Thursday, June 26, 2008

The Little Dipper

Now that you’re well acquainted with the Big Dipper and can use it to find the North Star, let’s visit its circumpolar companion, the Little Dipper. The Little Dipper is the defining asterism (recognizable star pattern) of the constellation Ursa Minor, the Little Bear. As discussed in an earlier post, circumpolar refers to stars that appear to circle the North Celestial Pole.

In the Northern Hemisphere, we’re fortunate to have a naked-eye star marking the position of our Celestial Pole. That all-important star is Polaris, the North Star. Also known as the Pole Star, the Lodestar, and the Steering Star, Polaris has been used from antiquity through modern times to point the way north. The seafaring Phoenicians may have been the first culture to rely heavily on its nearly fixed position in the sky to aid them in navigation.

I say “nearly fixed” because if you were to observe Polaris over a 24-hour period, you would discover that it appears to move, inscribing a very small circle in the sky. This is because Polaris and the North Celestial Pole are not one and the same. Polaris simply happens to be the star that’s currently closest to the North Celestial Pole, that spot in the sky where the Earth’s axis points. But Polaris does not line up precisely with the Pole. Will this small inconsistency matter when you’re lost in the woods and you’re using that old reliable, the North Star, to find your way out? Of course not!

Now that you’re feeling confident again that Polaris is as solid as the Rock of Gibraltar, I’ve got to pull the celestial rug out from under you. Polaris wasn’t always the North Star, and it won’t continue to be the North Star! You see, our planet may spin reliably on its axis 24/7, like a top, but that top has developed a bit of a wobble.

Precession of Earth’s axis
Image by Robert Simmon, NASA GSFC

The gravitational pull of both the Sun and Moon on planet Earth has caused a phenomenon called precession. Precession is the change in the alignment of Earth’s axis. Because our spinning top is wobbling, our axis doesn’t always point in the same direction. The result is that the position of the North Celestial Pole--where Earth’s axis points--moves over time against the backdrop of the stars, completing a circle in about 26,000 years. The North Celestial Pole will pass closest to Polaris around the year 2100, after which it will begin to move away from it, toward the stars of the constellation Cepheus the King. The good news is, if we wait just 26,000 years, Polaris will be our North Star once more. What goes around, comes around.

Our current Steering Star marks the end of the handle of the Little Dipper. Like the Big Dipper, the Little Dipper has seven stars. But with four of its stars dimmer than the dimmest star in the Big Dipper, it is more challenging to see. Let‘s try to pick an observing site without a lot of light pollution, OK?

1) You’ll need to face north, so if you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your left shoulder to the west, and you’ll be facing approximately north.

2) Wait at least one hour after sunset to begin observing, so that twilight’s over and your sky’s good and dark. During this observing week, the Moon won’t rise to interfere with your stargazing until well after midnight. New Moon, when the Moon doesn‘t appear at all in the night sky, occurs on Wednesday, July 2.

3) Locate the Big Dipper and use the Pointer Stars to locate Polaris. If you need a refresher, review the earlier post.

4) The Little Dipper will be oriented as if it’s balancing on its handle, with Polaris at its southernmost point. Notice the Little Dipper’s size relative to the Big Dipper. Notice also the orientation of the handle and the bowl; they’re reversed from the Big Dipper. Finally, notice the curve of the Little Dipper’s handle relative to its bowl; it is the opposite of the Big Dipper’s handle curve. Noting these differences now may aid you in tracing the Little Dipper asterism when sky conditions aren’t so good.

The Little Dipper
Chart created with
Your Sky

5) After Polaris, the next star in the handle is Yildun (pronounced yill DUNN). Yildun is from the Turkish for star. It’s a fast-spinning but otherwise ordinary white star. The third star in the handle has no traditional name, so we simply call it Epsilon, its star catalog designation. Epsilon (pronounced EPP sill ahn) is a dying star, preparing to swell into a red giant.

6) With the Little Dipper oriented right side up, the top left bowl star is Alifa al Farkadain, a white dwarf star. Alifa al Farkadain is Arabic for the dim one of the two calves. The bottom left bowl star is Anwar al Farkadain, a yellowish-white dwarf. Anwar al Farkadain is Arabic for the bright one of the two calves. The names are a bit confusing, as Anwar is actually the dimmer of the two stars, as well as the dimmest of the Little Dipper’s seven stars.

7) The bottom right and top right bowl stars are Pherkad and Kochab, collectively known as the Guardians of the Pole, because they endlessly circle Polaris counterclockwise. Pherkad (pronounced FIRK uhd) is from the Arabic for two calves. Pherkad is a white giant, the hotter and more luminous of the two Guardians. Kochab (pronounced KOH kabb) is from the Arabic for star. This orange giant star was probably considered the North Star around 1100 BCE, when the North Celestial Pole drew close to it.

It’s comforting to know that, while Polaris shines on as our beloved guiding star, it has two beefy bodyguards who are always on duty, relentlessly sweeping the perimeter in a tight, protective circle.

On thy unaltering blaze
The half-wrecked mariner, his compass lost,
Fixes his steady gaze,
And steers, undoubting, to the friendly coast;
And they who stray in perilous wastes, by night,
Are glad when thou dost shine to guide their footsteps right.

~ from “Hymn to the North Star” by William Cullen Bryant