t forget the Pleiades cluster, the tiny dipper-looking thing just past the Hyades. As far as star-forming regions, you’ll have to wait until I write about Orion. Soon, soon.

This group of stars stretches from almost completely overhead down to a good and comfortable distance above the horizon, perfect to put Orion in prime viewing. Every time I see it, I’m struck by how bright these stars are, and how much of what I can see it takes over. It’s really an inspiring sight, especially if you take yourself outside, go through the whole ordeal of bundling up, and stand there in the cold. The payoff is remarkable, and there are few things in the sky I’d rather look at, or write about. If you can, step outside. You won’t regret having a look.

Clear skies, everyone!


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