Ursa Minor

The Little Bear and the pole

Ursa Minor, the Little Bear, is a small northern constellation built around one famous star: Polaris, the North Star. Its seven main stars form the Little Dipper, a smaller and fainter copy of the Big Dipper in Ursa Major.

Polaris marks the end of the bear's long tail, or the end of the dipper's handle. Because it lies very close to the north celestial pole, the point in the sky directly above Earth's North Pole, every other star appears to circle around it. From the northern hemisphere, the whole constellation stays above the horizon all night throughout the year.

How to find it

Begin with the Big Dipper. The two stars at the outer edge of its bowl, Dubhe and Merak, point to Polaris. Follow that line about five times the distance between them and you reach a moderately bright star standing alone: that is Polaris.

From Polaris the handle of the Little Dipper curves away toward the bowl. The two brightest bowl stars, orange Kochab and white Pherkad, are easy to spot even from town. They are sometimes called the Guardians of the Pole, because they wheel around Polaris like sentries through the night.

To check your direction, face Polaris: you are looking north. Its height above the horizon roughly equals your latitude, so from about 41°N it stands about 41 degrees up, a little less than halfway to the zenith.

The main stars and their names

Polaris (Alpha Ursae Minoris) shines at magnitude about 2.0 and lies about 430 light-years away, though some measurements give a larger figure. It is a yellow-white supergiant more than 2,000 times as luminous as the Sun. It is also the nearest Cepheid, a pulsating star whose brightness rises and falls in a regular cycle, here of about four days; the change is too small to notice with the eye.

Polaris has two companions. One, at about magnitude 9, can be seen in a small telescope; the other orbits so close that it was first imaged directly by the Hubble Space Telescope.

Kochab (Beta UMi), magnitude about 2.1 and about 130 light-years away, is an orange giant almost as bright as Polaris. Its name is of uncertain Arabic origin. Pherkad (Gamma UMi), magnitude about 3.0, comes from an Arabic word for a calf, from an old picture of two calves circling the pole.

A pole star that changes

Earth's axis wobbles like a slowing spinning top, tracing a circle in the sky about once every 26,000 years. This motion, called precession, slowly moves the celestial pole among the stars.

Polaris is getting closer to the pole and will be nearest, less than half a degree away, around the year 2100. About 5,000 years ago, when the pyramids of Egypt were built, the pole star was Thuban in Draco. In roughly 12,000 years bright Vega will be close to the pole.

Today Polaris is less than 1 degree from the true pole, which is close enough for navigation and for aligning a telescope mount by eye.

Deep-sky objects and meteors

Ursa Minor has few bright deep-sky targets. In binoculars, look just next to Polaris for the Engagement Ring, a small circle of faint stars with Polaris as the diamond. The barred spiral galaxy NGC 6217, about magnitude 11 and roughly 67 million light-years away, needs a telescope.

Each December the Ursids appear to radiate from near Kochab. The shower peaks around December 21–22 and usually brings only about 10 meteors an hour, but it is a quiet reward on long winter nights. Its parent is comet 8P/Tuttle.

Mythology

In one Greek version, the two bears were nymphs who hid the infant Zeus on Crete from his father Kronos. The Little Bear was called Kynosoura, 'dog's tail', which gave English the word cynosure, a center of attention. Ancient writers say Phoenician sailors steered by the Little Bear, while Greek sailors used the Great Bear.

Several Turkic and Mongolic peoples saw Polaris as a stake or post in the sky, with nearby stars as horses tethered to it, turning around it through the night. In Turkish the constellation is Küçükayı, and Polaris is the Kutup Yıldızı.

Practical observing tips

The Little Dipper is visible on any clear night, but it is best placed on early summer evenings, when it stands above Polaris. Use only a dim red light and give your eyes 20 minutes to adapt.

Count the stars of the dipper. If you can see all seven, including the faint ones at about magnitude 5, your sky is fairly dark. If you see only Polaris, Kochab and Pherkad, light pollution is strong. Repeating this test through the year is an easy way to track sky quality at home.