Delta Ursae Majoris
 Location of δ Ursae Majoris (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] |
| Astrometry | |
| Radial velocity (Rv) | -20.2[4] km/s |
| Proper motion (μ) | RA: +143.42[1] mas/yr Dec.: -129.88[1] mas/yr |
| Parallax (π) | 55.82 ± 0.25[1] mas |
| Distance | 58.4 ± 0.3 ly (17.91 ± 0.08 pc) |
| Details | |
| Mass | 1.63[5] M☉ |
| Radius | 1.4[6] R☉ |
| Luminosity | 14[6] L☉ |
| Surface gravity (log g) | 4.49[6] |
| Temperature | 9,480 ± 570[6] K |
| Rotational velocity (v sin i) | 233[7] km/s |
| Age | 0.3[8] Gyr |
| Other designations | |
Delta Ursae Majoris (δ UMa, δ Ursae Majoris) is a star in the constellation Ursa Major. It has the traditional names Megrez (pronounced /ˈmɛɡrɛz/), from the Arabic: المغرز al-maghriz "the base [of the bear's tail]", and Kaffa. Megrez has an apparent magnitude of +3.3,[2] making it the dimmest of the seven stars in the Big Dipper. Parallax measurements yield a distance estimate of 58.4 light-years (17.9 parsecs) from Earth.[1]
Megrez has 63%[5] more mass of the Sun and is about 1.4[6] times the Sun's radius. 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 shines at 14[6] times the luminosity of the Sun, with this energy being emitted from its outer envelope at an effective temperature of 9,480 K.[6] This gives it the white hue typical of an A-type star.[10]
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.[8]
It has two faint companions, an 11th magnitude star at an angular separation of 190 arcseconds, and a 10th magnitude star 186 arcseconds away.[citation needed]
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.[11]
[edit] Namesakes
USS Megrez (AK-126) was a United States Navy Crater class cargo ship named after the star.
[edit] References
- ^ a b c d e f van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics 474 (2): 653–664, Bibcode 2007A&A...474..653V, doi:10.1051/0004-6361:20078357
- ^ a b c d Oja, T., "UBV photometry of stars whose positions are accurately known. III", Astronomy and Astrophysics Supplement Series 65 (2): 405–4
- ^ a b Cowley, A. et al. (April 1969), "A study of the bright A stars. I. A catalogue of spectral classifications", Astronomical Journal 74: 375–406, Bibcode 1969AJ.....74..375C, doi:10.1086/110819
- ^ Wielen, R. et al. (1999), Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions, Astronomisches Rechen-Institut Heidelberg, Bibcode 1999VeARI..35....1W
- ^ a b Shaya, Ed J.; Olling, Rob P. (January 2011), "Very Wide Binaries and Other Comoving Stellar Companions: A Bayesian Analysis of the Hipparcos Catalogue", The Astrophysical Journal Supplement 192 (1): 2, Bibcode 2011ApJS..192....2S, doi:10.1088/0067-0049/192/1/2
- ^ a b c d e f g Malagnini, M. L.; Morossi, C. (November 1990), "Accurate absolute luminosities, effective temperatures, radii, masses and surface gravities for a selected sample of field stars", Astronomy and Astrophysics Supplement Series 85 (3): 1015–1019, Bibcode 1990A&AS...85.1015M
- ^ Royer, F. et al. (2002), "Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i in the northern hemisphere", Astronomy and Astrophysics 393 (3): 897–911, arXiv:astro-ph/0205255, Bibcode 2002A&A...393..897R, doi:10.1051/0004-6361:20020943
- ^ a b Wyatt, M. C. et al. (July 2007), "Steady State Evolution of Debris Disks around A Stars", The Astrophysical Journal 663 (1): 365–382, arXiv:astro-ph/0703608, Bibcode 2007ApJ...663..365W, doi:10.1086/518404
- ^ "MEGREZ -- Variable Star", SIMBAD (Centre de Données astronomiques de Strasbourg), http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Delta+Ursae+Majoris, retrieved 2012-02-04
- ^ "The Colour of Stars", Australia Telescope, Outreach and Education (Commonwealth Scientific and Industrial Research Organisation), December 21, 2004, http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_colour.html, retrieved 2012-01-16
- ^ King, Jeremy R. et al. (April 2003), "Stellar Kinematic Groups. II. A Reexamination of the Membership, Activity, and Age of the Ursa Major Group", The Astronomical Journal 125 (4): 1980–2017, Bibcode 2003AJ....125.1980K, doi:10.1086/368241
