Beta Ursae Minoris

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Beta Ursae Minoris
Diagram showing star positions and boundaries of the Ursa Minor constellation and its surroundings
Cercle rouge 100%.svg

Location of β Ursae Minoris (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Ursa Minor
Right ascension 14h 50m 42.32580s[1]
Declination +74° 09′ 19.8142″[1]
Apparent magnitude (V) 2.08[2]
Spectral type K4 III[3]
U−B color index +1.78[2]
B−V color index +1.47[2]
Radial velocity (Rv) +16.96[4] km/s
Proper motion (μ) RA: −32.61[1] mas/yr
Dec.: +11.42[1] mas/yr
Parallax (π) 24.91 ± 0.12[1] mas
Distance 130.9 ± 0.6 ly
(40.1 ± 0.2 pc)
Mass 2.2 ± 0.3[5] M
Radius 42.06 ± 0.91[6] R
Luminosity 390 ± 25[6] L
Surface gravity (log g) 1.83[6] cgs
Temperature 4,030[6] K
Metallicity [Fe/H] –0.29[6] dex
Rotational velocity (v sin i) 8[7] km/s
Other designations
Kochab, Kocab, Kochah, 7 Ursae Minoris, Al Kaukab al Shamaliyy, BD+74 595, FK5 550, GCTP 3373.00, HD 131873, HIP 72607, HR 5563, SAO 8102.[8]

Beta Ursae Minoris (β UMi, β Ursae Minoris) is the brightest star in the bowl of the "Little Dipper" (which is part of the constellation Ursa Minor), and only slightly fainter than Polaris, the northern pole star and brightest star in Ursa Minor. It has the traditional name Kochab. Kochab is 16 degrees from Polaris and has an apparent visual magnitude of 2.08.[2] The distance to this star can be deduced from the parallax measurements made during the Hipparcos, yielding a value of 130.9 light-years (40.1 parsecs).[1]

Kochab and its neighbor Pherkad served as twin pole stars, circling the North Pole, from 1500 BC until 500 AD. Ancient Egyptian astronomers referred to them as "The Indestructibles".[9] Neither star was as proximitous to the celestial north pole as Polaris is now.[10] Today, they are sometimes referred to as the "Guardians of the Pole."[10] Due to precession of the equinoxes, the previous holder of the title was Thuban, and the next was the present-day Polaris. This succession of pole stars is a result of Earth's precessional motion.

Amateur astronomers can use Kochab as a very precise guide for setting up a telescope, as the celestial north pole is located 43 arcminutes away from Polaris, very close to the line connecting Polaris with Kochab.[11]


This is a giant star with a stellar classification of K4 III.[3] It is 130 times more luminous than the Sun. Kochab has reached a state in its evolution where the outer envelope has expanded to 42[6] times the girth of the Sun. This enlarged atmosphere is radiating 390[6] times as much luminosity as the Sun from its outer atmosphere at an effective temperature of 4,030 K.[6] This heat gives the star the orange-hued glow of a K-type star.[12]

By modelling this star based upon evolutionary tracks, the mass of this star can be estimated as 2.2 ± 0.3 that of the Sun. A mass estimate using the interferometrically-measured radius of this star and its spectroscopically-determined surface gravity yields 2.5 ± 0.9 solar masses.[5] The star is known to undergo periodic variations in luminosity over roughly 4.6 days, with the astroseismic frequencies depending sensitively on the star's mass. From this, a much lower mass estimate of 1.3 ± 0.3 solar is reached.[5]

Planetary companion[edit]

Estimated to be around 2.95 billion years old, give or take 1 billion years, Kochab was announced to have a planetary companion around 6.1 times as massive as Jupiter with an orbit of 522 days.[13]


The origin of the name Kochab appeared in the Renaissance and its meaning is uncertain; it may be from Arabic الكوكب al-kawkab or Hebrew כוכב kōkhāv, both of which mean "star", or more likely derived from Alrucaba or Rucaba, a name applied to Theta Ursae Majoris.[14]

In Chinese, 北極 (Běi Jí), meaning North Pole, refers to an asterism consisting of β Ursae Minoris, γ Ursae Minoris, 5 Ursae Minoris, 4 Ursae Minoris and Σ 1694.[15] Consequently, β Ursae Minoris itself is known as 北極二 (Běi Jí èr, English: the Second Star of North Pole.),[16] representing (), meaning Emperor.[17]


  1. ^ a b c d e f van Leeuwen, F. (November 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 
  2. ^ a b c d Johnson, H. L.; et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory 4 (99), Bibcode:1966CoLPL...4...99J 
  3. ^ a b Morgan, W. W.; Keenan, P. C. (1973), "Spectral Classification", Annual Review of Astronomy and Astrophysics 11: 29, Bibcode:1973ARA&A..11...29M, doi:10.1146/annurev.aa.11.090173.000333 
  4. ^ Famaey, B.; et al. (January 2005), "Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters", Astronomy and Astrophysics 430: 165–186, arXiv:astro-ph/0409579, Bibcode:2005A&A...430..165F, doi:10.1051/0004-6361:20041272 
  5. ^ a b c Tarrant, N. J.; et al. (June 2008), "Oscillations in ß Ursae Minoris. Observations with SMEI", Astronomy and Astrophysics 483 (3, 2008): L43–L46, arXiv:0804.3253, Bibcode:2008A&A...483L..43T, doi:10.1051/0004-6361:200809738 
  6. ^ a b c d e f g h Piau, L.; et al. (February 2011), "Surface convection and red-giant radius measurements", Astronomy and Astrophysics 526: A100, arXiv:1010.3649, Bibcode:2011A&A...526A.100P, doi:10.1051/0004-6361/201014442 
  7. ^ Bernacca, P. L.; Perinotto, M. (1970), "A catalogue of stellar rotational velocities", Contributi Osservatorio Astronomico di Padova in Asiago 239 (1), Bibcode:1970CoAsi.239....1B 
  8. ^ "KOCHAB -- Variable Star", SIMBAD (Centre de Données astronomiques de Strasbourg), retrieved 2012-01-11 
  9. ^ article "Planet Earth: Ancient Astronomy Calendars, Navigation, Predictions"
  10. ^ a b Benningfield, Damond (14 June 2015). "Kochab". Stardate Magazine (University of Texas McDonald Observatory). Retrieved 14 June 2015. 
  11. ^ Sherrod, Clay (2001). ""Clay's Kochab Clock": Precise Portable Polar Alignment EVERY Time!". Arkansas Sky Observatories. Retrieved 2013-07-16. 
  12. ^ "The Colour of Stars", Australia Telescope, Outreach and Education (Commonwealth Scientific and Industrial Research Organisation), December 21, 2004, retrieved 2012-01-16 
  13. ^ Lee, B.-C.; Han, I.; Park, M.-G.; Mkrtichian, D.E.; Hatzes, A.P.; Kim, K.-M. (2014). "Planetary Companions in K giants β Cancri, μ Leonis, and β Ursae Minoris". Astronomy & Astrophysics 566: 7. arXiv:1405.2127. Bibcode:2014A&A...566A..67L. doi:10.1051/0004-6361/201322608. A67. 
  14. ^ Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub. p. 58. ISBN 978-1-931559-44-7. 
  15. ^ (Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
  16. ^ (Chinese)北極 (星官)
  17. ^ (Chinese) 香港太空館 - 研究資源 - 亮星中英對照表, Hong Kong Space Museum. Accessed on line November 23, 2010.
Preceded by
Pole Star
1900 BC–500
Succeeded by

Coordinates: Sky map 14h 50m 42.3s, +74° 09′ 20″