Sigma2 Ursae Majoris

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Sigma2 Ursae Majoris
Sigma2 Ursae Majoris is located in 100x100
Sigma2 Ursae Majoris
Location of σ2 Ursae Majoris (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Ursa Major
σ2 UMa A
Right ascension 09h 10m 23.538s[1]
Declination +67° 08′ 02.44″[1]
Apparent magnitude (V) +4.813[1]
σ2 UMa B
Right ascension 09h 10m 23.508s[1]
Declination +67° 08′ 06.58″[1]
Apparent magnitude (V) +10.26[1]
Spectral type F6IV-V / K2V[2]
U−B color index +0.01[3]
B−V color index +0.48[3]
Variable type Suspected[4]
σ2 UMa A
Radial velocity (Rv) −2.92 ± 0.12[5] km/s
Proper motion (μ) RA: 7.1[1] mas/yr
Dec.: −95.1[1] mas/yr
Parallax (π) 49.07 ± 0.37[6] mas
Distance 66.5 ± 0.5 ly
(20.4 ± 0.2 pc)
Absolute bolometric
σ2 UMa B
Proper motion (μ) RA: 4.1[1] mas/yr
Dec.: −30.0[1] mas/yr
Absolute magnitude (MV) 7.16[7]
Period (P) 970 ± 118 yr
Semi-major axis (a) 5.80 ± 0.14″
Eccentricity (e) 0.801 ± 0.017
Inclination (i) 145.4 ± 1.5°
Longitude of the node (Ω) 102.1 ± 1.9°
Periastron epoch (T) B 1917.39 ± 0.12
Argument of periastron (ω)
332.4 ± 1.9°
σ2 UMa A
Mass 1.31 M
Radius 1.75 ± 0.07 R
Surface gravity (log g) 4.00 ± 0.10 cgs
Temperature 6276 ± 80 K
Metallicity [Fe/H] −0.03 ± 0.07 dex
Rotational velocity (v sin i) 4.1 ± 0.8 km/s
σ2 UMa B
Mass ~0.73 M
Temperature ~4600 K
Other designations
BD+67° 577, Gl 335, HD 78154, HIP 45038, HR 3616, SAO 14788[4]
σ2 UMa A: TYC 4141-1496-1
σ2 UMa B: TYC 4141-1496-2
Database references

Sigma2 Ursae Majoris (σ2 Ursae Majoris, σ2 UMa) is a binary star in the constellation of Ursa Major. Parallax measurements made by the Hipparcos spacecraft put it at a distance of about 66.5 light years (20.4 parsecs) from Earth, making this a fairly nearby system. The primary component has an apparent magnitude of about 4.8,[1] meaning it can be seen with the naked eye (see Bortle scale).

This is a visual binary, meaning that the two components can be resolved, and the orbit is derived from the positions of the two stars. The primary component Sigma2 Ursae Majoris A, is a white-colored F-type subgiant. Its radius is about 1.75 times that of the Sun, and it is 31% more massive.[7] The companion is an orange K-type main-sequence star that is much fainter. The two stars are separated about 4 arcseconds away, and because of their slow orbital motion the orbit is poorly known: estimates of the orbital period range from 970 years[8] to over 1,500 years.[9] There is a third component, designated Sigma2 Ursae Majoris C. Located 205 arcseconds from the primary, it is thought to be a line-of-sight coincidence, and is not related to the system.


In Chinese, 三師 (Sān Shī), meaning Three Top Instructors, refers to an asterism consisting of σ2 Ursae Majoris and ρ Ursae Majoris. Consequently, σ2 Ursae Majoris itself is known as 三師三 (Sān Shī sān, English: the Third Star of Three Top Instructors.).[10]


  1. ^ a b c d e f g h i j k Høg, E.; et al. (2000). "The Tycho-2 catalogue of the 2.5 million brightest stars". Astronomy and Astrophysics. 355: L27–L30. Bibcode:2000A&A...355L..27H. 
  2. ^ Edwards, T. W. (1976). "MK classification for visual binary components". Astronomical Journal. 81: 245–249. Bibcode:1976AJ.....81..245E. doi:10.1086/111879. 
  3. ^ a b Mermilliod, J.-C. (1986). "Compilation of Eggen's UBV data, transformed to UBV (unpublished)". Bibcode:1986EgUBV........0M. 
  4. ^ a b "* sig02 UMa". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 27 March 2017. 
  5. ^ de Medeiros, J. R.; Mayor, M. (1999). "A catalog of rotational and radial velocities for evolved stars". Astronomy and Astrophysics Supplement. 139: 433–460. Bibcode:1999A&AS..139..433D. doi:10.1051/aas:1999401. 
  6. ^ van Leeuwen, F.; et al. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752Freely accessible. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. 
  7. ^ a b c d Fuhrmann, Klaus (2008). "Nearby stars of the Galactic disc and halo - IV". Monthly Notices of the Royal Astronomical Society. 384 (1): 173–224. Bibcode:2008MNRAS.384..173F. doi:10.1111/j.1365-2966.2007.12671.x. 
  8. ^ a b "Sixth Catalog of Orbits of Visual Binary Stars". United States Naval Observatory. 
  9. ^ Shklovskii, I. S.; Wenzel, W. (1980). "Book-Review - Stars Their Birth Life and Death". Astronomische Nachrichten. 301: 99. Bibcode:1980AN....301...99S. 
  10. ^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 16 日