Polaris

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Alpha Ursae Minoris
Polaris alpha ursae minoris.jpg
Polaris as seen by the Hubble Space Telescope.
Observation data
Epoch J2000      Equinox J2000
ConstellationUrsa Minor
Right ascension02h 31m 49.09s [1]
Declination+89° 15′ 50.8″ [1]
Apparent magnitude (V)1.985[2] / 8.5[2] / 8.21
Characteristics
Spectral typeF7 Ib-II[2] / F0V[2] / F3V
U−B color index0.38
B−V color index0.60
Variable typeCepheid variable
Astrometry
Radial velocity (Rv)-17 km/s
Proper motion (μ)RA: 44.48 ± 0.11 [1] mas/yr
Dec.: -11.85 ± 0.13 [1] mas/yr
Parallax (π)7.54 ± 0.11[1] mas
Distance433 ± 6 ly
(133 ± 2 pc)
Absolute magnitude (MV)-3.63 ± 0.14[2] (-3.62 / 2.9)
Details
α UMi A
Mass6.0 ± 0.5[2] M
Radius46 ± 3[3] R
Luminosity2200 L
Temperature7200 K
Metallicity112% solar[4]
Rotation~17 km/s
Age7×107[2] years
α UMi B
Mass1.38 ± 0.61[5] M
Luminosity3.868 L
Temperature6900 K
Orbit[2]
Primaryα UMi A
Period (P)29.59 ± 0.02 yr
Semimajor axis (a)142 ± 21 m
Eccentricity (e)0.608 ± 0.005
Inclination (i)130.2 ± 4.8°
Longitude of the node (Ω)167.1 ± 9.4°
Periastron epoch (T)1987.66 ± 0.13
Argument of periastron (ω)
(secondary)
303.01 ± 0.75°
Other designations
Polaris, North Star, 1 Ursae Minoris, HR 424, BD +88°8, HD 8890, SAO 308, FK5 907, GC 2243, ADS 1477, CCDM 02319+8915, HIP 11767, Cynosura, Alruccabah, Phoenice, Navigatoria, Star of Arcady, Yilduz, Mismar
 
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Alpha Ursae Minoris
Polaris alpha ursae minoris.jpg
Polaris as seen by the Hubble Space Telescope.
Observation data
Epoch J2000      Equinox J2000
ConstellationUrsa Minor
Right ascension02h 31m 49.09s [1]
Declination+89° 15′ 50.8″ [1]
Apparent magnitude (V)1.985[2] / 8.5[2] / 8.21
Characteristics
Spectral typeF7 Ib-II[2] / F0V[2] / F3V
U−B color index0.38
B−V color index0.60
Variable typeCepheid variable
Astrometry
Radial velocity (Rv)-17 km/s
Proper motion (μ)RA: 44.48 ± 0.11 [1] mas/yr
Dec.: -11.85 ± 0.13 [1] mas/yr
Parallax (π)7.54 ± 0.11[1] mas
Distance433 ± 6 ly
(133 ± 2 pc)
Absolute magnitude (MV)-3.63 ± 0.14[2] (-3.62 / 2.9)
Details
α UMi A
Mass6.0 ± 0.5[2] M
Radius46 ± 3[3] R
Luminosity2200 L
Temperature7200 K
Metallicity112% solar[4]
Rotation~17 km/s
Age7×107[2] years
α UMi B
Mass1.38 ± 0.61[5] M
Luminosity3.868 L
Temperature6900 K
Orbit[2]
Primaryα UMi A
Period (P)29.59 ± 0.02 yr
Semimajor axis (a)142 ± 21 m
Eccentricity (e)0.608 ± 0.005
Inclination (i)130.2 ± 4.8°
Longitude of the node (Ω)167.1 ± 9.4°
Periastron epoch (T)1987.66 ± 0.13
Argument of periastron (ω)
(secondary)
303.01 ± 0.75°
Other designations
Polaris, North Star, 1 Ursae Minoris, HR 424, BD +88°8, HD 8890, SAO 308, FK5 907, GC 2243, ADS 1477, CCDM 02319+8915, HIP 11767, Cynosura, Alruccabah, Phoenice, Navigatoria, Star of Arcady, Yilduz, Mismar

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. It is very close to the north celestial pole, making it the current northern pole star.

Based on measurements from the Hipparcos astrometry satellite,[6][7] Polaris is estimated to be at a distance of about 434 light-years from Earth.[1] It is a multiple star, consisting 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.

Contents

Star system

This artist's concept shows two class F stars: supergiant Polaris A and its distant companion Polaris B[8]

α UMi A is a six solar mass[2] F8 supergiant (Ib) or bright giant (II). The two smaller companions are: α UMi B, a 1.39 solar mass F3 main sequence star [5] orbiting at a distance of 2400 AU, and α UMi Ab (or P), a very close dwarf with an 18.8 AU radius orbit and mass of roughly 1.5 solar masses [2]. There are also two distant components α UMi C and α UMi D.[2]

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 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,[9] about the distance from our Sun to Uranus) from Polaris A, explaining why its light is swamped by its close and much brighter companion.[10]

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.[11] Around 1900,[citation needed] 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.[citation needed]

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.[12] 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

A typical star trail with Polaris in the center

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.[13][14] 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).[15] 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.[16]

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

Because in the current era[17] 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.[17] 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.[17], 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.[18] 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[19] 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.

See also


Notes

  1. ^ a b c d e f van Leeuwen, F. (2007). "HIP 11767". Hipparcos, the New Reduction. http://webviz.u-strasbg.fr/viz-bin/VizieR-5?-out.add=.&-source=I/311/hip2&recno=11739. Retrieved 2011-03-01. 
  2. ^ a b c d e f g h i j k Wielen, R.; et al. (2000). "Polaris: astrometric orbit, position, and proper motion". Astronomy and Astrophysics 360: 399-410. arXiv:astro-ph/0002406. Bibcode 2000A&A...360..399W. 
  3. ^ Nordgren, Tyler E. et al. (December 1999), "Stellar Angular Diameters of Late-Type Giants and Supergiants Measured with the Navy Prototype Optical Interferometer", The Astronomical Journal 118 (6): 3032–3038, Bibcode 1999AJ....118.3032N, doi:10.1086/301114 
  4. ^ Cayrel de Strobel, G.; Soubiran, C.; Ralite, N. (2001). " Catalogue of [Fe/H] determinations for FGK stars: 2001 edition ". Astronomy & Astrophysics 373: 159–163. arXiv:astro-ph/0106438. Bibcode 2001A&A...373..159C. doi:10.1051/0004-6361:20010525. 
  5. ^ a b "Polaris B, an optical companion of the Polaris (α UMi) system: atmospheric parameters, chemical composition, distance and mass". Royal Astronomical Society. 2008-04-29. http://onlinelibrary.wiley.com/doi/10.1111/j.1745-3933.2008.00426.x/abstract. Retrieved 2012-08-15. 
  6. ^ Perryman, M. A. C.; Lindegren, L.; Kovalevsky, J.; et al. (July 1997), "The Hipparcos Catalogue", Astronomy and Astrophysics 323: L49–L52, Bibcode 1997A&A...323L..49P 
  7. ^ Perryman, Michael (2010), The Making of History's Greatest Star Map, Heidelberg: Springer-Verlag, doi:10.1007/978-3-642-11602-5 
  8. ^ "SIMBAD Object query : CCDM J02319+8915". Centre de Données astronomiques de Strasbourg. http://simbad.u-strasbg.fr/simbad/sim-id?Ident=CCDM+J02319%2B8915&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id. Retrieved 2010-06-10. 
  9. ^ "There's More to the North Star Than Meets the Eye". Hubblesite.org. 2006-01-09. http://hubblesite.org/newscenter/archive/releases/2006/02/fastfacts/. Retrieved 2012-04-14. 
  10. ^ Evans, N. R.; Schaefer, G.; Bond, H.; Bono, G.; Karovska, M.; Nelan, E.; Sasselov, D. (January 9, 2006). "Direct detection of the close companion of Polaris with the Hubble Space Telescope". American Astronomical Society 207th Meeting. http://www.aas.org/publications/baas/v37n4/aas207/1130.htm. 
  11. ^ Hertzsprung, Ejnar (August 1911). "Nachweis der Veränderlichkeit von α Ursae minoris" (in German). Astronomische Nachrichten 189: 89. Bibcode 1911AN....189...89H. doi:10.1002/asna.19111890602. 
  12. ^ Irion, R. (2004). "AMERICAN ASTRONOMICAL SOCIETY MEETING: As Inconstant as the Northern Star". Science 304 (5678): 1740–1. doi:10.1126/science.304.5678.1740b. PMID 15205508. http://www.sciencemag.org/content/304/5678/1740.2.full. 
  13. ^  Chisholm, Hugh, ed. (1911). "Cynosure". Encyclopædia Britannica (11th ed.). Cambridge University Press. 
  14. ^ Allen, Richard Hinckley (1969). Star Names: Their Lore and Meaning. Dover Publications Inc. (Reprint of 1899 original). ISBN 0-486-21079-0. 
  15. ^ 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' ... [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
  16. ^ Richard Hinckley Allen, Star names: their lore and meaning (1899), p. 457.
  17. ^ a b c Norton, Arthur P. (1973). Norton's Star Atlas. Edinburgh: Sky Publishing. p. 10. ISBN 0-85248-900-5. "4500 years ago it was Thuban (α Draconis); 8000 years hence it will be Deneb" 
  18. ^ Nathaniel Bowditch: The American Practical Navigator, 2002 Bicentennial Ed., Chapter 15 Navigational Astronomy, page 248, Figure 1530a. Navigational stars and the planets
  19. ^ A visual method to correct a ship's compass using Polaris using Ursa Major as a point of reference [1]

External links

Preceded by
Kochab & Pherkad
Pole Star
500–3000
Succeeded by
Alrai

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