If your birthday falls around this time of year, namely May 21 through June 21, you are, according to the western zodiac, a Gemini. As a birthday present I’m going to present a star in the Gemini constellation. The star is named Castor.
First we’ll find it. Then we’ll unwrap it. And, guess what? There’s a surprise inside.
The birthday candle is burning short on Castor’s cake. Castor sets early these days, about three hours after sunset. And in a few weeks it will be gone for the season, so don’t come whining to me if you miss it.
Castor is roughly 300 trillion miles away. Its light takes 52 years to reach Earth. On the scale of the universe, this is next-door-neighbor close.
Fortunately, Castor is now in the west so it’s in the most viewable slice of the sky for most Saipan residents.
Here’s how to find Castor: If you wait until one hour after sunset this weekend, you’ll notice, looking due west of Saipan, a bright star roughly 30 degrees over the horizon. This star is named Procyon.
How do you measure 30 degrees? By using your fist: It’s a built-in degree measurer. If you hold a fist at arm’s length like you’re presenting someone the Olympic torch, your fist will cover 10 degrees of vertical arc. So if you walk it up from the horizon three times, you’ll be sighting 30 degrees over the horizon.
Once you’ve found Procyon, you can scan horizontally by 20 degrees (two fists) directly to the right, maintaining the same distance over the horizon. This will land you on a really bright object. This object is brighter than Procyon. It’s the planet Jupiter.
Jupiter, like all planets, is just passing through any given area of the sky. It’s not a permanent fixture. So, while the stars maintain essentially fixed positions relative to each other, at least over the human lifespan scale of things, the planets aren’t as well behaved. Therefore, you can’t always use Jupiter to find Castor. We’re just enjoying a chance alignment right now.
Anyway, it’s time to get diagonal now. Let’s slant up and to the right of Jupiter by one fist. This will land us near two bright stars, though they’re not as bright as Jupiter or even as bright as Procyon. These two stars are the Gemini twins. The brighter of them is Pollux, which is a little bit yellow or orange, and to the right of Pollux, and a little bit lower in the sky, is Castor.
Like many textual descriptions, this sounds like gobbledygook until you try it. Then it’s easy.
Easy, yes. Foolproof, no: Let’s not forget that it’s predicated on one hour after sunset. This region of the sky is setting, so if you’re late to the party, you’ll have to adjust your sights lower.
Anyway, now that we’ve got our hands on Castor, let’s unwrap it.
And, surprise, surprise! Castor isn’t really “a” star. No, it’s actually six stars and they’re doing a wacky cosmic dance.
Castor’s six-star array consists of three pairs of stars. Each pair is like a married couple: They are gravitationally bound to their counterpart. Furthermore, all the pairs are also gravitationally bound to the other pairs, so these stellar couples are doing the do-si-do around the dance floor, with little orbits spinning inside of bigger orbits.
Maybe “do-si-do” is a dated concept. So for you younger readers, I’ll note that it’s sort of like twerking, only different.
Anyway, marriages among paired stars, known as “binary” stars, aren’t always marriages of equals. One star is often brighter than its companion. Although they are presumably created at the same time and from the same blob of stuff (e.g. hydrogen), they often have nothing else in common. Some are big, some are small. Some are cooler, some are hotter. Some will burn out quickly, while others will become cosmic geezers.
Yes, there are a lot of odd couples up there. Physics, like Cupid, has a droll sense of humor.
Although Castor looks like just one star to the bare eyeball, an amateur telescope can see the brightest of each stellar pair. In this case, of these three visible stars, two are bright and very close to each other. The third is sort of an outcast, dimmer and distant, the true odd-man-out in an already odd arrangement. This weird scene is a cool sight.
But far cooler is simply knowing Castor’s story. Nobody needs a telescope for that, just as nobody needs a telescope to appreciate the night sky in general. The sky is, in fact, full of interesting stories like Castor’s. There is no price of admission for spectators. This is a present we can all enjoy, even if it’s not our birthday.