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Gamma Ursae Majoris (γ Ursae Majoris, abbreviated Gamma UMa, γ UMa), formally named Phecda /ˈfɛkdə/,[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]

Gamma Ursae Majoris
Gamma Ursae Majoris is located in 100x100
Gamma Ursae Majoris
Location of γ Ursae Majoris (circled)
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−B color index +0.008[2]
B−V color 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[1] mas
Distance83.2 ± 0.8 ly
(25.5 ± 0.3 pc)
Absolute magnitude (MV)+2.7[5]
Orbit[5]
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)
(secondary)
185 ± 37°
Details
γ UMa A
Mass2.94[3] M
Radius3.04 ± 0.08[6] R
Luminosity65.255[3] L
Surface gravity (log g)3.79[7] cgs
Temperature9,355[7] K
Rotational velocity (v sin i)178[8] km/s
Age0.3[9] Gyr
γ UMa B
Mass0.79[3] M
Luminosity0.397[3] L
Temperature4,780[3] K
Other designations
Phad, Phecda, Phekda, Phegda, Phekha, Phacd, Fekda,[10] 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.[11]
Database references
SIMBADdata

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 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).[17]

Contents

NomenclatureEdit

γ Ursae Majoris (Latinised to Gamma Ursae Majoris) is the star's Bayer designation.

It bore the traditional names Phecda or Phad, derived from the Arabic phrase فخذ الدب fakhth 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.

To the Hindus this star was known as Pulastya, one of the Seven Rishis.[10]

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]

PropertiesEdit

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;[22] hence the 'e' suffix in the stellar classification of A0 Ve.[17] It has 2.6[17] times the mass of the Sun, three times the Sun's radius,[6] and an effective temperature of 9,355 K in its outer atmosphere.[7] This star is rotating rapidly, with a projected rotational velocity of 178 km s−1.[8] The estimated angular diameter of this star is about 0.92 mas.[23] It has an estimated age of 300 million years.[9]

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]

See alsoEdit

ReferencesEdit

  1. ^ a b c d e f van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357
  2. ^ a b c Oja, T., "UBV photometry of stars whose positions are accurately known. III", Astronomy and Astrophysics Supplement Series, 65 (2): 405–4
  3. ^ a b c d e f g Eggl, S.; Pilat-Lohinger, E.; Funk, B.; Georgakarakos, N.; Haghighipour, N. (2012). "Circumstellar habitable zones of binary-star systems in the solar neighbourhood". Monthly Notices of the Royal Astronomical Society. 428 (4): 3104. arXiv:1210.5411. Bibcode:2013MNRAS.428.3104E. doi:10.1093/mnras/sts257.
  4. ^ Wielen, R.; et al. (1999), "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions", Veröff. Astron. Rechen-Inst. Heidelb, Astronomisches Rechen-Institut Heidelberg, 35 (35): 1, Bibcode:1999VeARI..35....1W
  5. ^ a b c Gontcharov, G.A.; Kiyaeva, O.V. (2010). "Photocentric orbits from a direct combination of ground-based astrometry with Hipparcos II. Preliminary orbits for six astrometric binaries". New Astronomy. 15 (3): 324–331. arXiv:1606.08182. Bibcode:2010NewA...15..324G. doi:10.1016/j.newast.2009.09.006.
  6. ^ a b Fitzpatrick, E. L.; Massa, D. (March 2005), "Determining the Physical Properties of the B Stars. II. Calibration of Synthetic Photometry", The Astronomical Journal, 129 (3): 1642–1662, arXiv:astro-ph/0412542, Bibcode:2005AJ....129.1642F, doi:10.1086/427855
  7. ^ a b c d King, Jeremy R.; et al. (April 2003), "Stellar Kinematic Groups. II. A Reexamination of the Membership, Activity, and Age of the Ursa Major Group", The Astronomical Journal, 125 (4): 1980–2017, Bibcode:2003AJ....125.1980K, doi:10.1086/368241
  8. ^ a b Royer, F.; Zorec, J.; Gómez, A. E. (February 2007), "Rotational velocities of A-type stars. III. Velocity distributions", Astronomy and Astrophysics, 463 (2): 671–682, arXiv:astro-ph/0610785, Bibcode:2007A&A...463..671R, doi:10.1051/0004-6361:20065224
  9. ^ a b Su, K. Y. L.; et al. (December 2006), "Debris Disk Evolution around A Stars", The Astrophysical Journal, 653 (1): 675–689, arXiv:astro-ph/0608563, Bibcode:2006ApJ...653..675S, doi:10.1086/508649
  10. ^ a b Allen, Richard Hinckley (1899), "Star-names and their meanings", New York, G. E. Stechert, Bibcode:1899sntm.book.....A
  11. ^ "PHECDA -- Emission-line Star", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2011-12-29
  12. ^ 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. ISBN 978-1-931559-44-7.
  13. ^ "IAU Catalog of Star Names". Retrieved 28 July 2016.
  14. ^ Garrison, R. F. (December 1993), "Anchor Points for the MK System of Spectral Classification", Bulletin of the American Astronomical Society, 25: 1319, Bibcode:1993AAS...183.1710G, retrieved 2012-02-04
  15. ^ 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
  16. ^ Perryman, Michael (2010), "The Making of History's Greatest Star Map", The Making of History's Greatest Star Map, Astronomers’ Universe, Heidelberg: Springer-Verlag, Bibcode:2010mhgs.book.....P, doi:10.1007/978-3-642-11602-5, ISBN 978-3-642-11601-8
  17. ^ a b c Shaya, Ed J.; Olling, Rob P. (January 2011), "Very Wide Binaries and Other Comoving Stellar Companions: A Bayesian Analysis of the Hipparcos Catalogue", The Astrophysical Journal Supplement, 192 (1): 2, arXiv:1007.0425, Bibcode:2011ApJS..192....2S, doi:10.1088/0067-0049/192/1/2
  18. ^ Garfinkle, Robert A. (1997), Star-Hopping: Your Visa to Viewing the Universe, Cambridge University Press, p. 118, ISBN 0-521-59889-3
  19. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  20. ^ "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016.
  21. ^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 15 日
  22. ^ Jaschek, C.; Andrillat, Y. (June 1998), "AE and A type shell stars", Astronomy and Astrophysics Supplement, 130 (3): 507–512, Bibcode:1998A&AS..130..507J, doi:10.1051/aas:1998101
  23. ^ Nordgren, Tyler E.; et al. (December 1999), "Stellar Angular Diameters of Late-Type Giants and Supergiants Measured with the Navy Prototype Optical Interferometer", The Astronomical Journal, 118 (6): 3032–3038, Bibcode:1999AJ....118.3032N, doi:10.1086/301114