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||A2 III|
|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.
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