Megrez

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Megrez
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 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]
Distance
80.5 ± 0.3 ly
(24.69 ± 0.09 pc)
Absolute magnitude (MV)+1.39[5]
Details
Gyr
HIP 59774, HR 4660, IDS 12105+5735 A, PPM 33469, SAO 28315, WDS J12154+5702A.[10]
Database references
SIMBADdata

Megrez /ˈmiːɡrɛz/, also called Delta Ursae Majoris (δ Ursae Majoris, abbreviated Delta UMa, δ UMa),[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

The Big Dipper with Delta Ursae Majoris

Delta Ursae Majoris has 63% more mass than the Sun

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

arcminutes from the primary.[14]

Delta Ursae Majoris is a member of 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]

Nomenclature

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

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

Seven Rishis.[17]

In

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]

Namesakes

Crater class cargo ship
named after the star.

References

  1. ^
    S2CID 18759600
  2. ^
    Bibcode:1986A&AS...65..405O
  3. ^
    doi:10.1086/110819
  4. Bibcode:1999VeARI..35....1W
  5. doi:10.1086/300465
    .
  6. ^
    S2CID 119226823
  7. ^
    Bibcode:1990A&AS...85.1015M
  8. S2CID 14070763
  9. ^
    S2CID 18883195
  10. ^ "MEGREZ -- Variable Star", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2012-02-04
  11. 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. doi:10.1086/323920
    .
  15. doi:10.1086/368241
  16. ISBN 978-1-931559-44-7
    .
  17. 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
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