Phecda /ˈfɛkdə/, also called Gamma Ursae Majoris (γ Ursae Majoris, abbreviated Gamma UMa, γ UMa),[10][11] 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.[12] Based upon parallax measurements with the Hipparcos astrometry satellite,[13][14] it is located at a distance of around 83.2 light-years (25.5 parsecs) from the Sun.[1]
  | |
| 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 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 ± 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[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 | |
| Database references | |
| SIMBAD | data |
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 Merak is separated from Phecda by 11.0 ly (3.4 pc).[15]
Nomenclature
editγ 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').[16] In 2016, the International Astronomical Union organized a Working 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 included Phecda for this star.
To the Hindus this star was known as Pulastya, one of the seven rishis.[8]
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).[19]
Properties
editPhecda is an Ae star, which is surrounded by an envelope of gas that is adding emission lines to the spectrum of the star;[20] hence the 'e' suffix in the stellar 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 a rotational velocity of 386 km/s at its equator, which causes it to have an oblate shape. The equatorial radius measures 3.39 R☉, while the polar radius measures 2.19 R☉. The effective temperature varies as well, from 6,750 K in the equator to 10,520 K in the poles.[6]
Phecda 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]
References
edit- ^ 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, S2CID 18759600
- ^ a b c Oja, T. (1986), "UBV photometry of stars whose positions are accurately known. III", Astronomy and Astrophysics Supplement Series, 65 (2): 405–4, Bibcode:1986A&AS...65..405O
- ^ a b c d e 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. S2CID 73534476.
- ^ 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
- ^ 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. S2CID 119252073.
- ^ a b Jones, Jeremy; White, R. J.; Boyajian, T.; Schaefer, G.; Baines, E.; Ireland, M.; Patience, J.; Brummelaar, T. ten; McAlister, H.; Ridgway, S. T.; Sturmann, J.; Sturmann, L.; Turner, N.; Farrington, C.; Goldfinger, P. J. (2015-10-28). "The Ages of A-Stars I: Interferometric Observations and Age Estimates for Stars in the Ursa Major Moving Group". The Astrophysical Journal. 813 (1): 58. arXiv:1508.05643. doi:10.1088/0004-637X/813/1/58. ISSN 1538-4357.
- ^ a b 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
- ^ a b Allen, Richard Hinckley (1899), "Star-names and their meanings", New York, G. E. Stechert, Bibcode:1899sntm.book.....A
- ^ "PHECDA -- Emission-line Star", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2011-12-29
- ^ 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.
- ^ "IAU Catalog of Star Names". Retrieved 28 July 2016.
- ^ 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, archived from the original on 2019-06-25, 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, Astronomers' Universe, Heidelberg: Springer-Verlag, Bibcode:2010mhgs.book.....P, doi:10.1007/978-3-642-11602-5, ISBN 978-3-642-11601-8
- ^ a b 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, S2CID 119226823
- ^ Garfinkle, Robert A. (1997), Star-Hopping: Your Visa to Viewing the Universe, Cambridge University Press, p. 118, ISBN 0-521-59889-3
- ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
- ^ "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016.
- ^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 15 日 Archived 2014-11-02 at the Wayback Machine
- ^ 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