Delta Ursae Majoris
Delta Ursae Majoris (δ Ursae Majoris, abbreviated Delta UMa, δ UMa), formally named Megrez //, is a star in the northern circumpolar constellation of Ursa Major. With an apparent magnitude of +3.3, 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.
Epoch J2000 Equinox J2000
|Right ascension||12h 15m 25.56063s|
|Declination||+57° 01′ 57.4156″|
|Apparent magnitude (V)||+3.312|
|Spectral type||A3 V|
|U−B color index||+0.067|
|B−V color index||+0.075|
|Radial velocity (Rv)||-20.2 km/s|
|Proper motion (μ)|| RA: +104.11 mas/yr |
Dec.: 7.30 mas/yr
|Parallax (π)||40.51 ± 0.15 mas|
|Distance||80.5 ± 0.3 ly |
(24.69 ± 0.09 pc)
|Absolute magnitude (MV)||+1.39|
|Surface gravity (log g)||4.49 cgs|
|Rotational velocity (v sin i)||233 km/s|
Megrez has 63% more mass than the Sun and is about 1.4 times its radius. It has a stellar classification of A3 V, 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 times the luminosity of the Sun, with this energy being emitted from its outer envelope at an effective temperature of 9,480 K. This gives it the white hue typical of an A-type star.
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.
Megrez is a marginally outlying 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.
It bore the traditional name Megrez // 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.
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).
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- (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 15 日