  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−Bcolor index | +0.067[2] |
| B−Vcolor index | +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] |
| Distance | 80.5 ± 0.3 ly (24.69 ± 0.09 pc) |
| Absolute magnitude (MV) | +1.39[5] |
| Details[6] | |
| Mass | 2.062+0.020 −0.033 M☉ |
| Radius | 2.512 (equatorial) 1.921 (polar) R☉ |
| Luminosity | 23+1.24 −1.21 L☉ |
| Surface gravity (log g) | 4.49[7] cgs |
| Temperature | 6,909 (equatorial) 10,030 (polar) K |
| Rotation | 3.1[6][a] hours |
| Rotational velocity (v sin i) | 244.6+11.6 −11.1 km/s |
| Age | 414+35 −43 Myr |
| 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.[8] | |
| Database references | |
| SIMBAD | data |
Megrez/ˈmiːɡrɛz/, also calledDelta Ursae Majoris (δ Ursae Majoris, abbreviatedDelta UMa,δ UMa),[9][10] is astar in the northernconstellation ofUrsa Major. With anapparent magnitude of +3.3,[2] it is the dimmest of the seven stars in theBig Dipperasterism.Parallax measurements yield a distance estimate of 80.5light-years (24.7parsecs) from theSun.[1]
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Megrez has two times more mass than the Sun and is about 23 times more luminous.[6] It has astellar classification of A3 V,[3] which means it is anA-type main sequence star that is generating energy at its core through thenuclear fusion of hydrogen. It rotates rapidly, taking 3.1 hours to complete a rotation across itsequator, causing Megrez to have an oblate shape and hotter temperatures at the poles due togravity darkening.[6]
This star has anexcess emission of infrared radiation, indicating the presence of circumstellar matter. This forms adebris 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 thePoynting–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 anangular separation of twoarcminutes from the primary.[12]
Delta Ursae Majoris is a member of theUrsa Major moving group, anassociation of stars that share a common motion through space and likely formed in the samemolecular cloud. Thespace velocity components of Delta Ursae Majoris in thegalactic coordinate system are [U, V, W] = [+15.35, +1.17, –11.52] km s−1.[13]
δ Ursae Majoris (Latinised toDelta Ursae Majoris) is the star'sBayer designation.
It bore the traditional nameMegrez/ˈmɛɡrɛz/ and the historical nameKaffa.Megrez comes from theArabic:المغرزal-maghriz 'the base [of the bear's tail]'. Professor Paul Kunitzch has been unable to find any clues as to the origin of the nameKaffa, which appeared in a 1951 publication,Atlas Coeli (Skalnate Pleso Atlas of the Heavens) by Czech astronomerAntonín Bečvář.[14]
TheHindus knew this star asAtri, one of theSeven Rishis.[15]
InChinese,北斗 (Běi Dǒu), meaningNorthern Dipper, refers to anasterism equivalent to the Big Dipper. Consequently, theChinese 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).[16]
USSMegrez (AK-126) was aUnited States NavyCrater class cargo ship named after the star.
