  | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Ursa Major |
| Right ascension | 11h 53m 49.84732s[1] |
| Declination | +53° 41′ 41.1350″[1] |
| Apparent magnitude (V) | 2.438[2] |
| Characteristics | |
| Spectral type | A0 Ve + K2 V[3] |
| U−Bcolor index | +0.008[2] |
| B−Vcolor index | –0.013[2] |
| Astrometry | |
| Radial velocity (Rv) | −12.6[4] km/s |
| Proper motion (μ) | RA: +107.68[1]mas/yr Dec.: +11.01[1]mas/yr |
| Parallax (π) | 39.21±0.40 mas[1] |
| Distance | 83.2 ± 0.8 ly (25.5 ± 0.3 pc) |
| Absolute magnitude (MV) | 0.4[5] |
| Orbit[5] | |
| Period (P) | 20.5 ± 1yr |
| Semi-major axis (a) | 0.460″ |
| Eccentricity (e) | 0.3 ± 0.3 |
| Inclination (i) | 51 ± 15° |
| Longitude of the node (Ω) | 6 ± 61° |
| Periastronepoch (T) | B 1984.0 ± 2.0 |
| Argument of periastron (ω) (secondary) | 185 ± 37° |
| Details[6] | |
| γ UMa A | |
| Mass | 2.412+0.053 −0.060 M☉ |
| Radius | 3.385 (equatorial) 2.186 (polar) R☉ |
| Luminosity | 44.57+3.39 −3.61 L☉ |
| Surface gravity (log g) | 3.79[7] cgs |
| Temperature | 6,751 (equatorial) 10,520 (polar) K |
| Rotational velocity (v sin i) | 184.5+32.6 −35.6 km/s |
| Age | 333+43 −83 Myr |
| γ UMa B | |
| Mass | 0.79[3] M☉ |
| Luminosity | 0.397[3] L☉ |
| Temperature | 4,780[3] K |
| Other designations | |
| Phad, Phecda, Phekda, Phegda, Phekha, Phacd, Fekda,[8] γ Ursae Majoris, γ UMa, Gamma UMa, 64 Ursae Majoris,BD+54 1475,FK5 447,GC 16268,HD 103287,HIP 58001,HR 4554,PPM 33292,SAO 28179.[9] | |
| Database references | |
| SIMBAD | data |
Phecda/ˈfɛkdə/, also calledGamma Ursae Majoris (γ Ursae Majoris, abbreviatedGamma UMa,γ UMa),[10][11] is astar in theconstellation ofUrsa Major. Since 1943, thespectrum of this star has served as one of the stable anchor points by which other stars are classified.[12] Based uponparallax measurements with theHipparcos astrometry satellite,[13][14] it is located at a distance of around 83.2light-years (25.5parsecs) from theSun.[1]
It is more familiar to most observers in thenorthern hemisphere as the lower-left star forming the bowl of theBig Dipper, together withAlpha Ursae Majoris (Dubhe, upper-right),Beta Ursae Majoris (Merak, lower-right) andDelta Ursae Majoris (Megrez, upper-left). Along with four other stars in this well-knownasterism, Phecda forms a loose association of stars known as theUrsa Major moving group.[7] Like the other stars in the group, it is amain sequence star, as the Sun is, although somewhat hotter, brighter and larger.
Phecda is located in relatively close physical proximity to the prominentMizar–Alcor star system. The two are separated by an estimated distance of 8.55 ly (2.62 pc); much closer than the two are from the Sun. The starMerak is separated from Phecda by 11.0 ly (3.4 pc).[15]
γ Ursae Majoris (Latinised toGamma Ursae Majoris) is the star'sBayer designation.
It bore the traditional namesPhecda orPhad, derived from theArabic phraseفخذ الدبfakhth al-dubb ('thigh of the bear').[16] In 2016, theInternational Astronomical Union organized aWorking Group on Star Names (WGSN)[17] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[18] included a table of the first two batches of names approved by the WGSN, which includedPhecda for this star.
To theHindus this star was known asPulastya, one of theseven rishis.[8]
InChinese,北斗 (Běi Dǒu), meaningNorthern Dipper, refers to an asterism equivalent to the Big Dipper. Consequently, theChinese name for Gamma Ursae Majoris itself is北斗三 (Běi Dǒu sān, English:the Third Star of Northern Dipper) and天璣 (Tiān Jī, English:Star of Celestial Shining Pearl).[19]
Phecda is anAe star, which is surrounded by an envelope of gas that is addingemission lines to thespectrum of the star;[20] hence the 'e' suffix in thestellar classification of A0 Ve.[15] It is 2.4 times more massive than the Sun and is 333 million years old. It rotates rapidly with arotational velocity of 386 km/s at itsequator, which causes it to have an oblate shape. The equatorial radius measures 3.39 R☉, while the polar radius measures 2.19 R☉. Theeffective temperature varies as well, from 6,750 K in the equator to 10,520 K in the poles.[6]
Phecda is also anastrometric binary: the companion star regularly perturbs the Ae-type primary star, causing the primary to wobble around thebarycenter. From this, anorbital period of 20.5 years has been calculated.[5] The secondary star is aK-type main-sequence star that is 0.79 times as massive as the Sun, and with a surface temperature of4,780 K.[3]
