Star in the constellation Ursa Major
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! style="text-align: center; background-color: #FFFFC0;" colspan="2" | Observation dataEpoch J2000      Equinox J2000
! style="text-align:left" | Constellation
! style="text-align:left" | Right ascension
! style="text-align:left" | Declination
! style="text-align:left" | Apparent magnitude (V)
! style="background-color: #FFFFC0; text-align: center;" colspan="2"| Characteristics
! style="text-align:left" | Spectral type
! style="text-align:left" | U−B 
! style="text-align:left" | B−V 
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Megrez , also called Delta Ursae Majoris (δ Ursae Majoris, abbreviated Delta UMa, δ UMa), is a star in the northern 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 from the Sun.
Stellar properties.
Megrez has two times more mass than the Sun and is about 23 times more luminous. 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 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.
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.
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.
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.
Nomenclature.
"δ Ursae Majoris" (Latinised to "Delta Ursae Majoris") is the star's Bayer designation.
It bore the traditional name "Megrez" and the historical name "Kaffa". "Megrez" comes from the "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.
The Hindus knew this star as "Atri", one of the Seven Rishis.
In Chinese, (), meaning "Northern Dipper", refers to an asterism equivalent to the Big Dipper. Consequently, the Chinese name for Delta Ursae Majoris itself is (, ) and (, ).
Namesakes.
USS "Megrez" (AK-126) was a United States Navy Crater class cargo ship named after the star.
References.
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