Phecda/ˈfɛkdə/, also called Gamma Ursae Majoris (γ Ursae Majoris, abbreviated Gamma UMa, γ UMa),[12][13] 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.[14] Based upon
parallax measurements with the Hipparcos astrometry satellite,[15][16] it is located at distance of around 83.2
light-years (25.5
parsecs) from the
Sun.[1]
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.[7] Like the other stars in the group, it is a
main sequence star, as the Sun is, 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).[17]
It bore the traditional names Phecda or Phad, derived from the
Arabic phrase فخذ الدبfakhth al-dubb ('thigh of the bear').[18] In 2016, the
International Astronomical Union organized a
Working Group on Star Names (WGSN)[19] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[20] 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).[21]
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.[5] 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.[3]
^Wielen, R.; et al. (1999), "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions", Veröff. Astron. Rechen-Inst. Heidelb, 35 (35), Astronomisches Rechen-Institut Heidelberg: 1,
Bibcode:
1999VeARI..35....1W
^Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub.
ISBN978-1-931559-44-7.
^Perryman, M. A. C.; Lindegren, L.; Kovalevsky, J.; et al. (July 1997), "The Hipparcos Catalogue", Astronomy and Astrophysics, 323: L49–L52,
Bibcode:
1997A&A...323L..49P