Polaris (star)

brightest star in the constellation Ursa Minor

Polaris (Alpha Ursae Minoris) is the Pole Star or North Star.

Chart showing star positions and boundaries of the Ursa Minor constellation
Polaris (α Ursae Minoris, circled in red) in the constellation Ursa Minor (white area)
Observation data
Epoch J2000      Equinox
Constellation Ursa Minor
Right ascension {{{ra}}}
Declination {{{dec}}}
α UMi Aa
Spectral type F7Ib[1]
U−B color index 0.38[2]
B−V color index 0.60[2]
Variable type Classical Cepheid[3]
α UMi Ab
Spectral type F6V[2]
α UMi B
Spectral type F3V[2]
U−B color index 0.01[4]
B−V color index 0.42[4]
Radial velocity (Rv)−17 km/s
Proper motion (μ) RA: 198.8±0.20 mas/yr
Dec.: -15±0.30 mas/yr
Parallax (π)7.54 ± 0.11[5] mas
Distance323–433[6] ly
(99–133[6] pc)
Absolute magnitude (MV)−3.6 (α UMi Aa)[2]
3.6 (α UMi Ab)[2]
3.1 (α UMi B)[2]
Primaryα UMi Aa
Companionα UMi Ab
Period (P)29.59 ± 0.02 yr
Semi-major axis (a)0.1204 ± 0.0059″
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)
123.01 ± 0.75°
Semi-amplitude (K1)
3.72 ± 0.03 km/s
α UMi Aa
Mass5.4[8] M
Radius37.5[8] R
Luminosity (bolometric)1,260[8] L
Surface gravity (log g)2.2[9] cgs
Temperature6015[4] K
Metallicity112% solar[10]
Rotation119 days[1]
Rotational velocity (v sin i)14[1] km/s
Age7×107[11] years
α UMi Ab
Mass1.26[2] M
Radius1.04[2] R
Luminosity (bolometric)3[2] L
Age7×107[11] years
α UMi B
Mass1.39[2] M
Radius1.38[4] R
Luminosity (bolometric)3.9[4] L
Surface gravity (log g)4.3[4] cgs
Temperature6900[4] K
Rotational velocity (v sin i)110[4] km/s
Age7×107[11] years
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°
Other designations
Polaris, North Star, 1 Ursae Minoris, HR 424, BD +88° 8, HD 8890, SAO 308, FK5 907, GC 2243, ADS 1477, CCDM J02319+8915, HIP 11767, Cynosura, Alruccabah, Phoenice, Navigatoria, Star of Arcady, Yilduz, Mismar
Database references
α UMi B
Polaris as seen by the Hubble Space Telescope

It is the brightest star in the constellation Ursa Minor. It is almost straight above Earth's North Pole. Because of this, when it is seen from Earth, it looks like it always stays in the same place in the sky. For centuries, sailors in the northern hemisphere used Polaris to help them figure out where they were on the ocean and what way they were moving.

Polaris is part of a triple star system. It has a very close dwarf binary, and a larger star, Polaris B, which orbits 2,400 AU away.[12]

The main star, Polaris A, is a giant with 4.5 times the mass of the Sun and a diameter of 45 million kilometers. It is a classic Cepheid variable, the closest to us in the whole Milky Way. Polaris B can be seen even with a modest telescope. It was found by William Herschel in 1780 using one of the most powerful telescopes of the time: his own reflecting telescope. The nearby dwarf star, Ab, was predicted in 1929, but only seen quite recently. The dwarf orbits as close to A as Uranus is to our sun.[13]

Even though Polaris is the North Star today, this has not always been the case. The place in the sky that the Earth's north pole points at changes slowly over time. This movement is called stellar precession. In 3000 BC, a faint star called Thuban in the constellation of Draco was the North Star. Polaris did not become the North Star until about AD 500. It will get closer to straight above the Earth's north pole until sometime in 2102. Then it will move away again. It will be the closest star to the pole until about AD 3000.

Surprisingly, considering its relative nearness, its distance is not yet known for certain. Many recent papers calculate the distance to Polaris at about 434 light-years (133 parsecs).[14] However, some suggest it may be as much as 30% closer. If correct, this would be especially notable because Polaris is the closest Cepheid variable to Earth. Its physical parameters are critical to the whole astronomical distance scale.[6]

There is no star in the southern hemisphere which plays a similar role to Polaris. The star at the celestial south pole, Sigma Octantis, is much dimmer, so it is basically useless.

References change

  1. 1.0 1.1 1.2 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.
  2. 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 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.
  3. Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007–2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1: 02025. Bibcode:2009yCat....102025S.
  4. 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 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.
  5. Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. S2CID 18759600.
  6. 6.0 6.1 6.2 Turner D.G. et al 2012. The pulsation mode of the Cepheid Polaris.
  7. Evans, N. R.; Marovska, M.; Bond, H. E.; Schaefer, G. H.; Sahu, K. C.; Mack, J.; Nelan, E. P.; Gallene, A.; Tingle, E. D. (2018). "The Orbit of the Close Companion of Polaris: Hubble Space Telescope Imaging, 2007 to 2014". The Astrophysical Journal. 863 (2): 187. arXiv:1807.06115. Bibcode:2018ApJ...863..187E. doi:10.3847/1538-4357/aad410. S2CID 119392532.
  8. 8.0 8.1 8.2 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.
  9. 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.
  10. 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.
  11. 11.0 11.1 11.2 Wielen, R.; Jahreiß, H.; Dettbarn, C.; Lenhardt, H.; Schwan, H. (2000). "Polaris: Astrometric orbit, position, and proper motion". Astronomy and Astrophysics. 360: 399. arXiv:astro-ph/0002406. Bibcode:2000A&A...360..399W.
  12. An anstronomical unit is the average distance of the Earth from the Sun.
  13. "There's more to the North Star than meets the eye". Hubblesite.org. 2006-01-09. Archived from the original on 2016-09-25. Retrieved 2012-04-14.
  14. Evans N.R. et al 2008. Direct detection of the close companion of Polaris with the Hubble pace elescope. The Astronomical Journal 136 (3): 1137.