Polaris
|
|
Polaris or Cynosura (α UMi / α Ursae Minoris / Alpha Ursae Minoris) is the brightest star in the constellation Ursa Minor. It is also very close to the north celestial pole, making it the current north pole star (but this is only a temporary situation due to precession).
Polaris is also known as the "North Star", the "Lode Star", or the "Pole star", or sometimes "Polaris Borealis".
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 under 1° away from the pole of rotation and hence revolves around the pole in a small circle almost 2° in diameter. Only twice during every sidereal day 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", Polaris will not always be the pole star. This is due to precession of the equinox over thousands of years. Other stars have been the pole star in the past and will be again in the future, including Thuban and Vega.
Polaris is due to become an even more accurate pole star in the near future — the distance between Polaris and the pole will reach a minimum (of just under 1/2 degree) in 2100.
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. As of 2005, 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. The 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 southern pole star. The star visible to the naked eye that is closest to the south celestial pole is the dim Sigma Octantis, sometimes called Polaris Australis. However, the bright Southern Cross (Crux) points towards the south celestial pole.
External links
- Info on Polaris (http://www.astro.uiuc.edu/~kaler/sow/polaris.html)ca:Polaris
de:Polarstern es:Polaris (estrella) fr:Polaris it:Stella Polare nl:Poolster ja:北極星 pl:Gwiazda Polarna ru:Полярная звезда sl:Severnica sv:Polstjärnan vi:Polaris