Gamma Ursae Majoris
Epoch J2000 Equinox J2000
|Right ascension||11h 53m 49.84732s|
|Declination||+53° 41′ 41.1350″|
|Apparent magnitude (V)||+2.438|
|Spectral type||A0 Ve|
|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)
|Radius||3.04 ± 0.08 R☉|
|Surface gravity (log g)||3.79 cgs|
|Rotational velocity (v sin i)||178 km/s|
Gamma Ursae Majoris (Gamma UMa, γ Ursae Majoris, γ UMa) is a star in the constellation Ursa Major It has the traditional name Phad, or Phecda. 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).
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 Dubhe (upper-right), Merak (lower-right) and Megrez (upper-left). Along with four other stars in this well-known asterism, Phecda forms an actual 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 our Sun, although somewhat hotter, brighter and larger.
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.
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 UMa by 11.0 ly (3.4 pc).
Name and etymology
- The traditional name Phad, or Phecda comes from the Arabic phrase فخذ الدب "fakhð ad-dubb" ("thigh of the bear").
- This star as Pulastya, one of the Seven Rishis.
- In Chinese, 北斗 (Běi Dǒu), meaning Northern Dipper, refers to an asterism consisting of γ Ursae Majoris, α Ursae Majoris, β Ursae Majoris, δ Ursae Majoris, ε Ursae Majoris, ζ Ursae Majoris and η Ursae Majoris. Consequently, γ Ursae Majoris itself is known as 北斗三 (Běi Dǒu sān, English: the Third Star of Northern Dipper) and 天璣 (Tiān Jī, English: Star of Celestial Shining Pearl).
- 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
- Oja, T., "UBV photometry of stars whose positions are accurately known. III", Astronomy and Astrophysics Supplement Series 65 (2): 405–4
- 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
- Wielen, R. et al. (1999), Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions (35), Astronomisches Rechen-Institut Heidelberg, Bibcode:1999VeARI..35....1W
- 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
- 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
- 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
- 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
- Allen, Richard Hinckley (1899), Star-names and their meanings, G. E. Stechert
- "PHECDA -- Emission-line Star", SIMBAD (Centre de Données astronomiques de Strasbourg), retrieved 2011-12-29
- 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
- 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
- Perryman, Michael (2010), The Making of History's Greatest Star Map, Heidelberg: Springer-Verlag, doi:10.1007/978-3-642-11602-5
- Jaschek, C.; Andrillat, Y. (June 1998), "AE and A type shell stars", Astronomy and Astrophysics Supplement 130: 507–512, Bibcode:1998A&AS..130..507J, doi:10.1051/aas:1998101
- 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
- Garfinkle, Robert A. (1997), Star-Hopping: Your Visa to Viewing the Universe, Cambridge University Press, p. 118, ISBN 0-521-59889-3
- (Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 15 日