Polaris

Polaris, Alpha Ursae Minoris, is the bright star closest to the north celestial pole, making it the current north pole star. It is also known as the North Star, the Lode Star, or the Pole star.

Because it lies nearly in a direct line with the axis of the Earth's rotation "above" the North Pole—the north celestial pole—Polaris is apparently motionless from the Earth, and all the stars of the Northern sky appear to rotate around it. Therefore, it makes an excellent fixed point from which to draw measurements for celestial navigation. The antiquity of the use of this star is attested to by the fact that it is found represented on the earliest known Assyrian tablets. At present, Polaris is slightly over 1° away from the pole of rotation and hence revolves around the pole in a small circle about 2° in diameter. Only twice during every 24 hours does Polaris accurately define the true north azimuth; the rest of the time it is only an approximation and must be corrected using tables.

Although Shakespeare wrote "I am as constant as the northern star", due to precession of the equinox, other stars in the northern hemisphere have been and will likely again become the north star over thousands of years; Thuban was pole star in the past, and Vega will be in the future.

It is easy to find Polaris by following the line traced from Merak to Dubhe (β and α Ursae Majoris, also known as the Pointers), the two stars at the end of the bowl of the Big Dipper. One can also follow the central point of the W shape of Cassiopeia.

Polaris's fame as the North Star has given rise to a persistent misconception that it is the brightest star in the sky. Although Polaris is a relatively bright and conspicuous star, it is nowhere near the brightest; it is actually the 51st brightest star. The brightest star in the sky (besides the Sun) is Sirius. See List of brightest stars.

Polaris is 431 light years (132 parsecs) from Earth, according to measurements made by the Hipparcos satellite. It is an F7 supergiant (Ib) or bright giant (II), with two smaller companions: an F3 V main sequence star about 2000 AU away and a close companion in an orbit with a 5 AU semi-major axis. The main star is a Population II cepheid variable, the pulsations of which cause it to cycle steadily. Around 1900, the star varied between being 8% brighter than its average luminosity and 8% dimmer (0.15 magnitudes in total) with a 3.97 day period. Now (2004), the variations are about 2% from peak to trough. The star is also about 15% brighter (on average) than it was in 1900; the period has also lengthened by about 8 seconds each year since then. Recent research reported in Science suggests that Polaris is 2.5 times brighter today than when Ptolemy observed it. Astronomer Edward Guinan considers this to be a remarkable rate of change and is on record as saying that "If they are real, these changes are 100 times larger than [those] predicted by current theories of stellar evolution".

There is no real south star. The star, visible to the naked eye, that is closest to the south celestial pole is the dim Sigma Octantis. However, the bright Southern Cross (Crux) points towards the south celestial pole.

External link

• Info on Polaris (http://www.astro.uiuc.edu/~kaler/sow/polaris.html)

ca:Polaris de:Polarstern fr:Polaris it:Stella Polare ja:北極星 nl:Poolster pl:Gwiazda Polarna