Gamma Ursae Minoris
Epoch J2000.0 Equinox J2000.0
|Right ascension||15h 20m 43.71604s|
|Declination||+71° 50′ 02.4596″|
|Apparent magnitude (V)||+3.05|
|Spectral type||A3 Iab:|
|U−B color index||+0.08|
|B−V color index||+0.09|
|Radial velocity (Rv)||−3.9 km/s|
|Proper motion (μ)||RA: −17.73 mas/yr
Dec.: +17.90 mas/yr
|Parallax (π)||6.70 ± 0.11 mas|
|Distance||487 ± 8 ly
(149 ± 2 pc)
|Absolute magnitude (MV)||–2.84|
|Surface gravity (log g)||2.53 cgs|
|Rotational velocity (v sin i)||180 km/s|
Gamma Ursae Minoris (γ UMi, γ Ursae Minoris; also 13 UMi) is a star in the northern circumpolar constellation of Ursa Minor. It has the traditional name Pherkad, which derives from the Arabic فرقد farqad "calf", short for aḫfa al farkadayn "the dim one of the two calves", Pherkad and Kochab. (The full name Ahfa al Farkadain is now applied to Zeta Ursae Minoris.) Gamma Ursae Minoris is sometimes called Pherkad Major to distinguish it from 11 Ursae Minoris Pherkad Minor. Together with Beta Ursae Minoris, or Kochab, Gamma forms the end of the dipper pan of the "Little Dipper", which is a colloquial name for the constellation. Pherkad and Kochab are sometimes called the "Guardians of the Pole" or the "Indestructibles", as they appear to circle around the north star Polaris as the Earth rotates.
In Chinese, 北極 (Běi Jí), meaning North Pole, refers to an asterism consisting of γ Ursae Minoris, β Ursae Minoris, 5 Ursae Minoris, 4 Ursae Minoris and Σ 1694. Consequently, γ Ursae Minoris itself is known as 北極一 (Běi Jí yī, English: the First Star of North Pole.), representing 太子 (Tàizǐ), meaning Crown Prince
Gamma Ursae Minoris has apparent magnitude +3.05 and can be readily observed with the naked eye even in a city-lit night sky. Based upon parallax measurements, it is approximately 487 light-years (149 parsecs) from Earth, giving it an absolute magnitude of –2.84. 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, giving it the typical white hue of an A-type star. It is rotating rapidly, with the projected rotational velocity of 180 km s−1 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. It is 1100 times more luminous than the Sun, and possesses a radius 15 times that of the Sun.
Pherkad in fiction
Star 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. This star is mentioned as the stellar abode of the flaming Outer God Yomagn'tho.
- 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
- 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
- "V* gam UMi -- Variable Star", Simbad (Centre de Données astronomiques de Strasbourg), retrieved 2012-12-31
- Wielen, R.; et al. (1999), Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions 35, Veröffentlichungen des Astronomisches Rechen-Institut Heidelberg, Bibcode:1999VeARI..35....1W
- Verdugo, E.; et al. (November 2005), "Do A-type Supergiants have Magnetic Fields?", in Ignace, Richard; Gayley, Kenneth G., The Nature and Evolution of Disks Around Hot Stars; Proceedings of a meeting held 7-9 July 2004 in Johnson City, Tennessee, USA, ASP Conference Series 337, p. 324, Bibcode:2005ASPC..337..324V
- Kaler, James B., "Pherkad (Gamma Ursae Minoris)", Stars (University of Illinois), retrieved 2007-10-05
- Hauck, B.; Jaschek, C. (February 2000), "A-shell stars in the Geneva system", Astronomy and Astrophysics 354: 157–162, Bibcode:2000A&A...354..157H
- 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, Bibcode:2009A&A...501..297Z, doi:10.1051/0004-6361/200811147
- 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: 897–911, arXiv:astro-ph/0205255, Bibcode:2002A&A...393..897R, doi:10.1051/0004-6361:20020943
- (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.
- (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.
- "The Colour of Stars", Australia Telescope, Outreach and Education (Commonwealth Scientific and Industrial Research Organisation), December 21, 2004, retrieved 2012-01-16
- Crispin Burnham & E.P. Berglund. "The Thing in the Library". Retrieved 2013-01-28.