Megrez /ˈmiːɡrɛz/, also called Delta Ursae Majoris (δ Ursae Majoris, abbreviated Delta UMa, δ UMa),[9][10] is a star in the northern constellation of Ursa Major. With an apparent magnitude of +3.3,[2] it is the dimmest of the seven stars in the Big Dipper asterism. Parallax measurements yield a distance estimate of 80.5 light-years (24.7 parsecs) from the Sun.[1]
  Location of Megrez (circled) | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Ursa Major |
| Right ascension | 12h 15m 25.56063s[1] |
| Declination | +57° 01′ 57.4156″[1] |
| Apparent magnitude (V) | +3.312[2] |
| Characteristics | |
| Spectral type | A3 V[3] |
| U−B color index | +0.067[2] |
| B−V color index | +0.075[2] |
| Astrometry | |
| Radial velocity (Rv) | -20.2[4] km/s |
| Proper motion (μ) | RA: +104.11[1] mas/yr Dec.: 7.30[1] mas/yr |
| Parallax (π) | 40.51 ± 0.15 mas[1] |
| Distance | 80.5 ± 0.3 ly (24.69 ± 0.09 pc) |
| Absolute magnitude (MV) | +1.39[5] |
| Details[6] | |
| Mass | 2.062+0.020 −0.033 M☉ |
| Radius | 2.512 (equatorial) 1.921 (polar) R☉ |
| Luminosity | 23+1.24 −1.21 L☉ |
| Surface gravity (log g) | 4.49[7] cgs |
| Temperature | 6,909 (equatorial) 10,030 (polar) K |
| Rotation | 3.1[6][a] hours |
| Rotational velocity (v sin i) | 244.6+11.6 −11.1 km/s |
| Age | 414+35 −43 Myr |
| Other designations | |
| Database references | |
| SIMBAD | data |
Stellar properties
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Megrez has two times more mass than the Sun and is about 23 times more luminous.[6] It has a stellar classification of A3 V,[3] which means it is an A-type main sequence star that is generating energy at its core through the nuclear fusion of hydrogen. It rotates rapidly, taking 3.1 hours to complete a rotation across its equator, causing Megrez to have an oblate shape and hotter temperatures at the poles due to gravity darkening.[6]
This star has an excess emission of infrared radiation, indicating the presence of circumstellar matter. This forms a debris disk around an orbital radius of 16 astronomical units from the star. This radius is unusually small for the estimated age of the disk, which may be explained by drag from the Poynting–Robertson effect causing the dust to spiral inward.[11]
It has two faint companions, a 10th magnitude star and an 11th magnitude star, both at an angular separation of two arcminutes from the primary.[12]
Delta Ursae Majoris is a member of the Ursa Major moving group, an association of stars that share a common motion through space and likely formed in the same molecular cloud. The space velocity components of Delta Ursae Majoris in the galactic coordinate system are [U, V, W] = [+15.35, +1.17, –11.52] km s−1.[13]
Nomenclature
edit
δ Ursae Majoris (Latinised to Delta Ursae Majoris) is the star's Bayer designation.
It bore the traditional name Megrez /ˈmɛɡrɛz/ and the historical name Kaffa. Megrez comes from the Arabic: المغرز al-maghriz 'the base [of the bear's tail]'. Professor Paul Kunitzch has been unable to find any clues as to the origin of the name Kaffa, which appeared in a 1951 publication, Atlas Coeli (Skalnate Pleso Atlas of the Heavens) by Czech astronomer Antonín Bečvář.[14]
The Hindus knew this star as Atri, one of the Seven Rishis.[15]
In Chinese, 北斗 (Běi Dǒu), meaning Northern Dipper, refers to an asterism equivalent to the Big Dipper. Consequently, the Chinese name for Delta Ursae Majoris itself is 北斗四 (Běi Dǒu sì, English: the Fourth Star of Northern Dipper) and 天權 (Tiān Quán, English: Star of Celestial Balance).[16]
Namesakes
editUSS Megrez (AK-126) was a United States Navy Crater class cargo ship named after the star.
Notes
edit- ^ From equatorial radius and equatorial velocity.
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 d 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 Cowley, A.; et al. (April 1969), "A study of the bright A stars. I. A catalogue of spectral classifications", Astronomical Journal, 74: 375–406, Bibcode:1969AJ.....74..375C, doi:10.1086/110819
- ^ 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
- ^ Eggen, Olin J. (August 1998), "The Sirius Supercluster and Missing Mass near the Sun", The Astronomical Journal, 116 (2): 782–788, Bibcode:1998AJ....116..782E, doi:10.1086/300465.
- ^ a b c d 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.
- ^ Malagnini, M. L.; Morossi, C. (November 1990), "Accurate absolute luminosities, effective temperatures, radii, masses and surface gravities for a selected sample of field stars", Astronomy and Astrophysics Supplement Series, 85 (3): 1015–1019, Bibcode:1990A&AS...85.1015M
- ^ "MEGREZ -- Variable Star", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2012-02-04
- ^ 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.
- ^ Wyatt, M. C.; et al. (July 2007), "Steady State Evolution of Debris Disks around A Stars", The Astrophysical Journal, 663 (1): 365–382, arXiv:astro-ph/0703608, Bibcode:2007ApJ...663..365W, doi:10.1086/518404, S2CID 18883195
- ^ Mason, Brian D.; Wycoff, Gary L.; Hartkopf, William I.; Douglass, Geoffrey G.; Worley, Charles E. (2001). "The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog". The Astronomical Journal. 122 (6): 3466. Bibcode:2001AJ....122.3466M. doi:10.1086/323920.
- ^ 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
- ^ Kunitzch, Paul; Smart, Tim (2006) [1986]. A Dictionary of Modern Star Names: A Short Guide to 254 Star Names and Their Derivations. Cambridge, Massachusetts: Sky Publishing Corporation. pp. 56, 62. ISBN 978-1-931559-44-7.
- ^ Allen, R. H. (1963). Star Names: Their Lore and Meaning (Reprint ed.). New York: Dover Publications Inc. p. 438. ISBN 0-486-21079-0. Retrieved 2012-09-04.
- ^ (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