Polaris

Alpha Ursae Minoris

Polaris as seen by the Hubble Space Telescope.
Observation data Epoch J2000      Equinox 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)	-3.63±0.14[1]
Details
Mass	7.54±0.6[1][2] M☉
Radius	30 R☉
Luminosity	2200 L☉
Temperature	7200 K
Metallicity	112% solar[3]
Rotation	~17 km/s
Age	? years
Other designations
Polaris, Cynosura, Alruccabah, Phoenice, Navigatoria, Star of Arcady, Yilduz, Mismar, Поля́рная звезда́ (Polyarnaya zvyezda), 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, commonly North(ern) Star or Pole Star, or Dhruva Tara and sometimes Lodestar) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star.

Polaris is about 430 light-years from Earth and is a multiple star. α UMi A is a six solar mass^[4] F7 bright giant (II) or supergiant (Ib). The two smaller companions are: α UMi B, a 1.5 solar mass^[4] F3V main sequence star orbiting at a sistance of 2400 AU, and α UMi Ab, a very close dwarf with an 18.5 AU radius orbit. There are also two distant components α UMi C and α UMi D.^[5]

Polaris B can be seen even with 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), which had been theorized in earlier observations (Moore, J.H and Kholodovsky, E. A.). In January 2006, NASA released images from the Hubble telescope, directly showing all three members of the Polaris ternary system. The nearer dwarf star is in an orbit of only 18.5 AU (2.8 billion km;^[6] about the distance from our Sun to Uranus) from Polaris A, explaining why its light is swamped by its close and much brighter companion.^[7]

Polaris is a classic 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 star's heat is 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). 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

Two Class F stars: Supergiant Polaris A and its distant companion Polaris B^[8]

Because in the current era^[9] 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. In more recent history it was referenced in Nathaniel Bowditch's 1802 book, The American Practical Navigator, where it is listed as one of the navigational stars.^[10] 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. The best approximation^[11] was made using the leading edge of the constellation Ursa Major as a point of reference. The leading edge (consisting of the star Dubhe) is referenced to a clock face and the Azimuth of Polaris worked out for different latitudes.

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 over 25,800 years.^[9] Other stars along this circle would have served as the pole star in the past and will again in the future, including Beta Ursae Minoris from 1500 BC to 500 AD, Thuban around 2500 BC and Vega 12000 BC.^[9]

Other names for Polaris

A typical star trail with Polaris in the centre

Polaris has numerous traditional names: Alruccabah, Cynosura, Dhruv, Phoenice, Tramontana, Angel Stern, Navigatoria, Star of Arcady, Yilduz, Mismar, Gwiazda Polarna, Polyarnaya, Midnight Star. Cynosūra is from the Greek κυνόσουρα "the dog’s tail" and is the source of the English word "cynosure".^[12] Yilduz is from the Turkish word for "star" (see Yildun). The most commonly known name (in English) is the Pole Star.

See also

• North celestial pole
• Polaris in fiction

Notes

References

Wieland, R.; H. Jahreiss, C. Dettbarn, H. Lenhardt, H. Schwan (2000). "Polaris: astrometric orbit, position, and proper motion". arΧiv:astro-ph/0002406 [astro-ph]. 

External links

• Info on Polaris
• Approximation for calculating Polaris Azimuth Visually given at www.kaizad.tk
• Polaris Ab imaged by Hubble
• Finding the Pole Star

Coordinates: 02^h 31^m 48.7^s, +89° 15′ 51″