The sad and solemn night
Hath yet her multitude of cheerful fires;
The glorious host of light
Walk the dark hemisphere till she retires....
And thou dost see them rise,
Star of the Pole! and thou dost see them set.
Alone, in thy cold skies,
Thou keep'st thy old unmoving station yet.
Hymn to the North Star
--William Cullen Bryant
DIM THOUGH IT MAY BE AT 2ND magnitude, the North Star is the most famous in the sky. Polaris stands essentially still above your landscape all night and all year (assuming you're in the Northern Hemisphere), holding its station while all others revolve around it. Polaris marks not only true north (to better than 3/4 [degrees]) but also your latitude on Earth--which nearly equals Polaris's altitude above your horizon.
Observationally, Polaris has more to offer us than marking the way north. A small telescope at low power will show that it's part of a 40' circlet of mostly 6th- through 9th-magnitude stars extending in the direction toward Perseus. This asterism is sometimes called the Engagement Ring, with Polaris as its sparkling diamond. The Engagement Ring seems to have suffered some rough treatment, however, being oval and badly dented on its south side.
Polaris itself is a double star; the 2nd-magnitude primary has a 9th-magnitude companion spark a generous 18" away. It seems our abused Engagement Ring also has a chip off its diamond. This would be an easy low-power split if the stars were more alike, but their difference of 7 magnitudes (a difference of about 600 times in brightness) means the companion is easily overpowered by the brighter star's glare. Try a magnification of at least 80x to bring it out. Polaris itself is pale yellowish. The faint companion looks pale blue by contrast, but measurements show that their colors are actually very similar.
Polaris enjoys its special position because the Earth's axis happens to point toward it. In other words, it stands almost directly above the North Pole. As the world turns, the geographic poles are the only places that face the same direction all night long. Consequently, the star straight above the pole is the only one that seems to stand still.
Yet Polaris is not quite the unmoving cynosure described by poets. The axis of our planet misses it slightly; the true north celestial pole is currently about 34[degrees] away, in the direction from Polaris toward the Little Dipper's other 2nd-magnitude star, Kochab. Our North Star travels in a little circle around the true pole each day. The circle will shrink to just under 1/2[degrees]
radius in the coming century. Why? The Sun and Moon tug on the equatorial bulge of the tipped Earth and try to pull it toward the ecliptic plane. The resulting torque on the spinning planet causes precession, a slow change in the orientation of the Earth's axis with respect to the stars. Precession carries the celestial pole around the sky in a large near-circle that takes some 26,000 years to complete.
So when will Polaris be closest to the north celestial pole? The answer isn't simple. Variations in the gravitational effects of the Moon add a tiny, 18.6-year nodding motion (nutation) to the celestial poles as they tour the precessional circle. Superimposed on nutation is a tiny annual shift in the apparent positions of all stars due to Earth's 30-kilometer-per-second orbital motion with respect to their incoming light (annual aberration). And Polaris changes its own position (proper motion) because of its sideways motion with respect to the Sun in space.
Taking all this into account, computational wizard Jean Meeus finds that Polaris will be closest to the north celestial pole on March 24, 2100, with an apparent separation of 27' 09". While this is probably not the final word on Polaris as the polestar, it shows how a simple question has an ever more complicated answer depending on how precise you want the answer to be.
Among the other stars making up the Little Dipper are four very wide pairs. None is a true binary, but two of them offer attractive color-contrast duos for binoculars or a small telescope.
One pair is formed by Zeta ([zeta]) and Theta ([theta]) Ursae Minoris, where the Little Dipper's handle joins its bowl. Fourth-magnitude Zeta Ursae Minoris is white; 5th-magnitude Theta is orange. Separated by 49', they will fit together only in a low-power, wide-angle view.
In the opposite corner of the Little Dipper's bowl are 3rd-magnitude Pherkad and 5th-magnitude Pherkad Minor, Gamma ([gamma]) and 11 Ursae Minoris, respectively. They're 17' apart and have hues similar to our previous pair, but the dimmer star is very slightly paler orange, with a color index of 1.4 compared to Theta's 1.6. (A color index of +0.2 is pure white; lower and negative values are slightly bluish, while larger values denote yellow, orange, and red.) Use your lowest power to keep each pair as close together as possible to heighten apparent color contrasts.
The corner of the bowl opposite bright Kochab is formed by 5th-magnitude Eta ([eta]) and 19 Ursae Minoris. They're 26' apart and tinted, respectively, white with the barest hint of yellow and white with a trace of blue. Can you detect the difference? Forming an isosceles triangle with them is 6th-magnitude 20 Ursae Minoris, which is yellow-orange.
The fourth pair is Delta ([delta]) and 24, the first stars down the handle from Polaris. They're magnitudes 4 and 6, separated by 23', and both white.
North of the Little Dipper's bowl is a pretty asterism discovered by Pennsylvania amateur Tom Whiting. He dubbed it the Mini-Coathanger for its likeness to the familiar Coathanger pattern in Vulpecula. It is composed of ten 9th- through 11th-magnitude stars 1.9[degrees] south-southwest of Epsilon ([epsilon]) Ursae Minoris. Like its larger cousin, the Mini-Coathanger resembles an old-fashioned coathanger with a straight wooden bar and metal hook. Seven stars make up the bar, which is about 17' long. Three faint stars comprise the hook.
Ursa Minor owes most of its recognition to the fact that it contains the North Star. But there are subtler treasures to be found here, and best of all, they can be seen by most readers all year long.
SUE FRENCH can be reached by e-mail at Sue_and_Alan_French@msn.com.
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|Title Annotation:||guidance on viewing Ursula Minor stars|
|Publication:||Sky & Telescope|
|Date:||Jun 1, 2001|
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