North Star - astronomy.

North Star - astronomy. North Star or Polestar, conspicuous star in the northern hemisphere, located closest to the point toward which the axis of the Earth is directed, thus roughly marking the location of the north celestial pole. A polestar has been used by navigators throughout recorded history for charting navigation routes and is still used for determining true azimuth and astronomic latitude. The positions of the celestial poles change as the Earth's axis moves with Earth's precessional motion (see Ecliptic), and as the north celestial pole assumes different positions relative to the constellations, different stars become the North Star. During the past 5,000 years the line of direction of the North Pole has moved from the star Thuban, or Alpha (?) Draconis, in the constellation Draco, to within one degree of the bright star Polaris, also known as Alpha (?) Ursae Minoris, in the constellation Ursa Minor (Little Dipper), which is now the North Star. Polaris, also known as Polaris A, is part of a triple star system and is located at a distance of about 430 light-years from Earth. A triple star, or trinary, system is one in which three stars orbit a common center of gravity. Polaris was long known to be part of a binary star system. Astronomers suspected the existence of a third star based on spectroscopic evidence. Polaris B can be easily seen with telescopes on Earth. The existence of the triple star system was confirmed in January 2006 when astronomers using the Hubble Space Telescope announced that they were able to resolve a third star. The third star, Polaris C, is so close to Polaris A and is so much dimmer that astronomers using Earth-based telescopes were unable to see it directly. Polaris A is easy to locate in the sky because of its apparent position near the Big Dipper. The two stars opposite the handle in the bowl of the Big Dipper, which are called the Pointers, point to the star Polaris A. See Big Dipper; Constellation; Little Dipper. In the year 7500 the brightest star in the constellation Cepheus, Alpha (?) Cephei, will mark the pole, and in the year 15,000 the star Vega, in the constellation Lyra, will be the North Star. About 9,000 years after that, Polaris will again become the North Star. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.