Polaris

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Polaris ( UMi, Ursae Minoris, Alpha Ursae Minoris, commonly North Star, Northern Star or Pole Star , also Lodestar, sometimes Guiding star) 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.

Based on measurements from the Hipparcos astrometry satellite, Polaris is estimated to be at a distance of about 434 light-years from Earth. It is a multiple star , consisting of the main star UMi A, two smaller companions, UMi B and UMi Ab, and two distant components UMi C and UMi D. UMi B was discovered in 1780 by William Herschel .

Star system

UMi A is a six solar mass F8 supergiant (Ib) or bright giant (II). The two smaller companions are: UMi B, a 1.5 solar mass F3 main sequence star orbiting at a distance of 2400& nbsp; Astronomical Unit|AU , and UMi Ab (or P), a very close dwarf with an 18.8& nbsp;AU radius orbit. There are also two distant components UMi C and UMi D.
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 spectroscopy|spectrum of Polaris A that it had another very close dwarf companion (variously UMi P, UMi a or 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 Space Telescope|Hubble telescope , directly showing all three members of the Polaris ternary system. The nearer dwarf star is in an orbit of only 18.5& nbsp;AU (2.8 billion km, about the distance from our Sun to Uranus ) from Polaris A, explaining why its light is swamped by its close and much brighter companion.

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. Around 1900, the star luminosity varied ±8% from its average (0.15 magnitudes in total) with a 3.97 day period. Since then, the star has brightened by 15% (on average), and the period has lengthened by about 8 seconds each year.
Research reported in Science (journal)|Science suggests that Polaris is 2.5 times brighter today than when Ptolemy observed it, changing from third to its current second magnitude.
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 predicted by current theories of stellar evolution ."

Names

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

One ancient name for Polaris was Cynosura , from the Greek "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 :wikt:cynosure|cynosure . Most other names are directly tied to its role as pole star.

In English, it was known as "pole star" or "north star", in Edmund Spenser|Spenser also "steadfast star".
An older English name, attested since the 14th century, is :wikt:lodestar|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).occasionally also as a title of Jesus. Robert Bellarmine deprecated this use of the title, writing ''Haec appellatio stelle maris tribui solet Beate Virgini. Fortasse melius de Christo diceretur 'stella splendida et matutina' ... am stella maris est stella polaris, quae exigua est. Stella splendida et matutina est stella omnium fulgentissima, quae ab astrologis dicitur stella Veneris ; cited after Peter Godman, The saint as censor: Robert Bellarmine between inquisition and index , Mnemosyne, Bibliotheca Classica Batava, BRILL, 2000, ISBN 978-90-04-11570-5,

In traditional Indian astronomy , its name in Sanskrit , literally "fixed star". Its name in Astronomy in medieval Islam|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.
Richard Hinckley Allen, Star names: their lore and meaning (1899), p. 457.

In the Old English rune poem , the Tiwaz rune|T-rune is identified with tir#Old English|tir "fame, honour", which is compared to the pole star, " is a sign, it keeps faith well".
Shakespeare 's sonnet 116 is an example of the symbolism of the north star as a guiding principle: " is the star to every wandering bark / Whose worth's unknown, although his height be taken."

Role as pole star

Because in the Age of Pisces|current era
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 .
The celestial pole will move away from UMi after the 21st century, passing close by Gamma Cephei by about the 41st century.
Historically, the celestial pole was close to Thuban around 2500 BC., and
during Classical Antiquity , it was closer to Kochab (ß UMi) than to a UMi. It was about the same angular distance from either ß UMi than to a UMi by the end of Late Antiquity .
The Greek navigator Pytheas in ca. 320 BC described the celestial pole as devoid of stars. However, as one of the brighter stars close to the celestial pole, Polaris was used for navigation at least from Late Antiquity , and described as ae? fa??? "always visible" by Stobaeus (5th century). a 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 . Nathaniel Bowditch : The American Practical Navigator , 2002 Bicentennial Ed., Chapter 15 Navigational Astronomy, page 248, Figure 1530a. Navigational stars and the planets 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 slightly displaced to East or West, and to bearing must be corrected using tables or a rough rule of thumb . The best approximateA visual method to correct a ship's compass using Polaris using Ursa Major as a point of reference was made using the leading edge of the " Big Dipper " asterism in the constellation Ursa Major as a point of reference. The leading edge (defined by the stars Dubhe and Merak ) was referenced to a clock face, and the true azimuth of Polaris worked out for different latitudes.

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Undetermined Music Artists

Polaris

Biography

Star system

Names

Role as pole star

See also

Notes

External links

Copyright Citations