Gamma Ursae Majoris
Gamma Ursae Majoris (γ Ursae Majoris, abbreviated Gamma UMa, γ UMa), also named Phecda, is a star in the constellation of Ursa Major. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified. Based upon parallax measurements with the Hipparcos astrometry satellite, it is located at distance of around 83.2 light-years (25.5 parsecs) from the Sun.
Epoch J2000 Equinox J2000
|Right ascension||11h 53m 49.84732s|
|Declination||+53° 41′ 41.1350″|
|Apparent magnitude (V)||+2.438|
|Spectral type||A0 Ve + K2 V|
|U−B color index||+0.008|
|B−V color index||–0.013|
|Radial velocity (Rv)||−12.6 km/s|
|Proper motion (μ)|| RA: +107.68 mas/yr |
Dec.: +11.01 mas/yr
|Parallax (π)||39.21 ± 0.40 mas|
|Distance||83.2 ± 0.8 ly |
(25.5 ± 0.3 pc)
|Absolute magnitude (MV)||+2.7|
|Period (P)||20.5 ± 1 yr|
|Semi-major axis (a)||0.460″|
|Eccentricity (e)||0.3 ± 0.3|
|Inclination (i)||51 ± 15°|
|Longitude of the node (Ω)||6 ± 61°|
|Periastron epoch (T)||B 1984.0 ± 2.0|
|Argument of periastron (ω)|
|185 ± 37°|
|γ UMa A|
|Radius||3.04 ± 0.08 R☉|
|Surface gravity (log g)||3.79 cgs|
|Rotational velocity (v sin i)||178 km/s|
|γ UMa B|
It is more familiar to most observers in the northern hemisphere as the lower-left star forming the bowl of the Big Dipper, together with Alpha Ursae Majoris (Dubhe, upper-right), Beta Ursae Majoris (Merak, lower-right) and Delta Ursae Majoris (Megrez, upper-left). Along with four other stars in this well-known asterism, Phecda forms a loose association of stars known as the Ursa Major moving group. Like the other stars in the group, it is a main sequence star not unlike the Sun, although somewhat hotter, brighter and larger.
Phecda is located in relatively close physical proximity to the prominent Mizar-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 star Beta Ursae Majoris is separated from Gamma Ursae Majoris by 11.0 ly (3.4 pc).
It bore the traditional names Phecda or Phad, derived from the Arabic phrase فخذ الدب fakhth al-dubb 'thigh of the bear'. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included Phecda for this star.
In Chinese, 北斗 (Běi Dǒu), meaning Northern Dipper, refers to an asterism equivalent to the Big Dipper. Consequently, the Chinese 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).
Gamma Ursae Majoris is an Ae star, which is surrounded by an envelope of gas that is adding emission lines to the spectrum of the star; hence the 'e' suffix in the stellar classification of A0 Ve. It has 2.6 times the mass of the Sun, three times the Sun's radius, and an effective temperature of 9,355 K in its outer atmosphere. This star is rotating rapidly, with a projected rotational velocity of 178 km s−1. The estimated angular diameter of this star is about 0.92 mas. It has an estimated age of 300 million years.
Gamma Ursae Majoris is also an astrometric binary: the companion star regularly perturbs the Ae-type primary star, causing the primary to wobble around the barycenter. From this, an orbital period of 20.5 years has been calculated. The secondary star is a K-type main-sequence star that is 0.79 times as massive as the Sun, and with a surface temperature of 4,780 K.
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