Epoch J2000 Equinox J2000
|Component||α UMi Aa||α UMi Ab||α UMi B|
|Right ascension||02h 31m 49.09s||02h 30m 41.63s|
|Declination||+89° 15′ 50.8″||+89° 15′ 38.1″|
|Apparent magnitude (V)||1.98v||9.2||8.7|
|U-B color index||0.38||0.01|
|B-V color index||0.60||0.42|
|Variable type||Classical Cepheid|
|Radial velocity (Rv)||-17 km/s|
|RA (μα cos δ)||44.48±0.11 mas/yr|
|Dec. (μδ)||-11.85±0.13 mas/yr|
|Parallax (π)||7.54 ± 0.11 mas|
|Distance||325 - 425 ly
(99 - 129.5 pc)
|Absolute magnitude (MV)||-3.6||3.6||3.1|
|Mass||4.5 M☉||1.26 M☉||1.39 M☉|
|Radius||46±3 R☉||1.38 R☉|
|Surface gravity (log g)||2.2||4.3|
|Luminosity (bolometric)||2500 L☉||3 L☉||3.9 L☉|
|Temperature||6015 K||6900 K|
|Rotational velocity (v sin i)||14km/s||110km/s|
|Period (P)||29.59 years||~100,000 years|
|Semimajor axis (a)||0.133"||32" (0.02 pc)|
|Longitude of node (Ω)||19°|
|Argument of periastron (ω)
|Position (relative to Aa)|
|Epoch of observation||2005.5880||2005.5880|
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.
It is a multiple star, consisting of the main star α UMi Aa, 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). 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.
History of observations
Polaris in stellar catalogues and atlases
|Ulugh Beg (1437)||Yes|
|De Houtman (1603)||No|
α 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: α UMi B, a 1.39 solar mass 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 solar masses. There are also two distant components α UMi C and α UMi D.
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 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, 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 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.
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--around 3.97 days--is now increasing. 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.
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. 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."
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. 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). 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.
In the Old English rune poem, the T-rune is identified with Tyr "fame, honour", which is compared to the pole star, ᛏ [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 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. 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., 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 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. 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 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.
|Year||Distance, ly (pc)||Notes|
|433 ly (133 pc)||Hipparcos|
|2006||330 ly (101 pc)||Turner|
|2008||359 ly (110 pc)||Usenko & Klochkova|
|2012||323 ly (99 pc)||Turner, et al.|
Many recent papers calculate the distance to Polaris at about 434 light-years (133 parsecs), 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). Polaris is the closest Cepheid variable to Earth so its physical parameters are of critical importance to the whole astronomical distance scale. 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. 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.
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 distances to within 20 microarcseconds (μas), with only 10% error for stars 8,000 pc (26 kly) away. 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.
- 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. arXiv:0806.4904. Bibcode:2008AJ....136.1137E. doi:10.1088/0004-6256/136/3/1137.
- 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. arXiv:0708.0333. Bibcode:2008MNRAS.387L...1U. doi:10.1111/j.1745-3933.2008.00426.x.
- Turner; Kovtyukh; Igor Usenko; Gorlova (2012). "The Pulsation Mode of the Cepheid Polaris". arXiv:1211.6103v1 [astro-ph.SR].
- Evans, N. R.; Sasselov, D. D.; Short, C. I. (2002). "Polaris: Amplitude, Period Change, and Companions". The Astrophysical Journal 567 (2): 1121. Bibcode:2002ApJ...567.1121E. doi:10.1086/338583.
- 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. Bibcode:2005MNRAS.362.1219U. doi:10.1111/j.1365-2966.2005.09353.x.
- 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: –. arXiv:0805.1165. Bibcode:2008MNRAS.388.1239S. doi:10.1111/j.1365-2966.2008.13439.x.
- Cayrel de Strobel, G.; Soubiran, C.; Ralite, N. (2001). "Catalogue of [Fe/H] determinations for FGK stars: 2001 edition". Astronomy and Astrophysics 373: 159. arXiv:astro-ph/0106438. Bibcode:2001A&A...373..159C. doi:10.1051/0004-6361:20010525.
- 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. arXiv:0804.2793. Bibcode:2008AJ....135.2240L. doi:10.1088/0004-6256/135/6/2240.
- Wielen; Jahreiss; Dettbarn; Lenhardt; Schwan (2000). "Polaris: Astrometric orbit, position, and proper motion". arXiv:astro-ph/0002406 [astro-ph].
- "There's More to the North Star Than Meets the Eye". Hubblesite.org. 2006-01-09. Retrieved 2012-04-14.
- Hertzsprung, Ejnar (August 1911). "Nachweis der Veränderlichkeit von α Ursae Minoris". Astronomische Nachrichten (in German) 189 (6): 89. Bibcode:1911AN....189...89H. doi:10.1002/asna.19111890602.
- 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. Bibcode:2005PASP..117..207T. doi:10.1086/427838.
- 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. arXiv:1201.0761. Bibcode:2012ApJ...745L..32N. doi:10.1088/2041-8205/745/2/L32.
- 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.
- Chisholm, Hugh, ed. (1911). "Cynosure". Encyclopædia Britannica (11th ed.). Cambridge University Press
- Allen, Richard Hinckley (1969). Star Names: Their Lore and Meaning. Dover Publications Inc. (Reprint of 1899 original). ISBN 0-486-21079-0.
- 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
- Richard Hinckley Allen, Star names: their lore and meaning (1899), p. 457.
- 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"
- Nathaniel Bowditch: The American Practical Navigator, 2002 Bicentennial Ed., Chapter 15 Navigational Astronomy, page 248, Figure 1530a. Navigational stars and the planets
- A visual method to correct a ship's compass using Polaris using Ursa Major as a point of reference 
- "The Hipparcos Space Astrometry Mission". Retrieved August 28, 2007.
- Catherine Turon, From Hipparchus to Hipparcos
- GAIA from ESA.
- Radio Telescopes' Precise Measurements Yield Rich Scientific Payoffs
Kochab & Pherkad