Polaris


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Polaris

(pōlâr`ĭs) or

North Star,

star nearest the north celestial pole (see equatorial coordinate systemequatorial coordinate system,
the most commonly used astronomical coordinate system for indicating the positions of stars or other celestial objects on the celestial sphere. The celestial sphere is an imaginary sphere with the observer at its center.
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). It is in the constellation Ursa Minor (see Ursa Major and Ursa MinorUrsa Major and Ursa Minor
[Lat.,=the great bear; the little bear], two conspicuous northern constellations. Known to many peoples from ancient times, these constellations have had various names; the configuration of the
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; Bayer designation Alpha Ursae Minoris) and marks the end of the handle of the Little Dipper. Polaris's location less than 1° from the pole (1992 position R.A. 2h23.3m, Dec. +89°14') makes it a very important navigational star even though it is only of second magnitude; it always marks due north from an observer. Polaris can be located by following the line upward from the two stars (the Pointers) at the right end of the bowl of the Big Dipper or, if the Big Dipper is not visible, by following the line through the left side of the square in Pegasus through the end star in Cassiopeia. The star is a Cepheid variableCepheid variables
, class of variable stars that brighten and dim in an extremely regular fashion. The periods of the fluctuations (the time to complete one cycle from bright to dim and back to bright) last several days, although they range from 1 to 50 days.
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 and oscillates in brightness roughly every four days. Because of the precession of the equinoxesprecession of the equinoxes,
westward motion of the equinoxes along the ecliptic. This motion was first noted by Hipparchus c.120 B.C. The precession is due to the gravitational attraction of the moon and sun on the equatorial bulge of the earth, which causes the earth's axis to
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, Polaris will not remain the polestar indefinitely; in 2300 B.C. the polestar was in the constellation DracoDraco
[Lat.,=the dragon], northern constellation lying SE of Ursa Minor and N of Lyra and Hercules. It is traditionally depicted as a dragon. Draco contains the bright star Eltanin (Gamma Draconis). Thuban (Alpha Draconis) was the polestar 5,000 years ago, i.e.
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, and by A.D. 12,000 the star VegaVega
, brightest star in the constellation Lyra; Bayer designation Alpha Lyrae; 1992 position R.A. 18h36.7m, Dec. +38°47'. A white main-sequence star of spectral class A0 V, its apparent magnitude is 0.1, making it the fifth brightest star in the sky.
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 in the constellation Lyra will be the polestar.

Polaris

(poh-lar -is, -la -ris) (North Star; α UMi) A remote creamy-yellow supergiant that is the brightest star in the constellation Ursa Minor. It lies very close to the north celestial pole (dec: 89°16′) and is the present pole star. Its position is found by means of the Pointers in the Plough. Polaris is a classical Cepheid variable (period 3.97 days) but its pulsations have decreased rapidly since about the mid-1970s. It forms an optical double with a 9th-magnitude F3 V companion. mv : 2.0; Mv : -2.9; spectral type: F8 Ib; distance: 97 pc.

Polaris

 

(Alpha Ursae Minoris, the North Star), a bright star of the second magnitude near the north celestial pole and thus useful for determining the bearing of the sun and a particular area’s geographic latitude, which is approximately equal to the altitude of Polaris above the horizon. In 1975, Polaris was 51′ in angular distance from the celestial pole. As consequence of precession, this distance decreases almost 17″ per year and will attain its minimum (28′) around the year 2100. Polaris can be found in the sky by the method indicated in Figure 1. The precise azimuth and latitude of a location can be determined from Polaris using ephemerides published in astronomical annuals.

Figure 1. Method of determining the position of Polaris in the sky

Polaris is a triple star. Its bright component is a cepheid (variable star) with a brightness-variation amplitude of 0.14 stellar magnitude and a period of about four days.

Polaris

[pə′lar·əs]
(astronomy)
A creamy supergiant star of stellar magnitude 2.0, spectral classification F8, in the constellation Ursa Minor; marks the north celestial pole, being about 1° from this point; the star Ursae Minoris. Also known as North Star; Pole Star.
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