Α UMi

For other uses, see Polaris (disambiguation).
Alpha Ursae Minoris
HIP 11767, Cynosura, Alruccabah, Phoenice, Navigatoria, Star of Arcady, Yilduz, Mismar

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.

It is a multiple star, consisting of the main star UMi Aa, 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.

Many recent papers calculate the distance to Polaris at about 434 light-years (133 parsecs).[4] Some suggest it may be 30% closer which, if correct, is especially notable because Polaris is the closest Cepheid variable to Earth so its physical parameters are of critical importance to the whole astronomical distance scale.[3]

Star system

UMi Aa is a 4.5 solar mass F7 supergiant (Ib). This is the first classical Cepheid to have a dynamical mass determined from its orbit. The two smaller companions are: UMi B, a 1.39 solar mass F3 main sequence star orbiting at a distance of 2400 AU, and UMi Ab (or P), a very close F6 main sequence star with an 18.8 AU radius orbit and 1.26 solar masses. There are also two distant components UMi C and UMi D.[9]

Polaris B can be seen even with a modest telescope. It was found by William Herschel in 1780 using one of the most powerful telescopes at the time: a reflecting telescope that he had made. In 1929, it was discovered by examining the spectrum of Polaris A that it was a very close binary with the secondary being a dwarf (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 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,[10] about the distance from our Sun to Uranus) from Polaris A, explaining why its light is swamped by its close and much brighter companion.[4]

Variable star

Polaris A, the supergiant primary component, 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.[11]

Both the amplitude and period of the variations have changed since discovery. Prior to 1963 the amplitude was over 0.1 magnitude and decreasing very gradually. After 1966 it decreased very rapidly until it was less than 0.05 magnitude and since then has varied erratically near that range. It has been reported that the period is now increasing.[8] The period increased fairly steadily by around 4 seconds per year until 1963. It then stayed constant for 3 years, but began to increase again from 1966 onwards. Current measurements show a consistent increase of 3.2 seconds per year in the period. This was originally thought to be due to secular red-ward evolution across the instability strip, but is now considered to be interference between the primary and first overtone pulsation modes. Comparison of the period luminosity relationship and the observed luminosity indicate that the main pulsations are the first overtone.[4][12][13]

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.[14] 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

Further information: Axis mundi

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

One ancient name for Polaris 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.[15][16] 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).[17] 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.[18]

In the Old English rune poem, the T-rune is identified with Tyr "fame, honour", which is compared to the pole star, [tir] biþ tacna sum, healdeð trywa wel "[fame] 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: "[Love] is the star to every wandering bark / Whose worth's unknown, although his height be taken."


Role as pole star

Further information: Pole star

Because in the current era[19] 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.[19] 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.,[19] and during Classical Antiquity, it 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. 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 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.[20] 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 approximate[21] 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.

Distance

Selected distance estimates to Polaris
Year Distance, ly (pc) Notes
433 ly (133 pc) Hipparcos
2006 330 ly (101 pc) Turner[12]
2008 359 ly (110 pc) Usenko & Klochkova[2]
2012 323 ly (99 pc) Turner, et al.[3]

Many recent papers calculate the distance to Polaris at about 434 light-years (133 parsecs),[4] in agreement with parallax measurements from the Hipparcos astrometry satellite. Older distance estimates were often slightly less, and recent research based on high resolution spectral analysis suggests it is about 100 light years closer (323 ly/99 pc).[3] Polaris is the closest Cepheid variable to Earth so its physical parameters are of critical importance to the whole astronomical distance scale.[3] It is also the only one with a dynamically measured mass.

The Hipparcos spacecraft used stellar parallax to take measurements from 1989 and 1993 with an accuracy of 0.97 milliarcseconds (970 microarcseconds), and it obtained accurate measurements for stellar distances up to 1,000 pc away.[22][23] Despite the advantages of Hipparcos astrometry, the uncertainty in its Polaris data has been pointed out and some researches have questioned the accuracy of Hipparcos when measuring binary Cephids like Polaris.[3]

The next major step in high precision parallax measurements will come from Gaia, a space astrometry mission due to launch in 2013 intended to measure stellar distances to within 20 microarcseconds (μas), with only 10% error for stars 8,000 pc (26 kly) away.[24] Gaia will not be able to take measurements on bright stars like Polaris, but it may help with measurements of other members of assumed associations and with the general galactic distance scale. Radio telescopes have also been used to produce accurate parallax measurements at large distances, but these require a compact radio source in close association with the star which is typically only the case for cool supergiants with masers in their circumstellar material.[25]

See also

Notes

External links

  • Info on Polaris
  • Finding the Pole Star
Preceded by
Kochab & Pherkad
Pole Star
500–3000
Succeeded by
Alrai

, +89° 15′ 51″

hi:ध्रुव तारा

li:Polaris ml:ധ്രുവനക്ഷത്രം zh-yue:勾陳一

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