Polaris

 

(pōlâr'ĭs) or North Star, star nearest the north celestial pole (see equatorial coordinate system). It is in the constellation Ursa Minor (see Ursa Major and Ursa Minor; 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. 2^h23.3^m, 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 variable and oscillates in brightness roughly every four days. Because of the precession of the equinoxes, Polaris will not remain the polestar indefinitely; in 2300 B.C. the polestar was in the constellation Draco, and by A.D. 12,000 the star Vega in the constellation Lyra will be the polestar.

Alpha Ursae Minoris

Polaris as seen by the Hubble Space Telescope.
Observation data Epoch J2000
Constellation	Ursa Minor
Right ascension	02h 31m 48.7s
Declination	+89° 15′ 51″
Apparent magnitude (V)	1.97
Characteristics
Spectral type	F7 Ib-II SB
U-B color index	0.38
B-V color index	0.60
Variable type	Cepheid variable
Astrometry
Radial velocity (Rv)	-17 km/s
Proper motion (μ)	RA: 44.22 mas/yr Dec.: -11.74 mas/yr
Parallax (π)	7.56 ± 0.48 mas
Distance	430 ± 30 ly (132 ± 8 pc)
Absolute magnitude (MV)	-3.64
Details
Mass	5.0 or 6.0 M☉
Radius	30 R☉
Luminosity	2200 L☉
Temperature	7200 K
Metallicity	?
Rotation	~17 km/s
Age	? years
Other designations
Polaris, Cynosura, Alruccabah, Phoenice, Lodestar, Pole Star, Tramontana, Angel Stern, Navigatoria, Star of Arcady, Yilduz, Mismar, Polyarnaya, 1 Ursae Minoris, HR 424, BD +88°8, HD 8890, SAO 308, FK5 907, GC 2243, ADS 1477, CCDM 02319+8915, HIP 11767.

Polaris (α UMi / α Ursae Minoris / Alpha Ursae Minoris), more commonly known as The North Star or simply North Star, is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole (42′ away as of 2006), making it the current northern pole star.

Name

"Polaris" comes from Stella polaris, the Latin form of its common name "Polar Star". The rarely used Cynosura from the Greek name Κυνόσουρα means "the dog-tailed one" and is the source of the English word cynosure. Other names include the "North Star / Northern Star", the "Lode Star", or sometimes "Polaris Borealis".

Physical properties

Polaris is a trinary system, consisting of a large yellow Cepheid variable (α UMi A), orbited by a bright yellow dwarf (α UMi B) at a distance of about 2400 AU (360 billion kilometers, or 224 billion miles). Polaris B can be seen with even a modest telescope and was first noticed by William Herschel in 1780. In 1929, it was discovered by examining the spectrum of Polaris A that it had another very close dwarf companion (variously α UMi P, α UMi a or α UMi Ab). In January 2006, NASA released images from the Hubble telescope, directly showing all three members of the Polaris trinary system. The nearer dwarf star is in an orbit of only 18.5 AU (2.8 billion km ^[1]; about the distance from our Sun to Uranus) from Polaris A, explaining why its light is swamped by its close and much brighter companion.^[2]

Polaris is 431 light years (132 pc) from Earth, according to astrometric measurements of the Hipparcos satellite. Concerning the detailed physics, α UMi A is an F7 bright giant (II) or supergiant (Ib). The two smaller companions are: α UMi B, an F3V main sequence star orbiting at a distance of 2400 AU, and α UMi Ab, a very close dwarf with an 18.5 AU radius orbit. Recent observations show that Polaris may be part of a loose open cluster of type A and F stars.

The giant star of Polaris is a classical Population I Cepheid variable (although it was once thought to be Population II due to its high galactic latitude). Since Cepheids are an important standard candle for determining distance, Polaris (as the closest such star) is heavily studied. Around 1900, the star luminosity varied ±8% from its average (0.15 magnitudes in total) with a 3.97 day period; however, the amplitude of its variation has been quickly declining since the middle of the 20th century. The variation reached a minimum of 1% in the mid 1990s and has remained at a low level. Over the same period, the star has brightened by 15% (on average), and the period has lengthened by about 8 seconds each year.

Recent research reported in Science suggests that Polaris is 2.5 times brighter today than when Ptolemy observed it (now 2mag, antiquity 3mag). 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."

Pole Star

Because α UMi lies nearly in a direct line with the axis of the Earth's rotation "above" the North Pole — the north celestial pole — Polaris stands almost motionless on the sky, 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 and for astrometry. The antiquity of its use is attested by the fact that it is found represented on the earliest known Assyrian tablets. At present, Polaris is 0.7° away from the pole of rotation (1.4 times the Moon disc) and hence revolves around the pole in a small circle 1½° 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 or a rough rule of thumb.

Due to the precession of the equinoxes, Polaris will not always be the pole star. Over tens of thousands of years, perturbations to the Earth's axis of rotation will cause it to point to other regions of the sky, tracing out a circle. Other stars along this circle were the pole star in the past and will be again in the future, including Thuban and Vega. Polaris has been close to the actual position of the north pole for over 1000 years and during the course of the 21st century it will continue to approach the exact theoretical position, reaching its closest approach on 24 March 2100 (almost 0.45° away). Subsequently it will begin to pull away.^[3]

In the Northern Hemisphere, 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 is not visible from the Southern Hemisphere except from an elevated position near the equator.

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 star and is conspicuous since no other stars of similar brightness are close to it, it is nowhere near the brightest; it is actually the 48th brightest star in the night sky. The brightest star in the night sky is Sirius (see Winter sky and List of brightest stars).

There is no real southern pole star. The only star visible to the naked eye that is close to the south celestial pole is the dim Sigma Octantis, sometimes called Polaris Australis. However, the bright Southern Cross (Crux) points fairly accurately towards the south celestial pole.

See also

• Polaris in fiction

References

External links

• Info on Polaris
• Polaris: astrometric orbit, position, and proper motion(mass = 6.0±0.5 M)
• Polaris Ab imaged by Hubble
• Polaris: Mass and Multiplicity (mass = 5.0±1.5 M)

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