Polaris

Alpha Ursae Minoris

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	325[6] - 425[7] ly (99[6] - 129.5[7] 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[8] M☉
Radius	37.5[8] R☉
Luminosity (bolometric)	1,260[8] L☉
Surface gravity (log g)	2.2[9] cgs
Temperature	6015[4] K
Metallicity	112% solar[10]
Rotation	119 days[2]
Rotational velocity (v sin i)	14[2] km/s
Age	7×107[11] years
α UMi Ab
Mass	1.26[1] M☉
Radius	1.04[1] R☉
Luminosity (bolometric)	3[1] L☉
Age	7×107[11] 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[11] 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 (α Ursae Minoris, α UMi, commonly the North Star or Pole Star) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star.

It is a multiple star, comprising the main star, α UMi Aa, which is a supergiant; two smaller companions, α UMi B and α UMi Ab; and two distant components, α UMi C and α UMi D. α UMi B was discovered in 1780 by William Herschel.

The revised Hipparcos parallax calculates the distance to Polaris at about 434 light-years (133 parsecs). Many recent papers derive distances up to 30% closer, particularly by assuming that its pulsation is in the fundamental mode. Deriving an accurate distance is especially important because Polaris is the closest Cepheid variable to Earth, so its physical parameters are critically important to the whole astronomical distance scale.^[6]

History of observations

Polaris in stellar catalogues and atlases

Star system

α UMi Aa is a 4.5 solar mass (M_☉) F7 yellow supergiant (Ib). This is the first classical Cepheid to have a dynamical mass determined from its orbit. The two smaller companions are α UMi B, a 1.39 M_☉ F3 main-sequence star orbiting at a distance of 2400 AU, and α UMi Ab (or P), a very close F6 main- sequence star with an 18.8 AU radius orbit and 1.26 M_☉. There are also two distant components: α UMi C and α UMi D which have been shown to not be physically associated with the other three stars.^[11][12]

Polaris B can be seen even with a modest telescope. William Herschel discovered the star in 1780 while using a handbuilt 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 nearer dwarf star is in an orbit of only 18.5 AU (2.8 billion km from Polaris A,^[13] about the distance between our Sun and Uranus), which explains why its light is swamped by its close and much brighter companion.^[7]

Variable star

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.^[7][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 of amplitude of temperature changes during each cycle, from less than 50K to at least 170K, may be related to the orbit with Polaris Ab.^[9]

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

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]

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.^[19][20] 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).^[21]

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.^[22]

In the Old English rune poem, the T-rune is identified with Tyr "fame, honour", which is compared to the pole star, ᛏ [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."

Role as pole star

A typical Northern Hemisphere star trail with Polaris in the center

Because α UMi 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.^[23] 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 2500 BCE,^[23] and during classical antiquity, it was closer to Kochab (β UMi) than to α UMi. It was about the same angular distance from either β UMi than 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). α UMi could reasonably be described as stella polaris from about the High Middle Ages.

Ursa Major and Ursa Minor in relation to Polaris

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.^[24] 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^[25] 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

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 434 light-years (133 parsecs),^[7] 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.

The Hipparcos spacecraft used stellar parallax to take measurements from 1989 and 1993 with an accuracy of 0.97 milliarcseconds (970 microarcseconds), and it obtained accurate measurements for stellar distances up to 1,000 pc away.^[27] 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.^[28]

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).^[29] Gaia will 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.^[30]

See also

• Polar alignment
• Polaris in fiction

References

External links

Wikimedia Commons has media related to Polaris.

• Info on Polaris
• Finding the Pole Star
• Polaris at Constellation Guide

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