Polaris

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 + F6V[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]
Distance	446.5±1.1 ly (136.90±0.34 pc)[9]
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)
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°
Orbit[9]
Primary	α UMi Aa
Companion	α UMi Ab
Period (P)	29.416±0.028 yr
Semi-major axis (a)	0.12955±0.00205" (≥2.90±0.03 AU[10])
Eccentricity (e)	0.6354±0.0066
Inclination (i)	127.57±1.22°
Longitude of the node (Ω)	201.28±1.18°
Periastron epoch (T)	2016.831±0.044
Argument of periastron (ω) (primary)	304.54±0.84°
Semi-amplitude (K1) (primary)	3.762±0.025 km/s
Details
α UMi Aa
Mass	5.13±0.28[9] M☉
Radius	46.27±0.42[9] R☉
Luminosity (bolometric)	1,260[11] L☉
Surface gravity (log g)	2.2[12] cgs
Temperature	6015[7] K
Metallicity	112% solar[13]
Rotation	119 days[6]
Rotational velocity (v sin i)	14[6] km/s
Age	45 - 67[14] Myr
α UMi Ab
Mass	1.316[9] M☉
Radius	1.04[3] R☉
Luminosity (bolometric)	3[3] L☉
Age	>500[14] Myr
α UMi B
Mass	1.39[3] M☉
Radius	1.38[7] R☉
Luminosity (bolometric)	3.9[7] L☉
Surface gravity (log g)	4.3[7] cgs
Temperature	6900[7] K
Rotational velocity (v sin i)	110[7] km/s
Age	1.5[14] Gyr
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
SIMBAD	α UMi A
	α UMi B

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.^[15] 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.^[16]

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.