Polaris

Polaris (α UMi, α Ursae Minoris, Alpha Ursae Minoris, commonly North(ern) Star or Pole Star, also 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 434 light-years from Earth^[1] and is a multiple star. It consists of the main star α UMi A, two smaller companions, α UMi B and α UMi Ab, and two distant components α UMi C and α UMi D. α UMi B was discovered in 1780 by William Herschel.

Star system

α UMi A is a six solar mass^[5] F7 bright giant (II) or supergiant (Ib). The two smaller companions are: α UMi B, a 1.5 solar mass^[5] 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 α UMi C and α UMi D.^[6]

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,^[7] about the distance from our Sun to Uranus) from Polaris A, explaining why its light is swamped by its close and much brighter companion.^[8]

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. The variability of Polaris had been suspected since 1852; this variation was confirmed by Ejnar Hertzsprung in 1911.^[9] 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.

Research reported in Science suggests that Polaris is 2.5 times brighter today than when Ptolemy observed it, changing from third to its current second magnitude.^[10] 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."

Names

Because of its importance in celestial navigation, Polaris is known by numerous names.

Its ancient name was Cynosūra, from the Greek κυνόσουρα "the dog’s tail" (reflecting a time when the constellation of Ursa Minor "Little Bear" was taken to represent a dog), whence the English word cynosure.^[11][12]. Most other names are directly tied to its role as pole star.

In English, it was known as "pole star" or "north star", in Spenser also "steadfast star". An older English name, attested since the 14th century, is lodestar "guiding star", cognate with the Old Norse leiðarstjarna, Middle High German leitsterne. Use of the name Polaris in English dates to the 17th century. It is an ellipsis for the Latin stella polaris "pole star". Another Latin name is stella maris "sea-star", from an early time also used as a title of the Blessed Virgin Mary, popularized in the hymn Ave Maris Stella (8th century).^[13] In traditional Indian astronomy, its name in Sanskrit dhruva tāra, literally "fixed star". Its name in medieval Islamic astronomy was variously reported as Mismar "needle, nail", al-kutb al-shamaliyy "the northern axle/spindle", al-kaukab al-shamaliyy "north star". The name Alruccabah or Ruccabah reported in 16th century western sources was that of the constellation.^[14]

In the Old English rune poem, the T-rune is identified as the pole star, ᛏ Tir biþ tacna sum, healdeð trywa wel "Tir is a star, it keeps faith well". Shakespeare's sonnet 116 is an example of the symbolism of the north star as a guiding principle: "[Love] is the star to every wandering bark / Whose worth's unknown, although his height be taken."

Role as pole star

Because in the current era^[16] 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 in 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 moving of Polaris towards, and in the future away from, the celestial pole, is due to the precession of the equinoxes.^[16] The celestial pole will move away from α UMi after the 21st century, passing close by Gamma Cephei by about the 41st century. Historically, it was close to Thuban around 2500 BC.^[16] During Classical Antiquity, the celestial pole was closer to Kochab (β UMi) than to α UMi. It was about the same angular distance from either β UMi than to α UMi by the end of Late Antiquity. The Greek navigator Pytheas in ca. 320 BC described the celestial pole as devoid of stars. As one of the brighter stars close to the celestial pole, it was still used for navigation at least from Late Antiquity, and described as αει φανης "always visible" by Stobaeus (5th century). α UMi could reasonably be described as stella polaris from about the High Middle Ages.

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.^[17] 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^[18] 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.

See also

• Polaris in fiction

Notes

References

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

External links

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

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