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Polaris is a
star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
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, it is the brightest star in the constellation and is readily visible to the naked eye at night. The position of the star lies less than 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. As the closest Cepheid variable its distance is used as part of the cosmic distance ladder. The revised ''
Hipparcos ''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial obj ...
''
stellar parallax Stellar parallax is the apparent shift of position of any nearby star (or other object) against the background of distant objects, and a basis for determining (through trigonometry) the distance of the object. Created by the different orbital p ...
gives a distance to Polaris of about , while the successor mission ''Gaia'' gives a distance of about . Calculations by other methods vary widely. Although appearing to the naked eye as a single point of light, Polaris is a triple
star system A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a '' star cluster'' or '' galaxy'', although, broadly speak ...
, 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 system

Polaris Aa is an evolved yellow supergiant of spectral type F7Ib with 5.4
solar mass The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...
es (). It is the first classical Cepheid to have a mass determined from its orbit. The two smaller companions are Polaris B, a F3 main-sequence star orbiting at a distance of (AU), and Polaris Ab (or P), a very close F6 main-sequence star with a mass of . Polaris B can be resolved 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. In January 2006, NASA released images, from the
Hubble telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...
, that showed the three members of the Polaris ternary system. The variable
radial velocity The radial velocity or line-of-sight velocity, also known as radial speed or range rate, of a target with respect to an observer is the temporal rate of change, rate of change of the distance or Slant range, range between the two points. It is e ...
of Polaris A was reported by
W. W. Campbell William Wallace Campbell (April 11, 1862 – June 14, 1938) was an American astronomer, and director of Lick Observatory from 1901 to 1930. He specialized in spectroscopy. He was the tenth president of the University of California from 1923 to ...
in 1899, which suggested this star is a binary system. 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 A pulse, in physiology, is the throbbing of arteries resulting from heartbeat. Pulse, The Pulse or Pulses may also refer to: Botany * Pulse (legume), any agriculturally significant annual leguminous food crop, such as peas, beans, lentils, and ...
period combined with a much longer orbital period and a large eccentricity of around 0.6. Moore published preliminary
orbital elements Orbital elements are the parameters required to uniquely identify a specific orbit. In celestial mechanics these elements are considered in two-body systems using a Kepler orbit. There are many different ways to mathematically describe the same ...
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 Proper motion is the astrometric measure of the observed changes in the apparent places of stars or other celestial objects in the sky, as seen from the center of mass of the Solar System, compared to the abstract background of the more dista ...
studies performed by B. P. Gerasimovič in 1939. 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. K. W. Kamper in 1996 produced refined elements with a period of and an eccentricity of . In 2019, a study by R. I. Anderson gave a period of with an eccentricity of . 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.


Observation


Variability

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, 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 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 (lower temperature) 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 Edward F. Guinan is a professor in Villanova University's Department of Astronomy and Astrophysics. He and two colleagues observed evidence of Neptune's ring system in 1968, which was later discovered by Voyager 2 in 1989. He was also involved in ...
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
hose A hose is a flexible hollow tube designed to carry fluids from one location to another. Hoses are also sometimes called ''pipes'' (the word ''pipe'' usually refers to a rigid tube, whereas a hose is usually a flexible one), or more generally '' ...
predicted by current theories of
stellar evolution Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is cons ...
".


Role as pole star

Because Polaris lies nearly in a direct line with the
Earth's rotation Earth's rotation or Earth's spin is the rotation of planet Earth around its own Rotation around a fixed axis, axis, as well as changes in the orientation (geometry), orientation of the rotation axis in space. Earth rotates eastward, in retrograd ...
al axis "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 Celestial navigation, also known as astronavigation, is the practice of position fixing using stars and other celestial bodies that enables a navigator to accurately determine their actual current physical position in space (or on the surface of ...
and for
astrometry Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, the Milky Way. His ...
. The elevation of the star above the horizon gives the approximate latitude of the observer. In 2018 Polaris was 0.66° 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) soon after the year 2100. Because it is so close to the celestial north pole, its
right ascension Right ascension (abbreviated RA; symbol ) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point in question above the earth. When paired w ...
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' 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 In English, the phrase ''rule of thumb'' refers to an approximate method for doing something, based on practical experience rather than theory. This usage of the phrase can be traced back to the 17th century and has been associated with various t ...
. The best approximation 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. 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. The celestial pole was close to Thuban around 2750 BC, and during classical antiquity it was slightly closer to
Kochab Beta Ursae Minoris (β Ursae Minoris, abbreviated β UMi, Beta UMi), formally named Kochab , is the brightest star in the bowl of the Little Dipper asterism (which is part of the constellation of Ursa Minor), and only slightly fainter tha ...
(β UMi) than to Polaris, although still about from either star. It was about the same angular distance from β UMi as to α UMi by the end of late antiquity. The Greek navigator
Pytheas Pytheas of Massalia (; Ancient Greek: Πυθέας ὁ Μασσαλιώτης ''Pythéas ho Massaliōtēs''; Latin: ''Pytheas Massiliensis''; born 350 BC, 320–306 BC) was a Greeks, Greek List of Graeco-Roman geographers, geographer, explor ...
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. On his first trans-Atlantic voyage in 1492, Christopher Columbus had to correct for the "circle described by the pole star about the pole". In Shakespeare's play
Julius Caesar Gaius Julius Caesar (; ; 12 July 100 BC – 15 March 44 BC), was a Roman general and statesman. A member of the First Triumvirate, Caesar led the Roman armies in the Gallic Wars before defeating his political rival Pompey in a civil war, and ...
, 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. Polaris was referenced in Nathaniel Bowditch's 1802 book, '' American Practical Navigator'', where it is listed as one of the navigational stars.


Names

The modern name ''Polaris''
IAU Working Group on Star Names The International Astronomical Union (IAU) established a Working Group on Star Names (WGSN) in May 2016 to catalog and standardize proper names for stars for the international astronomical community. It operates under Division C – Education ...
is shortened from New Latin ''stella 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 as ''stella illa quae polaris dicitur'' ("that star which is called 'polar'"), placing it 3° 8' from the celestial pole. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) 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. 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 Old English rune poem, dated to the 8th or 9th century, has stanzas on 29 Anglo-Saxon runes. It stands alongside younger rune poems from Scandinavia, which record the names of the 16 Younger Futhark runes. The poem is a product of the perio ...
, the T-rune is apparently associated with "a circumpolar constellation", or the planet Mars. In the Hindu
Puranas Purana (; sa, , '; literally meaning "ancient, old"Merriam-Webster's Encyclopedia of Literature (1995 Edition), Article on Puranas, , page 915) is a vast genre of Indian literature about a wide range of topics, particularly about legends an ...
, it became personified under the name '' Dhruva'' ("immovable, fixed"). In the later medieval period, it became associated with the
Marian title Mary, the mother of Jesus in Christianity, is known by many different titles (Blessed Mother, Virgin Mary, Mother of God, Our Lady, Holy Virgin), epithets ( Star of the Sea, Queen of Heaven, Cause of Our Joy), invocations (''Panagia'', Mother o ...
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 "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 ''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. 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), and that name was used in
medieval Islamic astronomy Islamic astronomy comprises the astronomical developments made in the Islamic world, particularly during the Islamic Golden Age (9th–13th centuries), and mostly written in the Arabic language. These developments mostly took place in the Middle ...
as well. 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" by Spenser. Shakespeare's sonnet 116 is an example of the symbolism of the north star as a guiding principle: " oveis the star to every wandering bark / Whose worth's unknown, although his height be taken." In ''
Julius Caesar Gaius Julius Caesar (; ; 12 July 100 BC – 15 March 44 BC), was a Roman general and statesman. A member of the First Triumvirate, Caesar led the Roman armies in the Gallic Wars before defeating his political rival Pompey in a civil war, and ...
'', he has
Caesar Gaius Julius Caesar (; ; 12 July 100 BC – 15 March 44 BC), was a Roman people, Roman general and statesman. A member of the First Triumvirate, Caesar led the Roman armies in the Gallic Wars before defeating his political rival Pompey in Caes ...
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. In
Inuit astronomy Inuit astronomy is centered around the ''Qilak,'' the Inuit name for the celestial sphere and the home for souls of departed people. Inuit beliefs about astronomy are the shaped by the harsh climate in the Arctic and the resulting difficulties o ...
, Polaris is known as ''Niqirtsuituq'' ( syllabics: ). It is depicted on the
flag A flag is a piece of fabric (most often rectangular or quadrilateral) with a distinctive design and colours. It is used as a symbol, a signalling device, or for decoration. The term ''flag'' is also used to refer to the graphic design empl ...
and coat of arms of the Canadian Inuit territory of
Nunavut Nunavut ( , ; iu, ᓄᓇᕗᑦ , ; ) is the largest and northernmost Provinces and territories of Canada#Territories, territory of Canada. It was separated officially from the Northwest Territories on April 1, 1999, via the ''Nunavut Act'' ...
, as well as on the
flag A flag is a piece of fabric (most often rectangular or quadrilateral) with a distinctive design and colours. It is used as a symbol, a signalling device, or for decoration. The term ''flag'' is also used to refer to the graphic design empl ...
of the U.S. state of Alaska. 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 Tapun San Win "Red Cheeked Woman". However she fell from the heavens, and in his grief he stared down from "waŋkátu" (the above land) forever. The
Plains Cree Plains Cree may refer to: * Plains Cree language * Plains Cree people Plains Indians or Indigenous peoples of the Great Plains and Canadian Prairies are the Native American tribes and First Nation band governments who have historically liv ...
call the star in Nehiyawewin: ''acâhkos êkâ kâ-âhcît'' "the star that does not move" ( syllabics: ). In Mi'kmawi'simk the star is named ''Tatapn''.


Distance

Many recent papers calculate the distance to Polaris at about 433 light-years (133 parsecs), based on parallax measurements from the
Hipparcos ''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial obj ...
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). 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 Stellar parallax is the apparent shift of position of any nearby star (or other object) against the background of distant objects, and a basis for determining (through trigonometry) the distance of the object. Created by the different orbital p ...
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 Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, the Milky Way. His ...
, 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. 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 comes from ''Gaia'', a space astrometry mission launched in 2013 and intended to measure stellar parallax to within 25 microarcseconds (μas). 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". Gaia Data Release 2 does not include a parallax for Polaris, but a distance inferred from it is (445.5 ly) for Polaris B, somewhat further than most previous estimates and several times more accurate. This was further improved to (447.6 ly), upon publication of the
Gaia Data Release 3 The ''Gaia'' catalogues are star catalogues created using the results obtained by ''Gaia'' space telescope. The catalogues are released in stages that will contain increasing amounts of information; the early releases also miss some stars, especia ...
catalog on 13 June 2022 which superseded Gaia Data Release 2.


Gallery

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 Star Trail above Beccles - geograph.org.uk - 1855505.jpg, A typical Northern Hemisphere
star trail A star trail is a type of photograph that uses long exposure times to capture '' diurnal circles'', the apparent motion of stars in the night sky due to Earth's rotation. A star-trail photograph shows individual stars as streaks across the i ...
with Polaris in the center. 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. File:Polaris time-lapse illustrating Cepheid type variability.gif, A 4 day time lapse of Polaris illustrating its Cepheid type variability. Utsjoki.vaakuna.svg, Polaris pictured in the coat of arms of Utsjoki


See also

* Extraterrestrial sky (for the pole stars of other celestial bodies) * Polar alignment *
Polaris Australis Sigma Octantis is a solitary star in the Octans constellation that forms the pole star of the Southern Hemisphere. Its name is also written as σ Octantis, abbreviated as Sigma Oct or σ Oct, and it is officially named Polaris Australi ...
* Polaris Flare * Regiment of the North Pole


References

{{Authority control F-type supergiants F-type main-sequence stars Classical Cepheid variables Triple star systems Northern pole stars Ursa Minor (constellation) Ursae Minoris, Alpha Durchmusterung objects Ursae Minoris, 01 008890 011767 0424 Stars with proper names Suspected variables