Polaris

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,[3] it is the brightest star in the constellation and is readily visible to the naked eye at night.[16] 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.

Polaris
Location of Polaris (circled)
Observation data
Epoch J2000      Equinox
Constellation Ursa Minor
Pronunciation /pəˈlɛərɪs, -ˈlær-/;
UK: /pəˈlɑːrɪs/[1]
α UMi A
Right ascension 02h 31m 49.09s[2]
Declination +89° 15 50.8[2]
Apparent magnitude (V) 1.98[3] (1.86  2.13)[4]
α UMi B
Right ascension 02h 30m 41.63s[5]
Declination +89° 15 38.1[5]
Apparent magnitude (V) 8.7[3]
Characteristics
α UMi A
Spectral type F7Ib[6]
U−B color index 0.38[3]
B−V color index 0.60[3]
Variable type Classical Cepheid[4]
α UMi B
Spectral type F3V[3]
U−B color index 0.01[7]
B−V color index 0.42[7]
Variable type suspected[4]
Astrometry
Radial velocity (Rv)−17[8] km/s
Proper motion (μ) RA: 198.8±0.20[2] mas/yr
Dec.: −15±0.30[2] mas/yr
Parallax (π)7.54 ± 0.11 mas[2]
Distance323–433[9] ly
(99–133[9] pc)
Absolute magnitude (MV)−3.6 (α UMi Aa)[3]
3.6 (α UMi Ab)[3]
3.1 (α UMi B)[3]
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)29.59±0.02 yr
Semi-major axis (a)0.1204±0.0059"
(≥2.90±0.03 AU[11])
Eccentricity (e)0.608±0.005
Inclination (i)146.2±10.9°
Longitude of the node (Ω)191.4±4.9°
Periastron epoch (T)1987.66±0.13
Argument of periastron (ω)
(secondary)
123.01±0.75°
Semi-amplitude (K1)
(primary)
3.72±0.03 km/s
Details
α UMi Aa
Mass5.4[12] M
Radius37.5[12] R
Luminosity (bolometric)1,260[12] L
Surface gravity (log g)2.2[13] cgs
Temperature6015[7] K
Metallicity112% solar[14]
Rotation119 days[6]
Rotational velocity (v sin i)14[6] km/s
Age70[15] Myr
α UMi Ab
Mass1.26[3] M
Radius1.04[3] R
Luminosity (bolometric)3[3] L
Age70[15] Myr
α UMi B
Mass1.39[3] M
Radius1.38[7] R
Luminosity (bolometric)3.9[7] L
Surface gravity (log g)4.3[7] cgs
Temperature6900[7] K
Rotational velocity (v sin i)110[7] km/s
Other designations
Polaris, North Star, Cynosura, Alpha UMi, α UMi, ADS 1477, CCDM J02319+8915
α UMi A: 1 Ursae Minoris, BD+88°8, FK5 907, GC 2243, HD 8890, HIP 11767, HR 424, SAO 308
α UMi B: NSV 631, BD+88°7, GC 2226, SAO 305
Database references
SIMBADα UMi A
α UMi B

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 433 light-years (133 parsecs), while the successor mission Gaia gives a distance of about 448 light-years (137 parsecs). Calculations by other methods vary widely.

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.


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