Omicron Ursae Majoris

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Omicron Ursae Majoris
Diagram showing star positions and boundaries of the Ursa Major constellation and its surroundings
Diagram showing star positions and boundaries of the Ursa Major constellation and its surroundings

Location of ο Ursa Major (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Ursa Major
Right ascension 08h 30m 15.87064s[1]
Declination +60° 43′ 05.4115″[1]
Apparent magnitude (V) +3.35[2]
Spectral type G4 II–III[3]
U−B color index +0.52[4]
B−V color index +0.85[4]
Radial velocity (Rv) +19.8[5] km/s
Proper motion (μ) RA: –133.76[1] mas/yr
Dec.: –107.45[1] mas/yr
Parallax (π) 18.21 ± 0.16[1] mas
Distance 179 ± 2 ly
(54.9 ± 0.5 pc)
Absolute magnitude (MV) –0.40[3]
Mass 3.09[3] M
Radius 14[6] R
Luminosity 138[3] L
Surface gravity (log g) 2.64[3] cgs
Temperature 5,242[3] K
Metallicity [Fe/H] –0.09[3] dex
Rotational velocity (v sin i) 15[7] km/s
Age 360 ± 30[8] Myr
Other designations
Muscida, ο Ursae Majoris, ο UMa, Omicron UMa, 1 Ursae Majoris, BD+61 1054, CCDM J08303+6043A, FK5 317, GC 11593, HD 71369, HIP 41704, HR 3323, PPM 16654, SAO 14573, WDS J08303+6043A.[9]
Database references

Omicron Ursae Majoris (Omicron UMa, ο Ursae Majoris, ο UMa) is a star system in the northern circumpolar constellation of Ursa Major. It has an apparent visual magnitude of +3.35[2] and is located at a distance of around 179 light-years (55 parsecs) from Earth.[1] This star has the traditional name Muscida, which it shares with the optical double star Pi Ursae Majoris.

The stellar classification of this star, G4 II–III,[3] places it midway between the giant and bright giant stages of its evolution. The interferometry-measured angular diameter of this star is about 2.42 mas,[10] which, at its estimated distance, equates to a physical radius of about 14 times the radius of the Sun.[6] It has about three[3] times the mass of the Sun and radiates 138[3] times the Sun's luminosity from its outer atmosphere at an effective temperature of 5,282 K,[2] giving it the yellowish hue of a G-type star.[11]

In 1963, East German astronomer Gerhard Jakisch reported this star as a variable with a period of 358 days and an amplitude of 0.08 magnitude. The 1982 edition of the New Catalogue of Suspected Variable Stars listed it with a variability from 3.30 to 3.36 in the visual band. However, in 1992 American astronomer Dorrit Hoffleit noted that the two comparison stars used to determine the variability may themselves be variable. Hence the actual variability of this star may be suspect.[12]

Muscida has a magnitude 15.2 common proper motion companion at an angular separation of 7.1 arcseconds.[13] With a probability of 99.4%, this companion is the source for the X-ray emission from the system.[14] Omicron Ursae Majoris is sometimes listed with two more companions, but, based on proper motion data, these appear to be optical companions.

This system is a member of the thin disk population and is following an orbit through the Milky Way galaxy with an eccentricity of 0.12. It comes as close to the Galactic Center as 23.5 kly (7.2 kpc) and as distant as 30.2 kly (9.3 kpc). This orbit carries it no more than about 330 ly (100 pc) above the galactic plane.[8] It is considered a runaway star because it has a high peculiar velocity of 35.5 km s−1 relative to the typical motion of stars in its vicinity.[15]

In 2012, extrasolar planet Omicron Ursae Majoris Ab orbiting the primary at 3.9 astronomical units, was found. This gas giant (4.1 times as massive as Jupiter) completes orbit in 1630 days.[16]

The Omicron Ursae Majoris planetary system[16]
(in order from star)
Mass Semimajor axis
Orbital period
Eccentricity Inclination Radius
b >4.1 MJ 3.9 1630±35 0.130 ± 0.065


In Chinese, 內階 (Nèi Jiē), meaning Inner Steps, refers to an asterism consisting of ο Ursae Majoris, 16 Ursae Majoris, 6 Ursae Majoris, 23 Ursae Majoris, 5 Ursae Majoris and 17 Ursae Majoris. Consequently, ο Ursae Majoris itself is known as 內階一 (Nèi Jiē yī, English: the First Star of Inner Steps.).[17]


  1. ^ a b c d e f van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357 
  2. ^ a b c Mallik, Sushma V. (December 1999), "Lithium abundance and mass", Astronomy and Astrophysics 352: 495–507, Bibcode:1999A&A...352..495M 
  3. ^ a b c d e f g h i j Takeda, Yoichi; Sato, Bun'ei; Murata, Daisuke (August 2008), "Stellar parameters and elemental abundances of late-G giants", Publications of the Astronomical Society of Japan 60 (4): 781–802, arXiv:0805.2434, Bibcode:2008PASJ...60..781T, doi:10.1093/pasj/60.4.781 
  4. ^ a b Johnson, H. L.; et al. (1966). "UBVRIJKL photometry of the bright stars". Communications of the Lunar and Planetary Laboratory 4 (99). Bibcode:1966CoLPL...4...99J. 
  5. ^ Wilson, Ralph Elmer (1953), General Catalogue of Stellar Radial Velocities, Washington: Carnegie Institution of Washington, Bibcode:1953GCRV..C......0W 
  6. ^ a b Lang, Kenneth R. (2006), Astrophysical formulae, Astronomy and astrophysics library 1 (3rd ed.), Birkhäuser, ISBN 3-540-29692-1 . The radius (R*) is given by:
  7. ^ Bernacca, P. L.; Perinotto, M. (1970), "A catalogue of stellar rotational velocities", Contributi Osservatorio Astronomico di Padova in Asiago 239 (1), Bibcode:1970CoAsi.239....1B 
  8. ^ a b Soubiran, C.; et al. (2008), "Vertical distribution of Galactic disk stars. IV. AMR and AVR from clump giants", Astronomy and Astrophysics 480 (1): 91–101, arXiv:0712.1370, Bibcode:2008A&A...480...91S, doi:10.1051/0004-6361:20078788 
  9. ^ "bet CMi -- Be Star", SIMBAD Astronomical Object Database (Centre de Données astronomiques de Strasbourg), retrieved 2012-01-09 
  10. ^ Richichi, A.; Percheron, I.; Khristoforova, M. (February 2005), "CHARM2: An updated Catalog of High Angular Resolution Measurements", Astronomy and Astrophysics 431: 773–777, Bibcode:2005A&A...431..773R, doi:10.1051/0004-6361:20042039 
  11. ^ "The Colour of Stars", Australia Telescope, Outreach and Education (Commonwealth Scientific and Industrial Research Organisation), December 21, 2004, retrieved 2012-01-16 
  12. ^ Hoffleit, D. (October 1992), "Do all Three Vary: omicron UMa, 23 UMa and HR 3245?", Information Bulletin on Variable Stars 3789: 1, Bibcode:1992IBVS.3789....1H 
  13. ^ Eggleton, P. P.; Tokovinin, A. A. (September 2008). "A catalogue of multiplicity among bright stellar systems". Monthly Notices of the Royal Astronomical Society 389 (2): 869–879. arXiv:0806.2878. Bibcode:2008MNRAS.389..869E. doi:10.1111/j.1365-2966.2008.13596.x. 
  14. ^ Haakonsen, Christian Bernt; Rutledge, Robert E. (September 2009), "XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources", The Astrophysical Journal Supplement 184 (1): 138–151, arXiv:0910.3229, Bibcode:2009ApJS..184..138H, doi:10.1088/0067-0049/184/1/138 
  15. ^ Tetzlaff, N.; Neuhäuser, R.; Hohle, M. M. (January 2011), "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun", Monthly Notices of the Royal Astronomical Society 410 (1): 190–200, arXiv:1007.4883, Bibcode:2011MNRAS.410..190T, doi:10.1111/j.1365-2966.2010.17434.x 
  16. ^ a b Sato, Bun'ei; et al. (2012). "Substellar Companions to Seven Evolved Intermediate-Mass Stars". Publications of the Astronomical Society of Japan 64 (6). 135. arXiv:1207.3141. Bibcode:2012PASJ...64..135S. doi:10.1093/pasj/64.6.135. 
  17. ^ (Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 16 日