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 (42′ away as of 2006^{[update][citation needed]}), making it the current northern pole star.

Polaris is about 430 light-years from Earth. It 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 distance of 2400 AU, and α UMi Ab, a very close dwarf with an 18.5 AU radius orbit. There are also two distant components UMi C and UMi D.^[5] Recent observations show that Polaris may be part of a loose open cluster of type A and F stars.^{[clarification needed]}

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 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). 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."

[edit] Pole Star

An artist's concept of Polaris's system

Because in the current era^[8] 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.^{[citation needed]} 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.^[9] 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 over 25,800 years.^[8] 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.^[8]

[edit] Other names for Polaris

Polaris has numerous traditional names: Alruccabah, Cynosura, Phoenice, Tramontana, Angel Stern, Navigatoria, Star of Arcady, Yilduz, Mismar, Polyarnaya. Cynosūra is from the Greek κυνόσουρα "the dog’s tail" and is the source of the English word "cynosure."^[10] Yilduz is from the Turkish word for "star" (see Yildun).

[edit] Cultural significance

• To the Bedouin people of the Negev and Sinai, Polaris is known as الجديّ al-jadiyy, "the billy goat". It and Suhail (= Canopus, α Car) are the two principal stars used for nomadic wandering at night. Because it was circumpolar and hence always visible from the Northern Hemisphere, it became associated with a steadfast nature, as opposed to Suhail, which disappears below the horizon and hence 'flees'.^[11]

• A monkey's head is the emblem of the Mayan god of the pole star.

• In Hindu mythology Polaris is termed as Dhruva, meaning fixed.

• In Finnish mythology, Polaris (Pohjantähti) was called Taivaanneula, (Heaven's Needle), indicating the pillar which supported the heavens above Earth.

[edit] See also

• North celestial pole
• Polaris in fiction

[edit] Notes

[edit] References

• Wieland, R.; H. Jahreiss, C. Dettbarn, H. Lenhardt, H. Schwan, Polaris: astrometric orbit, position, and proper motion, http://arxiv.org/abs/astro-ph/0002406, retrieved 2009-04-13  (Link to download full paper as PDF)

[edit] External links

• Info on Polaris
• Polaris Ab imaged by Hubble

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