Gamma Ursae Minoris

Gamma Ursae Minoris (γ Ursae Minoris, abbreviated Gamma UMi, γ UMi), also named Pherkad /ˈfɜːrkæd/,[10][11] is a star in the northern circumpolar constellation of Ursa Minor. Together with Beta Ursae Minoris (Kochab), it forms the end of the dipper pan of the "Little Dipper", which is an asterism forming the tail of the bear. Based upon parallax measurements obtained during the Hipparcos mission, it is approximately 487 light-years (149 parsecs) from the Sun.[1]

γ Ursae Minoris
Location of γ Ursae Minoris (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Ursa Minor
Right ascension 15h 20m 43.71604s[1]
Declination +71° 50′ 02.4596″[1]
Apparent magnitude (V) +3.05[2]
Characteristics
Spectral type A2 III[3]
U−B color index +0.08[2]
B−V color index +0.09[2]
Astrometry
Radial velocity (Rv)−3.9[4] km/s
Proper motion (μ) RA: −17.73[1] mas/yr
Dec.: +17.90[1] mas/yr
Parallax (π)6.70 ± 0.11[1] mas
Distance487 ± 8 ly
(149 ± 2 pc)
Absolute magnitude (MV)–2.84[5]
Details
Radius15[6] R☉
Luminosity1,100[6] L☉
Surface gravity (log g)2.53[7] cgs
Temperature8,280[8] K
Rotational velocity (v sin i)180[9] km/s
Other designations
Pherkad, Pherkad Major, Gamma Ursae Minoris, 13 Ursae Minoris, HR 5735, BD+72°679, HD 137422, SAO 8220, HIP 75097
Database references
SIMBADdata

Nomenclature

γ Ursae Minoris (Latinised to Gamma Ursae Minoris) is the star's Bayer designation. The fainter 11 Ursae Minoris has been called γ¹ Ursae Minoris, in which case Gamma Ursae Minoris would be designated γ². However this usage is rarely seen.[12]

Gamma Ursae Minoris bore the traditional name Pherkad, which derived from the Arabic فرقد farqad "calf", short for aḫfa al farkadayn "the dim one of the two calves", that is Pherkad and Kochab (the full name Ahfa al Farkadain is traditionally applied to Zeta Ursae Minoris). Gamma Ursae Minoris was mostly called Pherkad Major to distinguish it from Pherkad Minor (11 Ursae Minoris). In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[13] to catalogue and standardize proper names for stars. The WGSN approved the name Pherkad for Gamma Ursae Minoris on 21 August 2016 and it is now so included in the List of IAU-approved Star Names.[11]

In Chinese, 北極 (Běi Jí), meaning North Pole, refers to an asterism consisting of Gamma Ursae Minoris, Beta Ursae Minoris, 5 Ursae Minoris, 4 Ursae Minoris and Σ 1694.[14] Consequently, the Chinese name for Gamma Ursae Minoris itself is 北極一 (Běi Jí yī, English: the First Star of North Pole), representing 太子 (Tàizǐ), meaning Crown Prince.[15]

Properties

Gamma Ursae Minoris has apparent magnitude +3.05[2] and can be readily observed with the naked eye even in a city-lit night sky. It has an absolute magnitude of –2.84.[5] Measurement of the star's spectrum resulted in a stellar classification of A3 Iab, with the luminosity class of 'Iab' indicating this is an intermediate luminosity supergiant star. The effective temperature of the star's outer envelope is 8,280 K,[8] giving it the typical white hue of an A-type star.[16] It is rotating rapidly, with the projected rotational velocity of 180 km s−1[9] providing a lower limit on the azimuthal velocity along the star's equator.

This is classified as a shell star that has a circumstellar disk of gas around the star's equator, which may be causing it to vary in magnitude.[7] It is 1100 times more luminous than the Sun, and possesses a radius 15 times that of the Sun.[6]

Pherkad in fiction

Pherkad (spelled as Pherkard) also features in Cthulhu Mythos, in the short story "The Thing in the Library", by Crispin Burnham and E.P. Berglund. It is mentioned as the stellar abode of the flaming Outer God Yomagn'tho.[17]

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, S2CID 18759600
  2. Fernie, J. D. (May 1983), "New UBVRI photometry for 900 supergiants", Astrophysical Journal Supplement Series, 52: 7–22, Bibcode:1983ApJS...52....7F, doi:10.1086/190856
  3. Abt, Helmut A.; Morrell, Nidia I. (1995). "The Relation between Rotational Velocities and Spectral Peculiarities among A-Type Stars". Astrophysical Journal Supplement Series. 99: 135. Bibcode:1995ApJS...99..135A. doi:10.1086/192182.
  4. Wielen, R.; et al. (1999), "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions", Veröff. Astron. Rechen-Inst. Heidelb, Veröffentlichungen des Astronomisches Rechen-Institut Heidelberg, 35: 1, Bibcode:1999VeARI..35....1W
  5. Verdugo, E.; et al. (November 2005). "Do A-type Supergiants have Magnetic Fields?". In Ignace, Richard; Gayley, Kenneth G. (eds.). The Nature and Evolution of Disks Around Hot Stars; Proceedings of a meeting held 7-9 July 2004 in Johnson City, Tennessee, USA. The Nature and Evolution of Disks Around Hot Stars. ASP Conference Series. 337. p. 324. Bibcode:2005ASPC..337..324V.
  6. Kaler, James B., "Pherkad (Gamma Ursae Minoris)", Stars, University of Illinois, retrieved 2007-10-05
  7. Hauck, B.; Jaschek, C. (February 2000), "A-shell stars in the Geneva system", Astronomy and Astrophysics, 354: 157–162, Bibcode:2000A&A...354..157H
  8. Zorec, J.; et al. (July 2009), "Fundamental parameters of B supergiants from the BCD system. I. Calibration of the (λ_1, D) parameters into Teff", Astronomy and Astrophysics, 501 (1): 297–320, arXiv:0903.5134, Bibcode:2009A&A...501..297Z, doi:10.1051/0004-6361/200811147, S2CID 14969137
  9. Royer, F.; et al. (October 2002), "Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i", Astronomy and Astrophysics, 393 (3): 897–911, arXiv:astro-ph/0205255, Bibcode:2002A&A...393..897R, doi:10.1051/0004-6361:20020943, S2CID 14070763
  10. Rumrill, H. B. (June 1936). "Star Name Pronunciation". Publications of the Astronomical Society of the Pacific. San Francisco, California. 48 (283): 139. Bibcode:1936PASP...48..139R. doi:10.1086/124681.
  11. "Naming Stars". IAU.org. Retrieved 4 March 2018.
  12. Kostjuk, N. D. (2004). "VizieR Online Data Catalog: HD-DM-GC-HR-HIP-Bayer-Flamsteed Cross Index (Kostjuk, 2002)". VizieR On-line Data Catalog: IV/27A. Originally Published in: Institute of Astronomy of Russian Academy of Sciences (2002). 4027. Bibcode:2004yCat.4027....0K.
  13. IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
  14. (in 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.
  15. (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.
  16. "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
  17. Crispin Burnham & E.P. Berglund. "The Thing in the Library". Archived from the original on 2005-03-22. Retrieved 2013-01-28.
Preceded by
Thuban		 Pole Star
1900–500 BCE]]		 Succeeded by
Polaris

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