Eye Astronomy #11: The Dragon

Around 9:00 PM to 10:00 PM in late May and early June, we at mid-northern latitudes can look almost straight up out of our galaxy. The densest parts of the Milky Way—its spiral arms and central bulge—then encircle us roughly along the horizon, giving us a clear view into extragalactic space. Amateur astronomers often train their telescopes on distant galaxies at this time of year.

Even with just your eyes, you can tell the difference between this “up and out” view and the view into the plane of our galaxy. When the band of the Milky Way rides high in the sky, we see large numbers of stars scattered through and alongside it. When it lies low in the sky, fewer stars shine overhead. The circumpolar constellations—those close enough to the north celestial pole to stay above the horizon all the time—are a case in point. With only two exceptions, they are dim and sparsely populated.

Cassiopeia, the Queen, is one exception. A bright, compact “W” or “M” of stars, she lies right in the Milky Way. (Right now she’s just barely above the northern horizon.) Ursa Major is the other exception. It contains the Big Dipper, a bright asterism of seven stars.

As for the rest…eh. Cephus, the King, can be a touch hard to find under suburban light pollution. Ursa Minor—home of the Little Dipper—is faint except for Polaris itself and the pair of stars forming the end of the dipper’s bowl. You’ll only see all seven of its stars under reasonably dark skies. Lynx—which is, yes, a lynx—is  all but invisible unless you escape to a rural area. And then there’s Camelopardalis, the giraffe, one of the faintest constellations in the sky. I don’t think I’ve ever seen a single star in that beast.

So yes, most of these constellations are unobtrusive. But there’s one edge case: Draco, the Dragon.

The circumpolar constellations, including Draco, the dragon, with Thuban circled in red.

Draco’s head lies close to Vega, the brightest summer star and the fifth brightest in the whole sky. Two of the stars in Draco’s head are second magnitude, similar in brightness to Polaris, while one is third magnitude. The remaining star is almost fifth magnitude, which can render it invisible depending on your local light pollution. Even so, it’s easy to locate the dragon’s maw. As it should be!

Draco’s body winds about, dipping toward Cephus and looping back around the bowl of the Little Dipper until its tail whips between the Little and Big Dippers. Most of the dragon isn’t that bright, but it’s bright enough to trace it’s general path if you look carefully. You might wonder, though, why you should care. It’s not that impressive.

There are, in fact, a couple of significant things about this constellation. Draco is among the oldest constellations on the books. Some constellations are very new, as you can tell by their names. The southern constellation Microscopium is one example. Others were introduced a few hundred years ago as astronomers systematized the study of the night sky. Remember that giraffe, Camelopardalis? It was introduced in 1612 by Dutch astronomer Petrus Plancius. But Draco appears in Ptolemy’s catalogue of stars, which you might know by its Arabic name, Almagest. That dragon has been flying among the stars for at least 2,000 years, maybe more.

Another interesting fact: Thuban, the star circled in red in the image, was once the pole star. Over the long periods of time, the Earth wobbles on its axis like a top, taking 26,000 years to complete one cycle. This causes the poles to wander through the stars in a circle. From roughly 3942 BCE to 1793 BCE, Thuban was the closest reasonably bright star to the north celestial pole. In the distant future—around A.D. 13,727, Vega will have that distinction, as it previously did around 12,000 BCE. And eventually, as the Earth wobbles on, Thuban will again become the pole star.

One final Thuban curiosity: it’s referred to as alpha Draconis. In 1603, German Astronomer Johann Bayer developed a system for identifying stars based on their home constellation. He assigned the brightest stars in each constellation a Greek letter, starting with alpha. Generally, alpha is the brightest in a constellation, beta the second brightest, and so on. Not that Bayer always got it right. Technology for accurately determining star brightnesses didn’t exist back then. Also, sometimes he used slightly different criteria in assigning Greek letters. So it’s not too surprising that alpha Draconis isn’t really the brightest star in Draco.

But Thuban isn’t even close. Seven of Draco’s fifteen brightest stars are brighter. So what gives? To be honest, I don’t know. The information I’ve found on the subject is a bit jumbled. One suggestion is that mistranslations during the Middle Ages led to confusion regarding which star in Draco was which, confusion only resolved after Bayer assigned his designations.

In any case, it may be fitting that the unimpressive Thuban is known as alpha Draconis. After all, it once was the pivot of the northern sky, something few stars can claim. And some day, it will be again.