Ursa Minor

Ursa Minor, Earth's north polar constellation, carves out a small area of sky well outside the plane of the Milky Way.

A small constellation of the northern sky, famous for the fact that it currently includes, close to the star Polaris, the Northern Celestial Pole.

Ursa Minor in the Sky

This little constellation has been associated with the shape of a bear since classical times, with the first recorded such references go back long before its association with the Northern Celestial Pole. Ursa Minor is surrounded to the east, west and south by the winding shape of Draco the Dragon, but it is more naturally associated with its fellow star-bear, Ursa Major, that lies beyond Draco to the southwest. Like the Plough within Ursa Major, Ursa Minor has a distinctive 'dipper' shape, and the two constellations are commonly referred to as the Big Dipper (the Plough) and the Little Dipper (Ursa Minor).

Ursa Major provides a simple method of locating Ursa Minor in the sky. The two western stars of the Plough are together known as the Pointers, because a line northwards through those stars points towards Polaris, the North Star that lies at the tip of the Little Bear's tail.

At present, the North Star falls extremely close to the Northern Celestial Pole, a fact that makes it remarkably useful in navgation (since Polaris always lies directly north from the perspective of any observer on Earth). It also means that the North Star remains almost motionless in the sky as Earth rotates on its axis, and the Lesser Bear rotates around the static tip of its own tail once in every twenty-four-hour period.

Stars

The brightest, most prominent and historically most important of Ursa Minor's stars is Polaris. Its significance as the Pole Star is due entirely to the orientation and orbit of the planet Earth, and it will hold that position only temporarily as Earth's precession slowly moves the Pole across the sky towards Errai in neighbouring Cepheus. Polaris itself is an immensely luminous F-type supergiant, more than 400 light years from the Solar System.

At the southern extent of Ursa Minor's 'Little Dipper' shape are two stars known collectively as the Guardians of the Pole (because they rotate around it once every day, to an observer on Earth). These are the orange giant Kochab, and the pulsating white star known as Pherkad. The quadrangle that forms the dipper's bowl is completed by two fainter stars, Alifa al Farkadain and Anwar al Farkadain. From Alifa al Farkadain, the dipper's 'handle' reaches northwards to Polaris, to which it is connected by Epsilon and Delta Ursae Minoris (Delta is a white star also known as Yildun or Pherkard).

Ursa Minor and Stellar Magnitude

The stars of Ursa Minor form a convenient reference point for stellar brightness, as many of the stars that make up the shape of the constellation are close to one of the integral degrees of apparent magnitude. Polaris, the brightest, has a magnitude of very nearly +2, as does the marginally fainter Kochab. Pherkad (Kochab's fellow Guardian of the Pole) is +3, and faint Anwar al Farkadain is almost exactly +5. Alifa al Farkadain breaks the pattern slightly (it would ideally be +4, whereas it is in fact +4.27) but in combination these stars give a useful visual reference to levels of apparent magnitude between +2 and +5.

The area of the sky around Polaris - specifically within two degrees of the Northern Celestial Pole - is home to a group of ninety-six stars known as the North Polar Sequence. These were at one time used as official reference points for both magnitude and spectral classification.