Polaris, designated Alpha Ursae Minoris (α Ursae Minoris, abbreviated Alpha UMi, α UMi), commonly the North Star or Pole Star, is the brightest star in the constellation of Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star. The revised Hipparcos parallax gives a distance to Polaris of about 433 light-years (133 parsecs), while calculations by other methods derive distances around 30% closer.
Chart showing location of Polaris (α Ursae Minoris) near the North Celestial Pole.
Epoch J2000 Equinox
|α UMi Aa|
|Right ascension||02h 31m 49.09s|
|Declination||+89° 15′ 50.8″|
|Apparent magnitude (V)||1.98|
|α UMi Ab|
|Apparent magnitude (V)||9.2|
|α UMi B|
|Right ascension||02h 30m 41.63s|
|Declination||+89° 15′ 38.1″|
|Apparent magnitude (V)||8.7|
|α UMi Aa|
|U−B color index||0.38|
|B−V color index||0.60|
|Variable type||Classical Cepheid|
|α UMi Ab|
|α UMi B|
|U−B color index||0.01|
|B−V color index||0.42|
|Radial velocity (Rv)||−17 km/s|
|Proper motion (μ)||RA: 44.48±0.11 mas/yr
Dec.: −11.85±0.13 mas/yr
|Parallax (π)||7.54 ± 0.11 mas|
|Absolute magnitude (MV)||−3.6 (α UMi Aa)
3.6 (α UMi Ab)
3.1 (α UMi B)
|Primary||α UMi Aa|
|Companion||α UMi Ab|
|Period (P)||29.59 yr|
|Semi-major axis (a)||0.133″|
|Longitude of the node (Ω)||19°|
|Argument of periastron (ω)
|α UMi Aa|
|Luminosity (bolometric)||1,260 L☉|
|Surface gravity (log g)||2.2 cgs|
|Rotational velocity (v sin i)||14 km/s|
|α UMi Ab|
|Luminosity (bolometric)||3 L☉|
|α UMi B|
|Luminosity (bolometric)||3.9 L☉|
|Surface gravity (log g)||4.3 cgs|
|Rotational velocity (v sin i)||110 km/s|
|Position (relative to α UMi Aa)|
|Component||α UMi Ab|
|Epoch of observation||2005.5880|
|Position (relative to α UMi Aa)|
|Component||α UMi B|
|Epoch of observation||2005.5880|
|SIMBAD||α UMa A|
|α UMa B|
Polaris is a multiple star, comprising the main star (Polaris Aa, a yellow supergiant) in orbit with a smaller companion (Polaris Ab); the pair in orbit with Polaris B (discovered in August 1779 by William Herschel). There were once thought to be two more distant components—Polaris C and Polaris D—but these have been shown not to be physically associated with the Polaris system.
Polaris Aa is a 5.4 solar mass (M☉) F7 yellow supergiant of spectral type Ib. This is the first classical Cepheid to have a mass determined from its orbit. The two smaller companions are Polaris B, a 1.39 M☉ F3 main-sequence star orbiting at a distance of 2400 astronomical units (au), and Polaris Ab (or P), a very close F6 main-sequence star with an 18.8 au radius orbit and 1.26 M☉.
Polaris B can be seen even with a modest telescope. William Herschel discovered the star in August 1779 using a reflecting telescope of his own, one of the best telescopes of the time. By examining the spectrum of Polaris A, it was also discovered in 1929 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 nearest dwarf star is in an orbit of only 18.5 au (2.8 billion km) from Polaris A, about the distance between the Sun and Uranus), which explains why its light is swamped by its close and much brighter companion.
Polaris A, the supergiant primary component, is a low-amplitude Population I classical Cepheid variable, although it was once thought to be a type II Cepheid due to its high galactic latitude. Cepheids constitute an important standard candle for determining distance, so 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.
The range of brightness of Polaris during its pulsations is given as 1.86–2.13, but the amplitude has 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 amplitude is now increasing again, a reversal not seen in any other Cepheid.
The period, roughly 4 days, has also changed over time. It has steadily increased by around 4.5 seconds per year except for a hiatus in 1963–1965. This was originally thought to be due to secular redward evolution across the Cepheid instability strip, but it may be due to interference between the primary and the first-overtone pulsation modes. Authors disagree on whether Polaris is a fundamental or first-overtone pulsator and on whether it is crossing the instability strip for the first time or not.
The temperature of Polaris varies by only a small amount during its pulsations, but the amount of this variation is variable and unpredictable. The erratic changes of temperature and the amplitude of temperature changes during each cycle, from less than 50 K to at least 170 K, may be related to the orbit with Polaris Ab.
Research reported in Science suggests that Polaris is 2.5 times brighter today than when Ptolemy observed it, changing from third to second magnitude. Astronomer Edward Guinan considers this to be a remarkable 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".
The modern name Polaris is shortened from New Latin stella polaris "polar star", coined in the Renaissance era, when the star had approached the celestial pole to within a few degrees. Gemma Frisius, writing in 1547, referred to it as stella illa quae polaris dicitur ("that star which is called 'polar'"), placing it 3° 7' from the celestial pole.
In antiquity, Polaris was not yet the closest naked-eye star to the celestial pole, and the entire constellation of Ursa Minor was used for navigation rather than any single star. Polaris moved close enough to the pole to be the closest naked-eye star, even though still at a distance of several degrees, in the early medieval period, and numerous names referring to this characteristic as polar star have been in use since the medieval period. In Old English, it was known as 'scip-steorra ("ship-star"); In the Old English rune poem, the T-rune is apparently associated with "a circumpolar constellation", compared to the quality of steadfastness or honour.
In the Hindu Puranas, it became personified under the name Dhruva ("immovable, fixed"). In the later medieval period, it became associated with the Marian title of Stella Maris "Star of the Sea" (so in Bartholomeus Anglicus, c. 1270s) An older English name, attested since the 14th century, is lodestar "guiding star", cognate with the Old Norse leiðarstjarna, Middle High German leitsterne.
The ancient name of the constellation Ursa Minor, Cynosura (from the Greek κυνόσουρα "the dog’s tail" ) became associated with the pole star in particular by the early modern period. An explicit identification of Mary as stella maris with the polar star (Stella Polaris), as well as the use of Cyonsura as a name of the star, is evident in the title Cynosura seu Mariana Stella Polaris (i.e. "Cynosure, or the Marian Polar Star"), a collection of Marian poetry published by Nicolaus Lucensis (Niccolo Barsotti de Lucca) in 1655.
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.
It as invoked as a symbol of steadfastness in poetry, as "steadfast star" by Spenser. 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." In Julius Caesar, he has Caesar explain his refusal to grant a pardon by saying, "I am as constant as the northern star/Of whose true-fixed and resting quality/There is no fellow in the firmament./The skies are painted with unnumbered sparks,/They are all fire and every one doth shine,/But there’s but one in all doth hold his place;/So in the world" (III, i, 65-71). Of course, Polaris will not "constantly" remain as the north star due to precession, but this is only noticeable over centuries.
Role as pole starEdit
Because Polaris 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 2750 BC, and during classical antiquity it was closer to Kochab (β UMi) than to Polaris. It was about the same angular distance from β UMi as 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), and it could reasonably be described as stella polaris from about the High Middle Ages.
In more recent history, in Shakespeare's play Julius Caesar, written around 1599, Caesar describes himself as being "as constant as the northern star", though in Caesar's time there was no constant northern star. It was referenced in Nathaniel Bowditch's 1802 book, American Practical Navigator, where it is listed as one of the navigational stars. 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 eastward or westward, and the 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.
Many recent papers calculate the distance to Polaris at about 433 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 may be up to 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.
|Year||Distance, ly (pc)||Notes|
|433 ly (133 pc)||Hipparcos|
|2006||330 ly (101 pc)||Turner|
|2008||359 ly (110 pc)||Usenko & Klochkova|
|2013||323 ly (99 pc)||Turner, et al.|
|2014||>=385 ly (>=118 pc)||Neilson|
|2015||346 ly (106 pc)||Fadeyev|
The Hipparcos spacecraft used stellar parallax to take measurements from 1989 and 1993 with the accuracy of 0.97 milliarcseconds (970 microarcseconds), and it obtained accurate measurements for stellar distances up to 1,000 pc away. The Hipparcos data was examined again with more advanced error correction and statistical techniques. 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 Cepheids like Polaris. The Hipparcos reduction specifically for Polaris has been re-examined and reaffirmed but there is still not widespread agreement about the distance.
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 25 microarcseconds (μas). It was expected that Gaia would 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. Gaia was launched in 2013 and began its mission to record data
Although it was originally planned to limit Gaia's observations to stars fainter than magnitude 5.7, tests carried out during the commissioning phase indicated that Gaia could autonomously identify stars as bright as magnitude 3. When Gaia entered regular scientific operations in July 2014, it was configured to routinely process stars in the magnitude range 3 – 20. Beyond that limit, special procedures are used to download raw scanning data for the remaining 230 stars brighter than magnitude 3; methods to reduce and analyse these data are being developed; and it is expected that there will be "complete sky coverage at the bright end" with standard errors of "a few dozen µas".
Determining an accurate distance to Polaris is important for the cosmic distance ladder because, until new data comes, it is the only Cepheid variable for which precision distance data exists, which has a ripple effect on distance measurements that use this "ruler".
History of observationsEdit
Polaris in stellar catalogues and atlases
- 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 The Hubble Space Telescope". The Astronomical Journal. 136 (3): 1137. Bibcode:2008AJ....136.1137E. arXiv: . doi:10.1088/0004-6256/136/3/1137.
- 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. Bibcode:2008AJ....135.2240L. arXiv: . doi:10.1088/0004-6256/135/6/2240.
- Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007–2013)". VizieR On-line Data Catalog: B/gcvs. Originally published in: 2009yCat....102025S. 1: 02025. Bibcode:2009yCat....102025S.
- 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. Bibcode:2008MNRAS.387L...1U. arXiv: . doi:10.1111/j.1745-3933.2008.00426.x.
- Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. Bibcode:2007A&A...474..653V. arXiv: . doi:10.1051/0004-6361:20078357.
- Turner, D. G.; Kovtyukh, V. V.; Usenko, I. A.; Gorlova, N. I. (2013). "The Pulsation Mode of the Cepheid Polaris". The Astrophysical Journal Letters. 762: L8. Bibcode:2013ApJ...762L...8T. arXiv: . doi:10.1088/2041-8205/762/1/L8.
- Fadeyev, Y. A. (2015). "Evolutionary status of Polaris". Monthly Notices of the Royal Astronomical Society. 449: 1011. Bibcode:2015MNRAS.449.1011F. arXiv: . doi:10.1093/mnras/stv412.
- 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.
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- Evans, Nancy Remage; Guinan, Edward; Engle, Scott; Wolk, Scott J.; Schlegel, Eric; Mason, Brian D.; Karovska, Margarita; Spitzbart, Bradley (2010). "Chandra Observation of Polaris: Census of Low-mass Companions". The Astronomical Journal. 139 (5): 1968. Bibcode:2010AJ....139.1968E. doi:10.1088/0004-6256/139/5/1968.
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- 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.
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Around 4800 years ago Thuban (α Draconis) lay a mere 0°.1 from the pole. Deneb (α Cygni) will be the brightest star near the pole in about 8000 years' time, at a distance of 7°.5.
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