Delta Ursae Majoris

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Delta Ursae Majoris
Diagram showing star positions and boundaries of the Ursa Major constellation and its surroundings
Diagram showing star positions and boundaries of the Ursa Major constellation and its surroundings

Location of δ Ursae Majoris (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]
Spectral type A3 V[3]
U−B color index +0.067[2]
B−V color index +0.075[2]
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]
Distance80.5 ± 0.3 ly
(24.69 ± 0.09 pc)
Absolute magnitude (MV)+1.39[5]
Mass1.63[6] M
Radius1.4[7] R
Luminosity14[7] L
Surface gravity (log g)4.49[7] cgs
Temperature9,480±570[7] K
Rotational velocity (v sin i)233[8] km/s
Age0.3[9] Gyr
Other designations
Megrez, δ Ursae Majoris, δ UMa, Delta UMa, 69 Ursae Majoris, BD+57 1363, CCDM J12155+5702A, FK5 456, GC 16736, HD 106591, HIP 59774, HR 4660, IDS 12105+5735 A, PPM 33469, SAO 28315, WDS J12154+5702A.[10]
Database references

Delta Ursae Majoris (δ Ursae Majoris, abbreviated Delta UMa, δ UMa), formally named Megrez /ˈmɡrɛz/,[11][12] 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]

Stellar properties[edit]

The Big Dipper with Delta Ursae Majoris

Delta Ursae Majoris has 63% more mass than the Sun[6] and is about 1.4 times its radius.[7] 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 shines at 14[7] times the luminosity of the Sun, with this energy being emitted from its outer envelope at an effective temperature of 9,480 K.[7] This gives it the white hue typical of an A-type star.[13]

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.[9]

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.[14]

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.[15]


Book plate by Sydney Hall depicting Ursa Major's stars

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.[16]

The Hindus knew this star as Atri, one of the Seven Rishis.[17]

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).[18]


USS Megrez (AK-126) was a United States Navy Crater class cargo ship named after the star.


  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, S2CID 18759600
  2. ^ 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
  3. ^ 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
  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. ^ 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.
  6. ^ 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
  7. ^ a b c d e f g 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
  8. ^ Royer, F.; et al. (2002), "Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i in the northern hemisphere", Astronomy and Astrophysics, 393 (3): 897–911, arXiv:astro-ph/0205255, Bibcode:2002A&A...393..897R, doi:10.1051/0004-6361:20020943, S2CID 14070763
  9. ^ a b 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
  10. ^ "MEGREZ -- Variable Star", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2012-02-04
  11. ^ 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.
  12. ^ "IAU Catalog of Star Names". Retrieved 28 July 2016.
  13. ^ "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on 2012-03-18, retrieved 2012-01-16
  14. ^ 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.
  15. ^ 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
  16. ^ 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.
  17. ^ 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.
  18. ^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 15 日