Polaris

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This article is about the star. For other uses, see Polaris (disambiguation).
Alpha Ursae Minoris
Diagram showing star positions and boundaries of the Ursa Minor constellation and its surroundings

Map showing location of α Ursae Minoris (Polaris) near North Celestial Pole.
Polaris alpha ursae minoris.jpg
Polaris as seen by the Hubble Space Telescope.
Observation data
Epoch J2000      Equinox
Constellation Ursa Minor
α UMi Aa
Right ascension 02h 31m 49.09s
Declination +89° 15′ 50.8″
Apparent magnitude (V) 1.98v[1]
α UMi Ab
Right ascension
Declination
Apparent magnitude (V) 9.2[1]
α UMi B
Right ascension 02h 30m 41.63s
Declination +89° 15′ 38.1″
Apparent magnitude (V) 8.7[1]
Characteristics
α UMi Aa
Spectral type F7Ib[2]
U−B color index 0.38[1]
B−V color index 0.60[1]
Variable type Classical Cepheid[3]
α UMi Ab
Spectral type F6V[1]
α UMi B
Spectral type F3V[1]
U−B color index 0.01[4]
B−V color index 0.42[4]
Astrometry
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[5] mas
Distance 323 - 433[6] ly
(99 - 133[6] pc)
Absolute magnitude (MV) -3.6(α UMi Aa)[1]
3.6(α UMi Ab)[1]
3.1(α UMi B)[1]
Orbit[1]
Primary α UMi Aa
Companion α UMi Ab
Period (P) 29.59 yr
Semi-major axis (a) 0.133"
Eccentricity (e) 0.608
Inclination (i) 128°
Longitude of the node (Ω) 19°
Argument of periastron (ω)
(secondary)
303°
Semi-amplitude (K1)
(primary)
3.72 km/s
Details
α UMi Aa
Mass 5.4[7] M
Radius 37.5[7] R
Luminosity (bolometric) 1,260[7] L
Surface gravity (log g) 2.2[8] cgs
Temperature 6015[4] K
Metallicity 112% solar[9]
Rotation 119 days[2]
Rotational velocity (v sin i) 14[2] km/s
Age 7×107[10] years
α UMi Ab
Mass 1.26[1] M
Radius 1.04[1] R
Luminosity (bolometric) 3[1] L
Age 7×107[10] years
α UMi B
Mass 1.39[1] M
Radius 1.38[4] R
Luminosity (bolometric) 3.9[4] L
Surface gravity (log g) 4.3[4] cgs
Temperature 6900[4] K
Rotational velocity (v sin i) 110[4] km/s
Age 7×107[10] years
Position (relative to α UMi Aa)
Component α UMi Ab
Epoch of observation 2005.5880
Angular distance 0.172
Position angle 231.4°
Position (relative to α UMi Aa)
Component α UMi B
Epoch of observation 2005.5880
Angular distance 18.217
Position angle 230.540°
Database references
α UMi Aa
SIMBAD data
α UMi B
SIMBAD data
Other designations
Polaris, North Star, 1 Ursae Minoris, HR 424, BD +88°8, HD 8890, SAO 308, FK5 907, GC 2243, ADS 1477, CCDM 02319+8915, HIP 11767, Cynosura, Alruccabah, Phoenice, Navigatoria, Star of Arcady, Yilduz, Mismar

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.

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 1780 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.[10][11]

Stellar system[edit]

Polaris Aa is a 4.5 solar mass (M) F7 yellow supergiant or spectral type Ib. This is the first classical Cepheid to have a dynamical 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 1780 while using a hand-built reflecting telescope, one of the most powerful telescopes at the time. In 1929, it was discovered, by examining the spectrum of Polaris A, 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,[12] about the distance between the Sun and Uranus), which explains why its light is swamped by its close and much brighter companion.[13]

Variable star[edit]

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.[14]

The range of brightness of Polaris during its pulsations is given as 1.86 - 2.13,[3] 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.[2]

The period, roughly four 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.[13][15][16] 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.[16]

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 50K to at least 170K, may be related to the orbit with Polaris Ab.[8]

Research reported in Science suggests that Polaris is 2.5 times brighter today than when Ptolemy observed it, changing from third to second magnitude.[17] 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."

Names[edit]

This artist's concept shows three class F stars: supergiant Polaris A, dwarf Polaris Ab, and its distant dwarf companion, Polaris B.

Because of its importance in celestial navigation, Polaris is known by numerous names. It became known as Polaris during the Renaissance, its name derived from the Latin polaris "of/near the (north) pole".[18] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[19] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[20] included a table of the first two batches of names approved by the WGSN; which included Polaris for this star. It is now so entered in the IAU Catalog of Star Names.[21]

One ancient name for Polaris was Cynosūra, from the Greek κυνόσουρα "the dog’s tail" (reflecting a time when the constellation of Ursa Minor "Little Bear" was taken to represent a dog), hence the English word cynosure.[22][23] Most other names are directly tied to its role as pole star.

In English, it was known as "pole star" or "north star"; in Spenser, also "steadfast star".

An older English name, attested since the 14th century, is lodestar "guiding star", cognate with the Old Norse leiðarstjarna, Middle High German leitsterne.

Use of the name Polaris in English dates to the 17th century. It is an ellipsis for the Latin stella polaris "pole star". Another Latin name is stella maris "sea-star", which, from an early time, was also used as a title of the Blessed Virgin Mary, popularized in the hymn Ave Maris Stella (8th century).[24]

In traditional Indian astronomy, its name in Sanskrit is dhruva tāra "fixed star". 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.[25]

In the Old English rune poem, the T-rune is identified with Tyr "fame, honour", which is compared to the pole star, [tir] biþ tacna sum, healdeð trywa wel "[fame] is a sign, it keeps faith well".

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.

In Inuit astronomy, Polaris is known as Niqirtsuituq. It is depicted on the flag and coat of arms of the Canadian Inuit territory of Nunavut, as well as on the flag of the U.S. state of Alaska.

Role as pole star[edit]

Ursa Major and Ursa Minor in relation to Polaris
A typical Northern Hemisphere star trail with Polaris in the center
Further information: Pole star

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.[26] 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 BCE,[26] and during classical antiquity it was closer to Kochab (β UMi) than to Polaris.[27] 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 BCE 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, it was referenced in Nathaniel Bowditch's 1802 book, American Practical Navigator, where it is listed as one of the navigational stars.[28] 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[29] 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.

Distance[edit]

Stellar parallax is the basis for the parsec, which is the distance from the Sun to an astronomical object which has a parallax angle of one arcsecond. (1 au and 1 pc are not to scale, 1 pc = about 206265 au)

Many recent papers calculate the distance to Polaris at about 433 light-years (133 parsecs),[13] 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).[6] Polaris is the closest Cepheid variable to Earth so its physical parameters are of critical importance to the whole astronomical distance scale.[6] It is also the only one with a dynamically measured mass.

Selected distance estimates to Polaris
Year Distance, ly (pc) Notes
433 ly (133 pc) Hipparcos[5]
2006 330 ly (101 pc) Turner[15]
2008 359 ly (110 pc) Usenko & Klochkova[4]
2013 323 ly (99 pc) Turner, et al.[6]
2014 >=385 ly (>=118 pc) Neilson[30]
2015 346 ly (106 pc) Fadeyev[7]

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.[31] The Hipparcos data was examined again with more advanced error correction and statistical techniques.[5] 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.[6] The Hipparcos reduction specifically for Polaris has been re-examined and reaffirmed but there is still not widespread agreement about the distance.[32]

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).[33] 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.[34] 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.[35] 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".[36]

Getting an accurate distance to Polaris is a big deal for the cosmic distance lander, 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".[37]

History of observations[edit]

Polaris in stellar catalogues and atlases

Source Presence
Ptolemy (~169) Yes
Al-Sufi (964) Yes
Al-Biruni (~1030) Yes
Khayyam (~1100) Yes
Al-Tusi (1272) No
Ulugh Beg (1437) Yes
Copernicus (1543) Yes
Schöner (1551) Yes
Brahe (1598) Yes
Brahe (1602) Yes
Source Presence
Bayer (1603) Yes
De Houtman (1603) No
Kepler (1627) Yes
Schiller (1627) Yes
Halley (1679) No
Hevelius (1690) Yes
Flamsteed (1725) Yes
Flamsteed (1729) Yes
Bode (1801a) Yes
Bode (1801b) Yes

See also[edit]

References[edit]

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  11. ^ 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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  16. ^ a b Neilson, H. R.; Engle, S. G.; Guinan, E.; Langer, N.; Wasatonic, R. P.; Williams, D. B. (2012). "The Period Change of the Cepheid Polaris Suggests Enhanced Mass Loss". The Astrophysical Journal. 745 (2): L32. arXiv:1201.0761Freely accessible. Bibcode:2012ApJ...745L..32N. doi:10.1088/2041-8205/745/2/L32. 
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  24. ^ occasionally also as a title of Jesus. Robert Bellarmine deprecated this use of the title: Haec appellatio stelle maris tribui solet Beate Virgini. Fortasse melius de Christo diceretur 'stella splendida et matutina' . . . . [N]am stella maris est stella polaris, quae exigua est. Stella splendida et matutina est stella omnium fulgentissima, quae ab astrologis dicitur stella Veneris; cited after Peter Godman, The saint as censor: Robert Bellarmine between inquisition and index, Mnemosyne, Bibliotheca Classica Batava, BRILL, 2000, ISBN 978-90-04-11570-5, p. 309
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  37. ^ [1]

External links[edit]

Preceded by
Kochab & Pherkad
Pole star
5003000
Succeeded by
Gamma Cephei

Coordinates: Sky map 02h 31m 48.7s, +89° 15′ 51″