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Polaris

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Polaris

Polaris (α UMi, α 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, and the 45th brightest star in the night sky. It is very close to the north celestial pole, making it the current northern pole star.

It is a multiple star, comprising the main star, α UMi Aa, which is a supergiant; 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.

Many recent papers calculate the distance to Polaris at about 434 light-years (133 parsecs).[4] Some suggest that 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 critically important to the whole astronomical distance scale.[3]

History of observations

Polaris in stellar catalogues and atlases
Source Presence
Ptolemy (~169) Yes
Al-Sufi (964) Yes
Al-Biruni (~1030) Yes
Khayyam (~1100) Yes
Al-Tusi (1272) No
Ulugh Beg (1437) Yes
Copernicus (1543) Yes
Schöner (1551) Yes
Brahe (1598) Yes
Brahe (1602) Yes
Bayer (1603) Yes
De Houtman (1603) No
Kepler (1627) Yes
Schiller (1627) Yes
Halley (1679) No
Hevelius (1690) Yes
Flamsteed (1725) Yes
Flamsteed (1729) Yes
Bode (1801a) Yes
Bode (1801b) Yes

Star system

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

Polaris B can be seen even with a modest telescope. William Herschel discovered the star in 1780 while using a handbuilt reflecting telescope, one of the most powerful telescopes at the time. 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 α UMi P, α UMi an 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, that showed the three members of the Polaris ternary system. The nearer dwarf star is in an orbit of only 18.5 AU (2.8 billion km from Polaris A,[10] about the distance between our Sun and Uranus), which explains 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 variable due to its high galactic latitude. Since Cepheids constitute 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 the period of the variations have changed since discovery. Prior to 1963, the amplitude was over 0.1 magnitude and was very gradually decreasing. After 1966, it very rapidly decreased until it was less than 0.05 magnitude; since then, it has erratically varied near that range. It has been reported that the period—around 3.97 days—is now increasing.[8] The period had steadily increased by around 4 seconds per year until 1963. It then stayed constant for 3 years but began to increase again from 1966. 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 the first overtone-pulsation modes. Comparison of the period/luminosity relationship and the observed luminosity indicate that the main pulsations constitute 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 second magnitude.[14] Astronomer Edward Guinan considers this to be a remarkable change and is on record as saying that, "[I]f they are real, these changes are 100 times larger than [those] predicted by current theories of stellar evolution."

Names

This artist's concept shows three class F stars: supergiant Polaris A, dwarf Polaris Ab, and its distant dwarf companion, Polaris B

Because of its importance in celestial navigation, Polaris is known by numerous names. It became known as Polaris during the Renaissance, its name derived from the Latin polaris "of/near the (north) pole".[15]

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), hence the English word cynosure.[16][17] 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", which, from an early time, was also used as a title of the Blessed Virgin Mary, popularized in the hymn Ave Maris Stella (8th century).[18] In traditional Indian astronomy, its name in Sanskrit is dhruva tāra "fixed star". Its name in medieval Islamic astronomy was variously reported as Mismar "needle, nail", al-kutb al-shamaliyy "the northern axle/spindle", and al-kaukab al-shamaliyy "north star". The name Alruccabah or Ruccabah that was reported in 16th century western sources was that of the constellation.[19]

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

A typical Northern Hemisphere star trail with Polaris in the center

Because 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.[20] The celestial pole will move away from α 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.,[20] 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 ἀεί φανής (aei phanēs) "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.[21] At present, Polaris is 0.75° away from the pole of rotation (1.4 times the Moon disc) and hence revolves around the pole in a small circle 1.5° 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[22] 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]
Stellar parallax is the basis for the parsec, which is the distance from the Sun to an astronomical object which has a parallax angle of one arcsecond. (1 AU and 1 pc are not to scale, 1 pc = about 206265 AU)

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.[23][24] 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 launched in 2013 and intended to measure stellar parallax to within 20 microarcseconds (μas), with only 10% error for stars 8,000 pc (26 kly) away.[25] 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.[26]

See also


References

  1. ^ a b c d e f g h i j k l m n o Evans, N. R.; Schaefer, G. H.; Bond, H. E.; Bono, G.; Karovska, M.; Nelan, E.; Sasselov, D.; Mason, B. D. (2008). "Direct Detection of the Close Companion of Polaris with Thehubble Space Telescope". The Astronomical Journal 136 (3): 1137.  
  2. ^ a b c d e f g h i Usenko, I. A.; Klochkova, V. G. (2008). "Polaris B, an optical companion of the Polaris (α UMi) system: Atmospheric parameters, chemical composition, distance and mass". Monthly Notices of the Royal Astronomical Society: Letters 387: L1.  
  3. ^ a b c d e f g Turner; Kovtyukh; Igor Usenko; Gorlova (2012). "The Pulsation Mode of the Cepheid Polaris". arXiv:1211.6103v1 [astro-ph.SR].
  4. ^ a b c d e f Evans, N. R.; Sasselov, D. D.; Short, C. I. (2002). "Polaris: Amplitude, Period Change, and Companions". The Astrophysical Journal 567 (2): 1121.  
  5. ^ Usenko, I. A.; Miroshnichenko, A. S.; Klochkova, V. G.; Yushkin, M. V. (2005). "Polaris, the nearest Cepheid in the Galaxy: Atmosphere parameters, reddening and chemical composition". Monthly Notices of the Royal Astronomical Society 362 (4): 1219.  
  6. ^ Spreckley, S. A.; Stevens, I. R. (2008). "The period and amplitude changes of Polaris (α UMi) from 2003 to 2007 measured with SMEI". Monthly Notices of the Royal Astronomical Society: –.  
  7. ^ Cayrel de Strobel, G.; Soubiran, C.; Ralite, N. (2001). "Catalogue of [Fe/H] determinations for FGK stars: 2001 edition". Astronomy and Astrophysics 373: 159.  
  8. ^ a b c Lee, B. C.; Mkrtichian, D. E.; Han, I.; Park, M. G.; Kim, K. M. (2008). "Precise Radial Velocities of Polaris: Detection of Amplitude Growth". The Astronomical Journal 135 (6): 2240.  
  9. ^ a b Wielen; Jahreiss; Dettbarn; Lenhardt; Schwan (2000). "Polaris: Astrometric orbit, position, and proper motion". arXiv:astro-ph/0002406 [astro-ph].
  10. ^ "There's More to the North Star Than Meets the Eye". Hubblesite.org. 2006-01-09. Retrieved 2012-04-14. 
  11. ^ Hertzsprung, Ejnar (August 1911). "Nachweis der Veränderlichkeit von α Ursae Minoris". Astronomische Nachrichten (in German) 189 (6): 89.  
  12. ^ a b Turner, D. G.; Savoy, J.; Derrah, J.; Abdel‐Sabour Abdel‐Latif, M.; Berdnikov, L. N. (2005). "The Period Changes of Polaris". Publications of the Astronomical Society of the Pacific 117 (828): 207.  
  13. ^ Neilson, H. R.; Engle, S. G.; Guinan, E.; Langer, N.; Wasatonic, R. P.; Williams, D. B. (2012). "The Period Change of the Cepheid Polaris Suggests Enhanced Mass Loss". The Astrophysical Journal 745 (2): L32.  
  14. ^ Irion, R. (2004). "AMERICAN ASTRONOMICAL SOCIETY MEETING: As Inconstant as the Northern Star". Science 304 (5678): 1740–1.  
  15. ^ Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts:  
  16. ^  
  17. ^ Allen, Richard Hinckley (1969). Star Names: Their Lore and Meaning. Dover Publications Inc. (Reprint of 1899 original).  
  18. ^ occasionally also as a title of Jesus. Robert Bellarmine deprecated this use of the title: Haec appellatio stelle maris tribui solet Beate Virgini. Fortasse melius de Christo diceretur 'stella splendida et matutina' . . . . [N]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, p. 309
  19. ^ Richard Hinckley Allen, Star names: their lore and meaning (1899), p. 457.
  20. ^ a b Norton, Arthur P. (1973). Norton's Star Atlas. Edinburgh:  
  21. ^ Nathaniel Bowditch: The American Practical Navigator, 2002 Bicentennial Ed., Chapter 15 Navigational Astronomy, page 248, Figure 1530a. Navigational stars and the planets
  22. ^ A visual method to correct a ship's compass using Polaris using Ursa Major as a point of reference [1]
  23. ^ "The Hipparcos Space Astrometry Mission". Retrieved August 28, 2007. 
  24. ^ From Hipparchus to HipparcosCatherine Turon,
  25. ^ GAIA from ESA.
  26. ^ Radio Telescopes' Precise Measurements Yield Rich Scientific Payoffs

External links

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

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