  | |
| 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−Bcolor index | +0.08[2] |
| B−Vcolor index | +0.09[2] |
| Variable type | δ Sct[4] |
| Astrometry | |
| Radial velocity (Rv) | −3.9[5] km/s |
| Proper motion (μ) | RA: −17.73[1]mas/yr Dec.: +17.90[1]mas/yr |
| Parallax (π) | 6.70±0.11 mas[1] |
| Distance | 487 ± 8 ly (149 ± 2 pc) |
| Absolute magnitude (MV) | –2.84[6] |
| Details | |
| Radius | 15[7] R☉ |
| Luminosity | 1,100[7] L☉ |
| Surface gravity (log g) | 2.53[8] cgs |
| Temperature | 8,280[9] K |
| Rotational velocity (v sin i) | 180[10] 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 | |
| SIMBAD | data |
Gamma Ursae Minoris (γ Ursae Minoris, abbreviatedGamma UMi,γ UMi), also namedPherkad/ˈfɜːrkæd/,[11][12] is astar in the northernconstellation ofUrsa Minor. Together withBeta Ursae Minoris (Kochab), it forms the end of the dipper pan of the "Little Dipper", which is anasterism forming the tail of the bear. Based uponparallax measurements obtained during theHipparcos mission, it is approximately 487light-years (149parsecs) from theSun.[1]
γ Ursae Minoris (Latinised toGamma Ursae Minoris) is the star'sBayer designation. The fainter11 Ursae Minoris has been calledγ1 Ursae Minoris, in which caseGamma Ursae Minoris would be designatedγ2. However this usage is rarely seen.[13]
Gamma Ursae Minoris bore the traditional namePherkad, which derived from the Arabic فرقدfarqad "calf", short foraḫfa al farkadayn "the dim one of the two calves", that is Pherkad and Kochab (the full nameAhfa al Farkadain is traditionally applied toZeta Ursae Minoris).[citation needed] Gamma Ursae Minoris was mostly calledPherkad Major to distinguish it fromPherkad Minor (11 Ursae Minoris). In 2016, theInternational Astronomical Union organized aWorking Group on Star Names (WGSN)[14] to catalogue and standardize proper names for stars. The WGSN approved the namePherkad for Gamma Ursae Minoris on 21 August 2016 and it is now so included in the List of IAU-approved Star Names.[12]
InChinese,北極 (Běi Jí), meaningNorth Pole, refers to an asterism consisting of Gamma Ursae Minoris,Beta Ursae Minoris,5 Ursae Minoris,4 Ursae Minoris andΣ 1694.[15] Consequently, theChinese name for Gamma Ursae Minoris itself is北極一 (Běi Jí yī, English:the First Star of North Pole), representing太子 (Tàizǐ), meaningCrown Prince.[16]
Gamma Ursae Minoris hasapparent magnitude +3.05[2] and can be readily observed with the naked eye even in acity-lit night sky. It has anabsolute magnitude of –2.84.[6] Measurement of the star'sspectrum resulted in astellar classification of A2 III,[3] with theluminosity class of 'III' indicating this is an expandinggiant star that left the main sequence after exhausting its core supply ofhydrogen. Based on stellar evolutionary theory, Pherkad might become aCepheid variable before settling as ared giant similar to its neighbor Kochab, before becoming awhite dwarf of roughly 0.85solar masses.[7] Theeffective temperature of the star's outer envelope is 8,280 K,[9] giving it the typical white hue of anA-type star.[17] It is rotating rapidly, with theprojected rotational velocity of180 km s−1[10] providing a lower limit on theazimuthal velocity along the star's equator.
This is classified as ashell star that has a circumstellar disk of gas around the star's equator, which may be causing it to vary in magnitude.[8] It is 1100 times more luminousthan the Sun, and possesses a radius 15 times thatof the Sun.[7]
Pherkad (spelled asPherkard) features inCthulhu Mythos.
| Preceded by | Pole Star 1900–500 BCE | Succeeded by |
