Beta Ursae Minoris

Beta Ursae Minoris (β Ursae Minoris, abbreviated Beta UMi, β UMi), formally named Kochab /ˈkoʊkæb/,[10][11] is the brightest star in the bowl of the Little Dipper asterism (which is part of the constellation of Ursa Minor), and only slightly fainter than Polaris, the northern pole star and brightest star in Ursa Minor. Kochab is 16 degrees from Polaris and has an apparent visual magnitude of 2.08.[2] The distance to this star from the Sun can be deduced from the parallax measurements made during the Hipparcos mission, yielding a value of 130.9 light-years (40.1 parsecs).[1]

Beta Ursae Minoris
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]
Characteristics
Spectral type K4 III[3]
U−B color index +1.78[2]
B−V color index +1.47[2]
Astrometry
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
Distance130.9 ± 0.6 ly
(40.1 ± 0.2 pc)
Absolute magnitude (MV)−0.83±0.010[5]
Details
Mass2.2 ± 0.3[6] M☉
Radius42.06 ± 0.91[7] R☉
Luminosity390 ± 25[7] L☉
Surface gravity (log g)1.83[7] cgs
Temperature4,030[7] K
Metallicity [Fe/H]–0.29[7] dex
Rotational velocity (v sin i)8[8] 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.[9]
Database references
SIMBADdata

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.[12]

Nomenclature

β Ursae Minoris (Latinised to Beta Ursae Minoris) is the star's Bayer designation.

It bore the traditional name Kochab which 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.[13] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[14] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[15] included a table of the first two batches of names approved by the WGSN; which included Kochab for this star.

In Chinese, 北極 (Běi Jí), meaning North Pole, refers to an asterism consisting of Beta Ursae Minoris, Gamma Ursae Minoris, 5 Ursae Minoris, 4 Ursae Minoris and Σ 1694.[16] Consequently, the Chinese name for Beta Ursae Minoris itself is 北極二 (Běi Jí èr, English: the Second Star of North Pole.),[17] representing 帝 (Dì), meaning Emperor.[18]

Properties

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[7] times the girth of the Sun. This enlarged atmosphere is radiating 390[7] times as much luminosity as the Sun from its outer atmosphere at an effective temperature of 4,030 K.[7] This heat gives the star the orange-hued glow of a K-type star.[19]

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.[6] 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.[6]

As the pole star

From around 2500 BCE, as Thuban became less and less aligned with the celestial north, Kochab became one pillar of the circumpolar stars first with Mizar, a star in the middle of the handle of the Big Dipper (Ursa Major), and later with Pherkad (in Ursa Minor).[20] In fact, circa the year 2467 BCE, the true north was best observed by drawing a plumb line between Mizar and Kochab, a fact with which the ancient Egyptians were well acquainted as they aligned the great Pyramid of Giza with it.[20] This cycle of the succession of pole stars occurs due to the precession of the equinoxes. Kochab and Mizar were referred to by Ancient Egyptian astronomers as "The Indestructibles" lighting the North.[20] As precession continued, by the year 1100 BCE Kochab was within roughly 7° of the northern celestial pole, with old references over emphasizing this near pass by mentioning Beta Ursae Minoris as "Polaris",[21] relating it to the current pole star, Polaris, which is slightly brighter and will have a much closer alignment of less than 0.5° by 2100 AD.[22]

This change in the identity of the pole stars is a result of Earth's precessional motion. After 2000 BCE, Kochab and a new star, its neighbor Pherkad, were closer to the pole and together served as twin pole stars, circling the North Pole, from around 1700 BCE until just after 300 AD. Neither star was as proximitous to the celestial north pole as Polaris is now.[23] Today, they are sometimes referred to as the "Guardians of the Pole."[23]

Planetary companion

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.[24]

The Beta Ursae Minoris planetary system[25]
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(days)		 Eccentricity Inclination Radius
b ≥6.1 ± 1.0 MJ 1.4 ± 0.1 522.3 ± 2.7 0.19 ± 0.02 — —
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References
  1. 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. 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. 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. Park, Sunkyung; et al. (2013), "Wilson-Bappu Effect: Extended to Surface Gravity", The Astronomical Journal, 146 (4): 73, arXiv:1307.0592, Bibcode:2013AJ....146...73P, doi:10.1088/0004-6256/146/4/73.
  6. Tarrant, N. J.; et al. (June 2008), "Oscillations in ß Ursae Minoris. Observations with SMEI", Astronomy and Astrophysics, 483 (3): L43–L46, arXiv:0804.3253, Bibcode:2008A&A...483L..43T, doi:10.1051/0004-6361:200809738
  7. 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
  8. 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
  9. "KOCHAB -- Variable Star", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2012-01-11
  10. 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. ISBN 978-1-931559-44-7.
  11. "IAU Catalog of Star Names". Retrieved 28 July 2016.
  12. Sherrod, Clay (2001). ""Clay's Kochab Clock": Precise Portable Polar Alignment EVERY Time!". Arkansas Sky Observatories. Retrieved 2013-07-16.
  13. 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.
  14. "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  15. "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016.
  16. 陳久金 (2005). 中國星座神話 [Chinese Constellation Myth] (in Chinese). 台灣書房出版有限公司. ISBN 978-986-7332-25-7.
  17. (in Chinese)北極 (星官)
  18. (in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表 Archived August 10, 2010, at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.
  19. "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on 2012-03-18, retrieved 2012-01-16
  20. article "Planet Earth: Ancient Astronomy Calendars, Navigation, Predictions"
  21. Kaler, James B., "KOCHAB (Beta Ursae Minoris)", Stars, University of Illinois, retrieved 2018-04-28
  22. Kaler, James B., "POLARIS (Alpha Ursae Minoris)", Stars, University of Illinois, retrieved 2018-04-28
  23. Benningfield, Damond (14 June 2015). "Kochab". Stardate Magazine. University of Texas McDonald Observatory. Retrieved 14 June 2015.
  24. 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.
  25. Schneider, Jean. "Notes on Planet beta Umi b". Extrasolar Planets Encyclopaedia. Retrieved 8 March 2017.
Preceded by
Thuban		 Pole Star
1900 BC–500		 Succeeded by
Polaris

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