Polaris
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Polaris is a star in the northern circumpolar constellation of Ursa Minor. It is designated α Ursae Minoris (Latinized to Alpha Ursae Minoris) and is commonly called the North Star or Pole Star. With an apparent magnitude that fluctuates around 1.98,<ref name=hst/> it is the brightest star in the constellation and is readily visible to the naked eye at night.<ref name=Kaler>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The position of the star lies less than 1° away from the north celestial pole, making it the current northern pole star. The stable position of the star in the Northern Sky makes it useful for navigation.<ref name="s936">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
As the closest Cepheid variable its distance is used as part of the cosmic distance ladder. The revised Hipparcos stellar parallax gives a distance to Polaris of about Template:Convert, while the successor mission Gaia gives a distance of about Template:Convert.
Although appearing to the naked eye as a single point of light, Polaris is a triple star system, composed of the primary, a yellow 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 by William Herschel, where the 'A' refers to what is now known to be the Aa/Ab pair.
Stellar systemEdit
Polaris Aa is an evolved yellow supergiant of spectral type F7Ib with 5.4 solar masses (Template:Solar mass). It is the first classical Cepheid to have a mass determined from its orbit. The two smaller companions are Polaris B, a Template:Solar mass F3 main-sequence star orbiting at a distance of Template:Val (AU),<ref name=Wielen>Template:Cite journal</ref> and Polaris Ab (or P), a very close F6 main-sequence star with a mass of Template:Solar mass.<ref name=hst/> Polaris B can be resolved with a modest telescope. William Herschel discovered the star in August 1779 using a reflecting telescope of his own,<ref>Template:Cite book</ref> one of the best telescopes of the time. In January 2006, NASA released images, from the Hubble telescope, that showed the three members of the Polaris ternary system.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=evans/>
The variable radial velocity of Polaris A was reported by W. W. Campbell in 1899, which suggested this star is a binary system.<ref>Template:Cite journal</ref> 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-day pulsation period combined with a much longer orbital period and a large eccentricity of around 0.6.<ref>Template:Cite journal</ref> Moore published preliminary orbital elements of the system in 1929, giving an orbital period of about 29.7 years with an eccentricity of 0.63. This period was confirmed by proper motion studies performed by B. P. Gerasimovič in 1939.<ref name=Roemer1965>Template:Cite journal</ref>
As part of her doctoral thesis, in 1955 E. 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.<ref>Template:Cite journal</ref> K. W. Kamper in 1996 produced refined elements with a period of Template:Val and an eccentricity of Template:Val.<ref name=Kamper1996>Template:Cite journal</ref> In 2019, a study by R. I. Anderson gave a period of Template:Val with an eccentricity of Template:Val.<ref name=Anderson2019>Template:Cite journal</ref>
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.<ref name=Wielen/><ref>Template:Cite journal</ref>
ObservationEdit
VariabilityEdit
Polaris Aa, 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,<ref name=Anderson2019/> is heavily studied. The variability of Polaris had been suspected since 1852; this variation was confirmed by Ejnar Hertzsprung in 1911.<ref>Template:Cite journal</ref>
The range of brightness of Polaris is given as 1.86–2.13,<ref name=gcvs/> 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.<ref name=lee/>
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 in redshift that causes light to stretch into longer wavelengths, causing it to appear red) evolution across the Cepheid instability strip, but it may be due to interference between the primary and the first-overtone pulsation modes.<ref name=evans>Template:Cite journal</ref><ref name=turner>Template:Cite journal</ref><ref name=neilson>Template:Cite journal</ref> 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.<ref name=fadeyev/><ref name=neilson/><ref name=engle>Template:Cite journal</ref>
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.<ref name=usenko2005/>
Research reported in Science suggests that Polaris is 2.5 times brighter today than when Ptolemy observed it, changing from third to second magnitude.<ref>Template:Cite journal</ref> 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".
In 2024, researchers led by Nancy Evans at the Harvard & Smithsonian, have studied with more accuracy the Polaris' smaller companion orbit using the CHARA 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 Template:Solar radius, using the Gaia distance of Template:Val light-years, and its mass was determined at Template:Solar mass.<ref name=evans2024>Template:Cite journal</ref>
Role as pole starEdit
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Because Polaris lies nearly in a direct line with the Earth's rotational axis above the North Pole, it stands almost motionless in the sky, and all the stars of the northern sky appear to rotate around it. It thus provides a nearly fixed point from which to draw measurements for celestial navigation and for astrometry. The elevation of the star above the horizon gives the approximate latitude of the observer.<ref name=Kaler/>
In 2018 Polaris was 0.66° (39.6 arcminutes) away from the pole of rotation (1.4 times the Moon 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.<ref>Template:Cite journal</ref> Because it is so close to the celestial north pole, its right ascension is changing rapidly due to the precession of Earth's axis, going from 2.5h in AD 2000 to 6h in AD 2100. Twice in each sidereal day Polaris's azimuth is true north; the rest of the time it is displaced eastward or westward, and the bearing must be corrected using tables or a rule of thumb. The best approximation<ref name="kaizad_co_uk" /> is made using the leading edge of the "Big Dipper" asterism in the constellation Ursa Major. The leading edge (defined by the stars Dubhe and Merak) 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 the precession of the equinoxes.<ref name="Nor">Template:Cite book</ref> The celestial pole will move away from α UMi after the 21st century, passing close by Gamma Cephei by about the 41st century, moving towards Deneb by about the 91st century.Template:Fact
The celestial pole was close to Thuban around 2750 BCE,<ref name="Nor" /> and during classical antiquity it was slightly closer to Kochab (β UMi) than to Polaris, although still about Template:Val from either star.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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), also termed Λύχνος (Lychnos) akin to a burner or lamp and would reasonably be described as stella polaris from about the High 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".<ref>Template:Cite book</ref> In Shakespeare's play Julius 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.<ref>Template:Cite news</ref>
Polaris was referenced in the classic Nathaniel Bowditch maritime navigation book American Practical Navigator (1802), where it is listed as one of the navigational stars.<ref name="BowditchAgency2002">Template:Cite book</ref>
NamesEdit
The modern name Polaris<ref name="IAU-CSN">IAU Working Group on Star Names {{#invoke:citation/CS1|citation |CitationClass=web }}</ref> is shortened from the Neo-Latin stella polaris ("polar star"), coined in the Renaissance when the star had approached the celestial pole to within a few degrees.<ref name="gemmaefrisii"/><ref name="kunitzch"/>
Gemma Frisius, writing in 1547, referred to it as stella illa quae polaris dicitur ("that star which is called 'polar'"), placing it 3° 8' from the celestial pole.<ref name="gemmaefrisii">Template:Cite book</ref><ref name="kunitzch">Template:Cite book</ref>
In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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 included Polaris for the star α Ursae Minoris Aa.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
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").Template:Citation needed
In the "Old English rune poem", the T-rune is apparently associated with "a circumpolar constellation", or the planet Mars.<ref>Template:Cite book; Dickins' "a circumpolar constellation" is attributed to L. Botkine, La Chanson des Runes (1879).</ref>
In the Hindu Puranas, it became personified under the name Dhruva ("immovable, fixed").<ref>Template:Cite book</ref>
In the later medieval period, it became associated with the Marian title of Stella Maris "Star of the Sea" (so in Bartholomaeus Anglicus, c. 1270s),<ref>Template:Cite book</ref> due to an earlier transcription error.<ref>{{#if:||{{#if:The Name of Mary|File:Wikisource-logo.svg|File:PD-icon.svg}} }}{{#invoke:template wrapper|{{#if:|list|wrap}}|_template=cite Catholic Encyclopedia
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An older English name, attested since the 14th century, is lodestar "guiding star", cognate with the Old Norse leiðarstjarna, Middle High German leitsterne.<ref>Template:Cite book</ref>
The ancient name of the constellation Ursa Minor, Cynosura (from the Greek {{#invoke:Lang|lang}} "the dog's tail"),<ref name="Ridpath2018">Template:Cite book</ref> 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 Cynosura 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. Template:Citation needed
Its name in traditional pre-Islamic Arab astronomy was al-Judayy الجدي ("the kid", in the sense of a juvenile goat ["le Chevreau"] in Description des Etoiles fixes),<ref>Template:Cite book</ref> and that name was used in medieval Islamic astronomy as well.<ref> {{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref> Template:Cite book</ref> 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.Template:Fact
It was 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."Template:Fact
In Julius Caesar, Shakespeare has Caesar 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 to precession, but this is only noticeable over centuries.Template:Citation needed
In Inuit astronomy, Polaris is known as Nuutuittuq (syllabics: {{#invoke:Lang|lang}}).<ref>Template:Cite book</ref>
In traditional Lakota star knowledge, Polaris is named "Wičháȟpi Owáŋžila". This translates to "The Star that Sits Still". This name comes from a Lakota 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
The Plains Cree call the star in Nehiyawewin: acâhkos êkâ kâ-âhcît "the star that does not move" (syllabics: {{#invoke:Lang|lang}}).<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
In Mi'kmawi'simk the star is named Tatapn.<ref>Template:Cite news</ref>
In the ancient Finnish worldview, the North Star has also been called taivaannapa and naulatähti ("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 were sky pin and world pin.Template:Citation needed
DistanceEdit
Many recent papers calculate the distance to Polaris at about 433 light-years (133 parsecs),<ref name="evans" /> based on parallax measurements from the Hipparcos 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).<ref name="turner2012">Template:Cite journal</ref> Polaris is the closest Cepheid variable to Earth so its physical parameters are of critical importance to the whole astronomical distance scale.<ref name="turner2012" /> It is also the only one with a dynamically measured mass.
| Year | Component | Distance, ly (pc) | Notes |
|---|---|---|---|
| 2006 | A | 330 ly (101 pc) | Turner<ref name=turner/> |
| 2007Template:Ref label | A | 433 ly (133 pc) | Hipparcos<ref name=hipparcos2/> |
| 2008 | B | 359 ly (110 pc) | Usenko & Klochkova<ref name=usenko/> |
| 2013 | B | 323 ly (99 pc) | Turner, et al.<ref name=turner2012/> |
| 2014 | A | ≥ 385 ly (≥ 118 pc) | Neilson<ref name=neilson2>Template:Cite journal</ref> |
| 2018 | B | 521 ly (160pc) | Bond et al.<ref name=bond>Template:Cite journal</ref> |
| 2018 | B | 445.3 ly (136.6 pc)Template:Ref label | Gaia DR2<ref name=bailer-jones/> |
| 2020 | B | 447.6 ly (137.2pc) | Gaia DR3<ref name="Gaia_DR3"/> |
| Template:Note label New revision of observations from 1989 to 1993, first published in 1997 |
| Template:Note label Statistical distance calculated using a weak distance prior |
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.<ref name=hipparcos>Template:Cite journal</ref> The Hipparcos data was examined again with more advanced error correction and statistical techniques.<ref name=hipparcos2>Template:Cite journal</ref> Despite the advantages of Hipparcos astrometry, 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.<ref name=turner2012/> The Hipparcos reduction specifically for Polaris has been re-examined and reaffirmed but there is still not widespread agreement about the distance.<ref name=polaris-hipparcos>Template:Cite journal</ref>
The next major step in high precision parallax measurements comes from Gaia, a space astrometry mission launched in 2013 and intended to measure stellar parallax to within 25 microarcseconds (μas).<ref name=gaia>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref> 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".<ref>Template:Citation</ref> Gaia Data Release 2 does not include a parallax for Polaris, but a distance inferred from it is Template:Val (445.5 ly) for Polaris B,<ref name=bailer-jones>Template:Cite journal</ref> somewhat further than most previous estimates and several times more accurate. This was further improved to Template:Val (447.6 ly), upon publication of the Gaia Data Release 3 catalog on 13 June 2022 which superseded Gaia Data Release 2.<ref name="Gaia_DR3">Template:Cite Gaia DR3</ref>
In popular cultureEdit
Polaris is depicted in the flag and coat of arms of the Canadian Inuit territory of Nunavut,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> the flag of the U.S. states of Alaska and Minnesota,<ref name="s027">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and the flag of the U.S. city of Duluth, Minnesota.<ref name="Fox21">Template:Cite news</ref><ref name="press">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
VexillologyEdit
- Flag of Nunavut.svg
- Flag of Alaska.svg
- Flag of Minnesota.svg
- Flag of Duluth, Minnesota.svg
- Flag of Maine.svg
- Flag of Maine (1901–1909).svg
- Pan American Exposition Flag.svg
CitationClass=web }}</ref>
- Francis Leopold McClintock's sledge flag (1852–1854).svg
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HeraldryEdit
- Coat of arms of Nunavut.svg
- Seal of Minnesota.svg
- Seal of Maine.svg
- Utsjoki.vaakuna.svg
Coat of arms of UtsjokiTemplate:Citation needed
GalleryEdit
- UrsaMinorCC.jpg
Polaris is the brightest star in the constellation of Ursa Minor (upper right).
- Ursa Major - Ursa Minor - Polaris.jpg
Big Dipper and Ursa Minor in relation to Polaris
- Polaris star and companion.jpg
A view of Polaris in a small telescope. Polaris B is separated by 18 arc seconds from the primary star, Polaris A.
- Polaris time-lapse illustrating Cepheid type variability.gif
A 4-day time lapse of Polaris illustrating its Cepheid type variability.
ShipsEdit
- The Chinese spy ship Beijixing is named after Polaris.
- USS Polaris is named after Polaris
See alsoEdit
- Extraterrestrial sky (for the pole stars of other celestial bodies)
- List of nearest supergiants
- Polar alignment
- Sigma Octantis
- Polaris Flare
- Regiment of the North Pole
ReferencesEdit
Template:Sky Template:S-start Template:S-bef Template:S-ttl Template:S-aft Template:S-end Template:Pole star Template:Stars of Ursa Minor Template:Portal bar Template:Authority control