Wikipedia

Polaris

This article is about the Earth's current north star. For such stars in general, seepole star.For other uses, seePolaris (disambiguation)andNorth Star (disambiguation).
"Stella Polaris" redirects here. For the military operation, seeOperation Stella Polaris.

Polarisis astarin the northerncircumpolar constellationofUrsa Minor.It is designatedα Ursae Minoris(LatinizedtoAlpha Ursae Minoris) and is commonly called theNorth StarorPole Star.With anapparent magnitudethat fluctuates around 1.98,[1]it is the brightest star in the constellation and is readily visible to thenaked eyeat night.[16]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 Skymakes it useful fornavigation.[17]

Polaris

Surface image of Polaris imaged by the CHARA Array showing star spots[1]
Observation data
EpochJ2000Equinox
Constellation Ursa Minor
Pronunciation /pəˈlɛərɪs,-ˈlær-/;
UK:/pəˈlɑːrɪs/[2]
α UMi A
Right ascension 02h31m49.09s[3]
Declination +89° 15′ 50.8″[3]
Apparent magnitude(V) 1.98[1](1.86 – 2.13)[4]
α UMi B
Right ascension 02h30m41.63s[5]
Declination +89° 15′ 38.1″[5]
Apparent magnitude(V) 8.7[1]
Characteristics
α UMi A
Spectral type F7Ib + F6[6]
U−Bcolor index 0.38[1]
B−Vcolor index 0.60[1]
Variable type Classical Cepheid[4]
α UMi B
Spectral type F3V[1]
U−Bcolor index 0.01[7]
B−Vcolor index 0.42[7]
Variable type suspected[4]
Astrometry
Radial velocity(Rv)−17[8]km/s
Proper motion(μ)RA:198.8±0.20[3]mas/yr
Dec.:−15±0.30[3]mas/yr
Parallax(π)7.54 ± 0.11mas[3]
Distance323–433[9]ly
(99–133[9]pc)
Absolute magnitude(MV)−3.6 (α UMi Aa)[1]
3.6 (α UMi Ab)[1]
3.1 (α UMi B)[1]
Position (relative to α UMi Aa)
Componentα UMi Ab
Epoch of observation2005.5880
Angular distance0.172″
Position angle231.4°
Position (relative to α UMi Aa)
Componentα UMi B
Epoch of observation2005.5880
Angular distance18.217″
Position angle230.540°
Orbit[10]
Primaryα UMi Aa
Companionα UMi Ab
Period(P)29.416±0.028yr
Semi-major axis(a)0.12955±0.00205"
(≥2.90±0.03AU[11])
Eccentricity(e)0.6354±0.0066
Inclination(i)127.57±1.22°
Longitude of the node(Ω)201.28±1.18°
Periastronepoch(T)2016.831±0.044
Argument of periastron(ω)
(primary)		304.54±0.84°
Semi-amplitude(K1)
(primary)		3.762±0.025km/s
Details
α UMi Aa
Mass5.13±0.28[10]M☉
Radius37.5[12]–46.27[10]R☉
Luminosity (bolometric)1,260[12]L☉
Surface gravity(logg)2.2[13]cgs
Temperature6015[7]K
Metallicity112% solar[14]
Rotation119 days[6]
Rotational velocity(vsini)14[6]km/s
Age45 - 67[15]Myr
α UMi Ab
Mass1.316[10]M☉
Radius1.04[1]R☉
Luminosity (bolometric)3[1]L☉
Age>500[15]Myr
α UMi B
Mass1.39[1]M☉
Radius1.38[7]R☉
Luminosity (bolometric)3.9[7]L☉
Surface gravity(logg)4.3[7]cgs
Temperature6900[7]K
Rotational velocity(vsini)110[7]km/s
Age1.5[15]Gyr
Other designations
Polaris, North Star, Cynosura,Alpha UMi, α UMi,ADS1477,CCDMJ02319+8915
α UMi A:1 Ursae Minoris,BD+88°8,FK5907,GC2243,HD8890,HIP11767,HR424,SAO308
α UMi B:NSV631,BD+88°7,GC2226,SAO305
Database references
SIMBADα UMi A
α UMi B

As the closestCepheid variableits distance is used as part of thecosmic distance ladder.The revisedHipparcosstellar parallaxgives a distance to Polaris of about 433light-years(133parsecs), while the successor missionGaiagives a distance of about 448light-years(137parsecs). Calculations by other methods vary widely.

Although appearing to the naked eye as a single point of light, Polaris is a triplestar system,composed of the primary, ayellow supergiantdesignated 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.

Stellar system

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Polaris components as seen by theHubble Space Telescope

Polaris Aa is anevolvedyellow supergiantofspectral typeF7Ib with 5.4solar masses(M☉). It is the first classicalCepheidto have a mass determined from its orbit. The two smaller companions are Polaris B, a 1.39M☉F3main-sequencestar orbiting at a distance of2,400astronomical units(AU),[18]and Polaris Ab (or P), a very close F6 main-sequence star with a mass of 1.26M☉.[1]Polaris B can be resolved with a modest telescope. William Herschel discovered the star in August 1779 using areflecting telescopeof his own,[19]one of the best telescopes of the time. In January 2006,NASAreleased images, from theHubble telescope,that showed the three members of the Polaris ternary system.[20][21]

The variableradial velocityof Polaris A was reported byW. W. Campbellin 1899, which suggested this star is a binary system.[22]Since Polaris A is a known cepheid variable,J. H. Moorein 1927 demonstrated that the changes in velocity along the line of sight were due to a combination of the four-daypulsationperiod combined with a much longerorbital periodand a largeeccentricityof around 0.6.[23]Moore published preliminaryorbital elementsof the system in 1929, giving anorbital periodof about 29.7 years with an eccentricity of 0.63. This period was confirmed byproper motionstudies performed byB. P. Gerasimovičin 1939.[24]

As part of her doctoral thesis, in 1955E. Roemerused radial velocity data to derive an orbital period of 30.46 y for the Polaris A system, with an eccentricity of 0.64.[25]K. W. Kamperin 1996 produced refined elements with a period of29.59±0.02 yearsand an eccentricity of0.608±0.005.[26]In 2019, a study by R. I. Anderson gave a period of29.32±0.11 yearswith an eccentricity of0.620±0.008.[11]

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.[18][27]

Observation

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Variability

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Alight curvefor Polaris, plotted fromTESSdata[28]

Polaris Aa, the supergiant primary component, is a low-amplitudePopulation Iclassical Cepheid variable,although it was once thought to be atype II Cepheiddue to its highgalactic latitude.Cepheids constitute an importantstandard candlefor determining distance, so Polaris, as the closest such star,[11]is heavily studied. Thevariabilityof Polaris had been suspected since 1852; this variation was confirmed byEjnar Hertzsprungin 1911.[29]

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]

Polaris and its surroundingintegrated flux nebula

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 inredshiftthat 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-overtonepulsation modes.[21][30][31]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.[12][31][32]

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 50Kto at least 170 K, may be related to the orbit with Polaris Ab.[13]

Research reported inSciencesuggests that Polaris is 2.5 times brighter today than whenPtolemyobserved it, changing from third to second magnitude.[33]AstronomerEdward Guinanconsiders 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 46R☉,using theGaiadistance of446±1light-years, and its mass was determined at 5.13M☉.[10]

Role as pole star

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Main article:Pole star
Polaris azimuths vis clock face analogy[34]
A typical Northern Hemispherestar trailwith Polaris in the center.

Because Polaris lies nearly in a direct line with theEarth's rotationalaxis "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 navigationand forastrometry.The elevation of the star above the horizon gives the approximatelatitudeof the observer.[16]

In 2018 Polaris was 0.66° (39.6 arcminutes) away from the pole of rotation (1.4 times theMoondisc) 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.[35]Because it is so close to the celestial north pole, itsright ascensionis changing rapidly due to theprecession of Earth's axis,going from 2.5h in AD 2000 to 6h in AD 2100. Twice in eachsidereal dayPolaris'sazimuthis 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[34]is made using the leading edge of the "Big Dipper"asterismin the constellation Ursa Major. The leading edge (defined by the starsDubheandMerak) is referenced to a clock face, and the true azimuth of Polaris worked out for different latitudes.

The apparent motion of Polaris towards and, in the future, away from the celestial pole, is due to theprecession of the equinoxes.[36]The celestial pole will move away from α UMi after the 21st century, passing close byGamma Cepheiby about the41st century,moving towardsDenebby about the91st century.

The celestial pole was close toThubanaround 2750 BC,[36]and duringclassical antiquityit was slightly closer toKochab(β UMi) than to Polaris, although still about10°from either star.[37]It was about the same angular distance from β UMi as to α UMi by the end oflate antiquity.The Greek navigatorPytheasin 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 polarisfrom about theHigh Middle Agesand onwards, both in Greek and Latin. On his first trans-Atlantic voyage in 1492,Christopher Columbushad to correct for the "circle described by the pole star about the pole".[38]InShakespeare'splayJulius 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. Despite its relative brightness, it is not, as is popularly believed, the brightest star in the sky.[39]

Polaris lying halfway between theasterismsCassiopeiaand theBig Dipper

Polaris was referenced inNathaniel Bowditch's 1802 book,American Practical Navigator,where it is listed as one of thenavigational stars.[40]

Names

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This artist's concept shows: supergiant Polaris Aa, dwarf Polaris Ab, and the distant dwarf companion Polaris B.

The modern namePolaris[41]is shortened fromNeo-Latinstella polaris"polar star",coined in the Renaissance when the star had approached the celestial pole to within a few degrees. 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.[42]

In 2016, theInternational Astronomical Unionorganized aWorking Group on Star Names(WGSN)[43]to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[44]included a table of the first two batches of names approved by the WGSN; which includedPolarisfor the star α Ursae Minoris Aa.

In antiquity, Polaris was not yet the closest naked-eye star to the celestial pole, and the entire constellation ofUrsa Minorwas 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 starhave been in use since the medieval period. In Old English, it was known asscip-steorra( "ship-star" )[citation needed]. In theOld English rune poem,theT-runeis apparently associated with "a circumpolar constellation", or the planet Mars.[45]

In the HinduPuranas,it became personified under the nameDhruva( "immovable, fixed" ).[46]In the later medieval period, it became associated with theMarian titleofStella Maris"Star of the Sea"(so inBartholomaeus Anglicus,c. 1270s),[47]due to an earlier transcription error.[48] 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 mariswith the polar star (Stella Polaris), as well as the use ofCynosuraas 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-Judayyالجدي ( "the kid", in the sense of a juvenilegoat[ "le Chevreau" ] in Description des Etoiles fixes),[51]and that name was used inmedieval Islamic astronomyas 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.

It was invoked as a symbol of steadfastness in poetry, as "steadfast star" bySpenser. Shakespeare'ssonnet 116is 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." InJulius Caesar,he hasCaesarexplain 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 toprecession,but this is only noticeable over centuries.[citation needed]

InInuit astronomy,Polaris is known asNuutuittuq(syllabics:ᓅᑐᐃᑦᑐᖅ).

In traditionalLakotastar knowledge, Polaris is named "Wičháȟpi Owáŋžila". This translates to "The Star that Sits Still". This name comes from aLakotastory 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]

ThePlains Creecall the star inNehiyawewin:acâhkos êkâ kâ-âhcît"the star that does not move" (syllabics:ᐊᒑᐦᑯᐢ ᐁᑳ ᑳ ᐋᐦᒌᐟ).[55]InMi'kmawi'simkthe star is namedTatapn.[56]

In the ancient Finnish worldview, the North Star has also been calledtaivaannapaandnaulatähti( "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 pinandworld pin.[citation needed]

Distance

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Stellar parallax is the basis for theparsec,which is the distance from theSunto anastronomical objectwhich has aparallaxangle of onearcsecond.(1AUand 1pcare not to scale, 1 pc = about 206265 AU)

Many recent papers calculate the distance to Polaris at about 433light-years(133 parsecs),[21]based on parallax measurements from theHipparcosastrometry 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).[9]Polaris is the closestCepheid variableto Earth so its physical parameters are of critical importance to the wholeastronomical distance scale.[9]It is also the only one with a dynamically measured mass.

Selected distance estimates to Polaris
Year Component Distance,ly(pc) Notes
2006 A 330 ly (101 pc) Turner[30]
2007[A] A 433 ly (133 pc) Hipparcos[3]
2008 B 359 ly (110 pc) Usenko & Klochkova[7]
2013 B 323 ly (99 pc) Turner, et al.[9]
2014 A ≥ 385 ly (≥ 118 pc) Neilson[57]
2018 B 521 ly (160pc) Bond et al.[58]
2018 B 445.3 ly (136.6 pc)[B] Gaia DR2[59]
2020 B 447.6 ly (137.2pc) Gaia DR3[5]
ANew revision of observations from 1989 to 1993, first published in 1997
BStatistical distance calculated using a weak distance prior

TheHipparcosspacecraft usedstellar parallaxto take measurements from 1989 and 1993 with the accuracy of 0.97milliarcseconds(970 microarcseconds), and it obtained accurate measurements for stellar distances up to 1,000 pc away.[60]The Hipparcos data was examined again with more advanced error correction and statistical techniques.[3]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.[9]The Hipparcos reduction specifically for Polaris has been re-examined and reaffirmed but there is still not widespread agreement about the distance.[61]

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).[62]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.[63]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".[64]Gaia Data Release 2does not include a parallax for Polaris, but a distance inferred from it is136.6±0.5pc(445.5 ly) for Polaris B,[59]somewhat further than most previous estimates and several times more accurate. This was further improved to137.2±0.3pc(447.6 ly), upon publication of theGaia Data Release 3catalog on 13 June 2022 which superseded Gaia Data Release 2.[5]

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Polaris is depicted in theflagandcoat of armsof theCanadianInuit territory ofNunavut,[65]the flag of theU.S.states ofAlaskaandMinnesota,[66]and the flag of the U.S. city ofDuluth, Minnesota.[67][68]

Vexillology

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Heraldry

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  • Polaris is the brightest star in the constellation of Ursa Minor (upper right).
  • Big DipperandUrsa Minorin relation to Polaris
  • A view of Polaris in a small telescope. Polaris B is separated by 18 arc seconds from the primary star, Polaris A.
  • A 4-day time lapse of Polaris illustrating its Cepheid type variability.

See also

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References

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  1. ^abcdefghijklmnEvans, 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.arXiv:0806.4904.Bibcode:2008AJ....136.1137E.doi:10.1088/0004-6256/136/3/1137.S2CID16966094.
  2. ^"Polaris | meaning in the Cambridge English Dictionary".Cambridge English Dictionary.Retrieved11 December2020.
  3. ^abcdefgVan Leeuwen, F. (2007). "Validation of the new Hipparcos reduction".Astronomy and Astrophysics.474(2): 653–664.arXiv:0708.1752.Bibcode:2007A&A...474..653V.doi:10.1051/0004-6361:20078357.S2CID18759600.
  4. ^abcdSamus, N. N.; Kazarovets, E. V.; et al. (2017). "General Catalogue of Variable Stars".Astronomy Reports.5.1.61(1): 80–88.Bibcode:2017ARep...61...80S.doi:10.1134/S1063772917010085.S2CID125853869.
  5. ^abcdVallenari, A.; et al. (Gaia collaboration) (2023)."GaiaData Release 3. Summary of the content and survey properties ".Astronomy and Astrophysics.674:A1.arXiv:2208.00211.Bibcode:2023A&A...674A...1G.doi:10.1051/0004-6361/202243940.S2CID244398875. Gaia DR3 record for this sourceatVizieR.
  6. ^abcdLee, 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.S2CID12176373.
  7. ^abcdefghiUsenko, 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(1): L1.arXiv:0708.0333.Bibcode:2008MNRAS.387L...1U.doi:10.1111/j.1745-3933.2008.00426.x.S2CID18848139.
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  11. ^abcAnderson, R. I. (March 2019). "Probing Polaris' puzzling radial velocity signals. Pulsational (in-)stability, orbital motion, and bisector variations".Astronomy & Astrophysics.623:17.arXiv:1902.08031.Bibcode:2019A&A...623A.146A.doi:10.1051/0004-6361/201834703.S2CID119467242.A146.
  12. ^abcFadeyev, Y. A. (2015)."Evolutionary status of Polaris".Monthly Notices of the Royal Astronomical Society.449(1): 1011–1017.arXiv:1502.06463.Bibcode:2015MNRAS.449.1011F.doi:10.1093/mnras/stv412.S2CID118517157.
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Preceded by Pole star
500–3000 		Succeeded by
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