Polaris

This article is about the Earth's current north star. For such stars in general, see pole star. For other uses, see Polaris (disambiguation) and North Star (disambiguation).
"Stella Polaris" redirects here. For the military operation, see Operation Stella 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 + F6[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)29.416±0.028 yr
Semi-major axis (a)0.12955±0.00205"
(≥2.90±0.03 AU[11])
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
Mass5.13±0.28[10] M☉
Radius37.5[12]–46.27[10] 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
Age45 - 67[15] Myr
α UMi Ab
Mass1.316[10] M☉
Radius1.04[3] R☉
Luminosity (bolometric)3[3] L☉
Age>500[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
Age1.5[15] 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
SIMBADα 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.[16] 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.[17]

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.

  1. ^ "Polaris | meaning in the Cambridge English Dictionary". Cambridge English Dictionary. Retrieved 11 December 2020.
  2. ^ a b c d e Cite error: The named reference hipparcos2 was invoked but never defined (see the help page).
  3. ^ a b c d e f g h i j k l Evans, N. R.; Schaefer, G. H.; Bond, H. E.; Bono, G.; Karovska, M.; Nelan, E.; Sasselov, D.; Mason, B. D. (2008). "Direct Detection of the Close Companion of Polaris with The Hubble Space Telescope". The Astronomical Journal. 136 (3): 1137. arXiv:0806.4904. Bibcode:2008AJ....136.1137E. doi:10.1088/0004-6256/136/3/1137. S2CID 16966094.
  4. ^ a b c Samus, N. N.; Kazarovets, E. V.; et al. (2017). "General Catalogue of Variable Stars". Astronomy Reports. 5.1. 61 (1): 80–88. Bibcode:2017ARep...61...80S. doi:10.1134/S1063772917010085. S2CID 125853869.
  5. ^ a b Cite error: The named reference Gaia_DR3 was invoked but never defined (see the help page).
  6. ^ a b c Lee, B. C.; Mkrtichian, D. E.; Han, I.; Park, M. G.; Kim, K. M. (2008). "Precise Radial Velocities of Polaris: Detection of Amplitude Growth". The Astronomical Journal. 135 (6): 2240. arXiv:0804.2793. Bibcode:2008AJ....135.2240L. doi:10.1088/0004-6256/135/6/2240. S2CID 12176373.
  7. ^ a b c d e f g h Usenko, I. A.; Klochkova, V. G. (2008). "Polaris B, an optical companion of the Polaris (α UMi) system: Atmospheric parameters, chemical composition, distance and mass". Monthly Notices of the Royal Astronomical Society: Letters. 387 (1): L1. arXiv:0708.0333. Bibcode:2008MNRAS.387L...1U. doi:10.1111/j.1745-3933.2008.00426.x. S2CID 18848139.
  8. ^ Campbell, William Wallace (1913). "The radial velocities of 915 stars". Lick Observatory Bulletin. 229: 113. Bibcode:1913LicOB...7..113C. doi:10.5479/ADS/bib/1913LicOB.7.113C.
  9. ^ a b Cite error: The named reference turner2012 was invoked but never defined (see the help page).
  10. ^ a b c d Cite error: The named reference evans2024 was invoked but never defined (see the help page).
  11. ^ Cite error: The named reference Anderson2019 was invoked but never defined (see the help page).
  12. ^ a b Fadeyev, Y. A. (2015). "Evolutionary status of Polaris". Monthly Notices of the Royal Astronomical Society. 449 (1): 1011–1017. arXiv:1502.06463. Bibcode:2015MNRAS.449.1011F. doi:10.1093/mnras/stv412. S2CID 118517157.
  13. ^ Usenko, I. A.; Miroshnichenko, A. S.; Klochkova, V. G.; Yushkin, M. V. (2005). "Polaris, the nearest Cepheid in the Galaxy: Atmosphere parameters, reddening and chemical composition". Monthly Notices of the Royal Astronomical Society. 362 (4): 1219. Bibcode:2005MNRAS.362.1219U. doi:10.1111/j.1365-2966.2005.09353.x.
  14. ^ Cayrel de Strobel, G.; Soubiran, C.; Ralite, N. (2001). "Catalogue of [Fe/H] determinations for FGK stars: 2001 edition". Astronomy and Astrophysics. 373: 159–163. arXiv:astro-ph/0106438. Bibcode:2001A&A...373..159C. doi:10.1051/0004-6361:20010525. S2CID 17519049.
  15. ^ a b c Neilson, H. R.; Blinn, H. (2021). The Curious Case of the North Star: The Continuing Tension Between Evolution Models and Measurements of Polaris. RR Lyrae/Cepheid 2019: Frontiers of Classical Pulsators. Vol. 529. p. 72. arXiv:2003.02326. Bibcode:2021ASPC..529...72N.
  16. ^ Kaler, Jim. "Polaris (Alpha Ursae Minoris)". Stars. Retrieved 2020-09-09.
  17. ^ McNamee, Gregory (2021-05-26). "How the stars, planets and other celestial objects got their names". CNN. Retrieved 2024-08-28.