Earlier this month, we spoke of Ursa Major, the Big Bear, so this week we take a look at the Little Bear, Ursa Minor. Astronomy neophytes sometimes mistake the Pleiades star cluster for the Little Dipper because the brightest Pleiades stars resemble a tiny, skewed dipper.
But in reality, most people have never seen the Little Dipper, because most of its stars are too dim to be seen through light-polluted skies.
The seven stars from which we derive a bear are also known as the Little Dipper. Polaris, the North Star, lies at the end of the handle of the Little Dipper, whose stars are rather faint. Its four faintest stars can be blotted out with very little moonlight or street lighting. The best way to find your way to Polaris is to use the so-called “Pointer” stars in the bowl of the Big Dipper, Dubhe and Merak. Just draw a line, between these two stars and prolong it about 5 times, and you will eventually arrive in the vicinity of Polaris.
Exactly where you see Polaris in your northern sky depends on your latitude. From Minneapolis, it stands halfway from the horizon to the overhead point (called the zenith). At the North Pole, you would find it directly overhead. At the equator, Polaris would appear to sit right on the horizon.
As you travel to the north, the North Star climbs progressively higher the farther north you go. When you head south, the star drops lower and ultimately disappears once you cross the equator and head into the Southern Hemisphere.
Celestial sentinel
Aside from the North Star the two stars at the front of the Little Dipper’s bowl are the only ones readily seen. These two are often referred to as the “Guardians of the Pole” because they appear to march around Polaris like sentries; the nearest of the bright stars to the celestial pole except for Polaris itself. Columbus mentioned these stars in the log of his famous journey across the ocean and many other navigators have found them useful in measuring the hour of the night and their place upon the sea.
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The brightest Guardian is Kochab, a second magnitude star with an orange hue. The other Guardian goes by an old Arabian name, Pherkad — the “Dim One of the Two Calves.” Pherkad is indeed dimmer than Kochab, shining at third magnitude. The two other stars that complete the pattern of the bowl of the Little Dipper are of fourth and fifth magnitude.
Thus, the bowl of the Little Dipper, which is visible at any hour on any night of the year from most localities in the Northern Hemisphere, can serve as an indicator for rating just how dark and clear your night sky really is. If, for example, you can readily see all four stars in the bowl, you’ve got yourself a good-to-excellent sky. Unfortunately, thanks to the spread of light pollution in recent years, only the Guardians are usually visible from most city and suburban sites, meaning the quality of the sky would rank fair-to-poor.
Interestingly, the Big and Little Dippers are arranged so that when one is upright, the other is upside down. In addition, their handles appear to extend in opposite directions. Of course, the Big Dipper is by far the brighter of the two, appearing as a long-handled pan, while the Little Dipper resembles a dim ladle.
A supergiant, yet not the brightest
Polaris is actually a triple star system; the primary star is a yellow supergiant 446 light years away, five times as massive, 46 times larger and nearly 1,300 times as luminous as our sun. There is a popular misconception in which many believe that the North Star is the brightest star in the sky. Yet, at a magnitude of +1.98, it actually ranks only 47th in brightness. This ranking can change by one or two places, because Polaris is a Cepheid variable star whose brightness can fluctuate by roughly 0.1 magnitude over an interval of about 4-days.
Polaris remains in very nearly the same spot in the sky year-round while the other stars circle around it. Only the apparent width of about 1.5 full moons separates Polaris from the pivot point directly in the north around which the stars go daily.
However, on account of the wobble of the Earth’s axis (called precession), the celestial pole shifts as the centuries go by. Polaris is actually still drawing closer to the pole and on March 24, 2100, it will be as close to it as it ever will come, just 27.15 arc-minutes or slightly less than the moon’s apparent diameter. Since it takes 25,800-years for the Earth’s axis to complete a single wobble, different stars have become the North Star at different times. In fact, the brightest Guardian, Kochab, was the North Star around the time of the start of the Iron Age, around 1200 B.C.
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Joe Rao serves as an instructor and guest lecturer at New York’s Hayden Planetarium. He writes about astronomy for Natural History magazine, Sky and Telescope and other publications.