Polaris is astar in the northerncircumpolar constellation ofUrsa Minor. It is designatedα Ursae Minoris (Latinized toAlpha Ursae Minoris) and is commonly called theNorth Star orPole Star. With anapparent magnitude that fluctuates around 1.98,[3] it is the brightest star in the constellation and is readily visible to thenaked eye at night.[15] The position of the star lies less than1° away from the northcelestial pole, making it the current northernpole star. The stable position of the star in theNorthern Sky makes it useful fornavigation.[16]
Although appearing to the naked eye as a single point of light, Polaris is a triplestar system, composed of the primary, ayellow supergiant designated Polaris Aa, in orbit with a smaller companion, Polaris Ab; the pair is in a wider orbit with Polaris B. The outer pair AB were discovered in August 1779 byWilliam Herschel, where the 'A' refers to what is now known to be the Aa/Ab pair.
Polaris Aa is anevolvedyellow supergiant ofspectral type F7Ib with 5.4solar masses (M☉). It is the first classicalCepheid to have a mass determined from its orbit. The two smaller companions are Polaris B, a 1.39 M☉ F3main-sequence star orbiting at a distance of2,400 astronomical units (AU),[17] and Polaris Ab (or P), a very close F6 main-sequence star with a mass of 1.26 M☉.[3] Polaris B can be resolved with a modest telescope. William Herschel discovered the star in August 1779 using areflecting telescope of his own,[18] one of the best telescopes of the time. In January 2006,NASA released images, from theHubble telescope, that showed the three members of the Polaris ternary system.[19][20]
The variableradial velocity of Polaris A was reported byW. W. Campbell in 1899, which suggested this star is a binary system.[21] Since Polaris A is a known cepheid variable,J. H. Moore in 1927 demonstrated that the changes in velocity along the line of sight were due to a combination of the four-daypulsation period combined with a much longerorbital period and a largeeccentricity of around 0.6.[22] Moore published preliminaryorbital elements of the system in 1929, giving anorbital period of about 29.7 years with an eccentricity of 0.63. This period was confirmed byproper motion studies performed byB. P. Gerasimovič in 1939.[23]
As part of her doctoral thesis, in 1955E. Roemer used radial velocity data to derive an orbital period of 30.46 y for the Polaris A system, with an eccentricity of 0.64.[24]K. W. Kamper in 1996 produced refined elements with a period of29.59±0.02 years and an eccentricity of0.608±0.005.[25] In 2019, a study by R. I. Anderson gave a period of29.32±0.11 years with an eccentricity of0.620±0.008.[10]
There were once thought to be two more widely separated components—Polaris C and Polaris D—but these have been shown not to be physically associated with the Polaris system.[17][26]
The range of brightness of Polaris is given as 1.86–2.13,[4] 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.[6]
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 (a long term change inredshift that causes light to stretch into longer wavelengths, causing it to appear red) evolution across the Cepheidinstability strip, but it may be due to interference between the primary and the first-overtone pulsation modes.[20][29][30] 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.[11][30][31]
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.[12]
Research reported inScience suggests that Polaris is 2.5 times brighter today than whenPtolemy observed it, changing from third to second magnitude.[32] AstronomerEdward 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 ofstellar evolution".
In 2024, researchers led by Nancy Evans at theHarvard & Smithsonian, have studied with more accuracy the Polaris' smaller companion orbit using theCHARA Array. During this observation campaign they have succeeded in shooting Polaris features on its surface; large bright places and dark ones have appeared in close-up images, changing over time. Further, Polaris diameter size has been re-measured to 46 R☉, using theGaia distance of446±1 light-years, and its mass was determined at 5.13 M☉.[9]
Polaris azimuths vis clock face analogy.[33]A typical Northern Hemispherestar trail with Polaris in the center.Polaris lying halfway between theasterismsCassiopeia and theBig Dipper.
Because Polaris lies nearly in a direct line with theEarth's rotational axis "above" theNorth 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 forcelestial navigation and forastrometry. The elevation of the star above the horizon gives the approximatelatitude of the observer.[15]
In 2018 Polaris was 0.66° (39.6 arcminutes) away from the pole of rotation (1.4 times theMoon disc) and so revolves around the pole in a small circle 1.3° in diameter. It will be closest to the pole (about 0.45 degree, or 27 arcminutes) soon after the year 2100.[34] Because it is so close to the celestial north pole, itsright ascension is changing rapidly due to theprecession of Earth's axis, going from 2.5h in AD 2000 to 6h in AD 2100. Twice in eachsidereal day Polaris'sazimuth is true north; the rest of the time it is displaced eastward or westward, and the bearing must be corrected using tables or arule of thumb. The best approximation[33] is made using the leading edge of the "Big Dipper"asterism in the constellation Ursa Major. The leading edge (defined by the starsDubhe andMerak) is referenced to a clock face, and the true azimuth of Polaris worked out for different latitudes.
The celestial pole was close toThuban around 2750 BCE,[35] and duringclassical antiquity it was slightly closer toKochab (β UMi) than to Polaris, although still about10° from either star.[36] It was about the same angular distance from β UMi as to α UMi by the end oflate antiquity. The Greek navigatorPytheas 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" byStobaeus (5th century), also termed Λύχνος (Lychnos) akin to a burner or lamp and would reasonably be described asstella polaris from about theHigh Middle Ages and onwards, both in Greek and Latin. On his first trans-Atlantic voyage in 1492,Christopher Columbus had to correct for the "circle described by the pole star about the pole".[37] InShakespeare's playJulius Caesar, written around 1599, Caesar describes himself as being "as constant as the northern star", although in Caesar's time there was no constant northern star. Despite its relative brightness, it is not, as is popularly believed, the brightest star in the sky.[38]
This artist's concept shows: supergiant Polaris Aa, dwarf Polaris Ab, and the distant dwarf companion Polaris B.
The modern namePolaris[40] is shortened from theNeo-Latinstella polaris ("polar star"), coined in the Renaissance when the star had approached the celestial pole to within a few degrees.[41][42]
Gemma Frisius, writing in 1547, referred to it asstella illa quae polaris dicitur ("that star which is called 'polar'"), placing it 3° 8' from the celestial pole.[41][42]
In 2016, theInternational Astronomical Union organized aWorking Group on Star Names (WGSN)[43] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which includedPolaris for the star α Ursae Minoris Aa.[44]
In antiquity, Polaris was not yet the closest naked-eye star to the celestial pole, and the entire constellation ofUrsa 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 aspolar star have been in use since the medieval period. In Old English, it was known asscip-steorra ("ship-star").[citation needed]
In the "Old English rune poem", theT-rune is apparently associated with "a circumpolar constellation", or the planet Mars.[45]
In the HinduPuranas, it became personified under the nameDhruva ("immovable, fixed").[46]
An older English name, attested since the 14th century, islodestar "guiding star", cognate with the Old Norseleiðarstjarna, Middle High Germanleitsterne.[49]
The ancient name of the constellation Ursa Minor,Cynosura (from the Greekκυνόσουρα "the dog's tail"),[50] became associated with the pole star in particular by the early modern period. An explicit identification ofMary asstella maris with the polar star (Stella Polaris), as well as the use ofCynosura as a name of the star, is evident in the titleCynosura 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.[citation needed]
Ursa Minor as depicted in the 964 Persian workBook of Fixed Stars, Polaris namedal-Judayy "الجدي" in the lower right.
Its name in traditional pre-Islamic Arab astronomy wasal-Judayy الجدي ("the kid", in the sense of a juvenilegoat ["le Chevreau"] in Description des Etoiles fixes),[51] and that name was used inmedieval Islamic astronomy as well.[52][53] In those times, it was not yet as close to the north celestial pole as it is now, and used to rotate around the pole.[citation needed]
It was invoked as a symbol of steadfastness in poetry, as "steadfast star" bySpenser.Shakespeare'ssonnet 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."[citation needed]
InJulius Caesar, Shakespeare hasCaesar explain his refusal to grant a pardon: "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 toprecession, but this is only noticeable over centuries.[citation needed]
In traditionalLakota star knowledge, Polaris is named "Wičháȟpi Owáŋžila". This translates to "The Star that Sits Still". This name comes from aLakota story in which he married Tȟapȟúŋ Šá Wíŋ, "Red Cheeked Woman". However, she fell from the heavens, and in his grief Wičháȟpi Owáŋžila stared down from "waŋkátu" (the above land) forever.[54]
In the ancient Finnish worldview, the North Star has also been calledtaivaannapa andnaulatähti ("the nailstar") because it seems to be attached to the firmament or even to act as a fastener for the sky when other stars orbit it. Since the starry sky seemed to rotate around it, the firmament is thought of as a wheel, with the star as the pivot on its axis. The names derived from it weresky pin andworld pin.[citation needed]
Stellar parallax is the basis for theparsec, which is the distance from theSun to anastronomical object which has aparallax angle of onearcsecond. (1AU and 1pc are not to scale, 1 pc = about 206265 AU)
Many recent papers calculate the distance to Polaris at about 433light-years (133 parsecs),[20] based on parallax measurements from theHipparcos astrometry satellite. Older distance estimates were often slightly less, and research based on high resolution spectral analysis suggests it may be up to 110 light years closer (323 ly/99 pc).[57] Polaris is the closestCepheid variable to Earth so its physical parameters are of critical importance to the wholeastronomical distance scale.[57] It is also the only one with a dynamically measured mass.
A New revision of observations from 1989 to 1993, first published in 1997
B Statistical distance calculated using a weak distance prior
TheHipparcos spacecraft usedstellar 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.[61] The Hipparcos data was examined again with more advanced error correction and statistical techniques.[2] Despite the advantages of Hipparcosastrometry, the uncertainty in its Polaris data has been pointed out and some researchers have questioned the accuracy of Hipparcos when measuring binary Cepheids like Polaris.[57] The Hipparcos reduction specifically for Polaris has been re-examined and reaffirmed but there is still not widespread agreement about the distance.[62]
The next major step in high precision parallax measurements comes fromGaia, a space astrometry mission launched in 2013 and intended to measure stellar parallax to within 25 microarcseconds (μas).[63] 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.[64] 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".[65]Gaia Data Release 2 does not include a parallax for Polaris, but a distance inferred from it is136.6±0.5 pc (445.5 ly) for Polaris B,[60] somewhat further than most previous estimates and several times more accurate. This was further improved to137.2±0.3 pc (447.6 ly), upon publication of theGaia Data Release 3 catalog on 13 June 2022 which superseded Gaia Data Release 2.[5]
^abcNeilson, H. R.; Blinn, H. (2021).The Curious Case of the North Star: The Continuing Tension Between Evolution Models and Measurements of Polaris. RR Lyrae/Cepheid 2019: Frontiers of Classical Pulsators. Vol. 529. p. 72.arXiv:2003.02326.Bibcode:2021ASPC..529...72N.
^Wyller, A. A. (December 1957). "Parallax and orbital motion of spectroscopic binary Polaris from photographs taken with the 24-inch Sproul refractor".Astronomical Journal.62:389–393.Bibcode:1957AJ.....62..389W.doi:10.1086/107559.
^Kamper, Karl W. (June 1996). "Polaris Today".Journal of the Royal Astronomical Society of Canada.90: 140.Bibcode:1996JRASC..90..140K.
^Meeus, J. (1990). "Polaris and the North Pole".Journal of the British Astronomical Association.100: 212.Bibcode:1990JBAA..100..212M.
^abRidpath, Ian, ed. (2004).Norton's Star Atlas. New York: Pearson Education. p. 5.ISBN978-0-13-145164-3.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.
^Columbus, Ferdinand (1960).The Life of the Admiral Christopher Columbus by His Son Fredinand. Translated byKeen, Benjamin. London: Folio Society. p. 74.
^Bowditch, Nathaniel; National Imagery and Mapping Agency (2002)."15".The American practical navigator : an epitome of navigation. Paradise Cay Publications. p. 248.ISBN978-0-939837-54-0.
^abKunitzsch, 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:Sky Publishing. p. 23.ISBN978-1-931559-44-7.
^Dickins, Bruce (1915).Runic and heroic poems of the old Teutonic peoples. p. 18.; Dickins' "a circumpolar constellation" is attributed to L. Botkine,La Chanson des Runes (1879).
^Martín-Fleitas, J.; Sahlmann, J.; Mora, A.; Kohley, R.; Massart, B.; l'Hermitte, J.; Le Roy, M.; Paulet, P. (2014). Oschmann, Jacobus M; Clampin, Mark; Fazio, Giovanni G; MacEwen, Howard A (eds.). "Enabling Gaia observations of naked-eye stars".Space Telescopes and Instrumentation 2014: Optical. Space Telescopes and Instrumentation 2014: Optical, Infrared, and Millimeter Wave.9143: 91430Y.arXiv:1408.3039.Bibcode:2014SPIE.9143E..0YM.doi:10.1117/12.2056325.S2CID119112009.
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