Megrez

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Template:Short description

Megrez
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Location of Megrez (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Ursa Major
Right ascension Template:RA[1]
Declination Template:DEC[1]
Apparent magnitude (V) +3.312[2]
Characteristics
Spectral type A3 V[3]
U−B Template:Engvar +0.067[2]
B−V Template:Engvar +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]
DistanceTemplate:Rnd ± Template:Rnd ly
(Template:Rnd ± Template:Rnd pc)
Absolute magnitude (MV)+1.39[5]
Details[6]
MassScript error: No such module "val". Template:Solar mass
Radius2.512 (equatorial)
1.921 (polar) Template:Solar radius
LuminosityScript error: No such module "val". Template:Solar luminosity
Surface gravity (log g)4.49[7] cgs
Temperature6,909 (equatorial)
10,030 (polar) K
Rotation3.1[6]Template:Efn hours
Rotational velocity (v sin i)Script error: No such module "val". km/s
AgeScript error: No such module "val". Myr
Metallicity [Fe/H]{{{metal_fe2}}} dex
Other designations
Template:Odlist, IDS 12105+5735 A[8]
Database references
SIMBADdata

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Megrez Template:IPAc-en, 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 Script error: No such module "convert". from the Sun.[1]

Stellar properties

File:Http---www.nasa.gov-feature-goddard-2016-hubble-team-breaks-cosmic-distance-record (25451077706).jpg
The Big Dipper with Delta Ursae Majoris

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

File:Sidney Hall - Urania's Mirror - Ursa Major.jpg
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 Template:IPAc-en and the historical name Kaffa. Megrez comes from the Template:Langx 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, Script error: No such module "Lang". (Script error: No such module "Lang".), meaning Northern Dipper, refers to an asterism equivalent to the Big Dipper. Consequently, the Chinese name for Delta Ursae Majoris itself is Script error: No such module "Lang". (Script error: No such module "Lang"., Template:Langx) and Script error: No such module "Lang". (Script error: No such module "Lang"., Template:Langx).[16]

Namesakes

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

Notes

Template:Notelist

References

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  1. ↑ a b c d e f Script error: No such module "citation/CS1".
  2. ↑ a b c d Script error: No such module "citation/CS1".
  3. ↑ a b Script error: No such module "citation/CS1".
  4. ↑ Script error: No such module "citation/CS1".
  5. ↑ Script error: No such module "citation/CS1".
  6. ↑ a b c d Script error: No such module "Citation/CS1".
  7. ↑ Script error: No such module "citation/CS1".
  8. ↑ Script error: No such module "citation/CS1".
  9. ↑ Script error: No such module "citation/CS1".
  10. ↑ Script error: No such module "citation/CS1".
  11. ↑ Script error: No such module "citation/CS1".
  12. ↑ Script error: No such module "Citation/CS1".
  13. ↑ Script error: No such module "citation/CS1".
  14. ↑ Script error: No such module "citation/CS1".
  15. ↑ Script error: No such module "citation/CS1".
  16. ↑ Template:In lang AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 15 日 Script error: No such module "webarchive".

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Template:Stars of Ursa Major

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