Theta Ursae Majoris
Epoch J2000 Equinox J2000
|Right ascension||09h 32m 51.43390s|
|Declination||+51° 40′ 38.2811″|
|Apparent magnitude (V)||3.166|
|Spectral type||F6 IV|
|U−B color index||+0.03|
|B−V color index||+0.46|
|Radial velocity (Rv)||+14.6 km/s|
|Proper motion (μ)||RA: -947.46 mas/yr
Dec.: -535.60 mas/yr
|Parallax (π)||74.19 ± 0.14 mas|
|Distance||43.96 ± 0.08 ly
(13.48 ± 0.03 pc)
|Absolute magnitude (MV)||2.43|
|Radius||2.365 ± 0.008 R☉|
|Luminosity||7.871 ± 0.158 L☉|
|Surface gravity (log g)||3.80 ± 0.10 cgs|
|Temperature||6,300 ± 33 K|
|Metallicity [Fe/H]||–0.18 ± 0.07 dex|
|Rotational velocity (v sin i)||6.8 km/s|
Theta Ursae Majoris (Theta UMa, θ Ursae Majoris, θ UMa) is a suspected spectroscopic binary star system in the northern circumpolar constellation of Ursa Major. It has an apparent visual magnitude of 3.17, placing it among the brighter members of this constellation. The distance to this star has been measured directly using the parallax method, yielding an estimated value of 43.96 light-years (13.48 parsecs).
In 1976, this was reported as a spectroscopic binary system by Helmut A. Abt and Saul G. Levy, giving it an orbital period of 371 days. This has been listed as a spectroscopic binary system with an orbital period of 371 days. However, this was brought into question by Christopher L. Morbey and Roger F. Griffin in 1987, who suggested that the data could be explained by random chance. Further observations in 2009 with observations with the Bok Telescope in Arizona did show changes of 180 m/s in radial velocity, although there was not sufficient evidence to support a Keplerian orbit. There is a 14th-magnitude common proper motion companion to Theta Ursae Majoris at an angular separation of 4.1 arcseconds, so this may potentially be a triple star system.
The primary component of this putative system has a published stellar classification of F6 IV, indicating it is a subgiant star that is evolving away from the main sequence. In 2009, Helmut A. Abt listed it with a stellar classification of F7 V, suggesting that it is still on the main sequence. It is larger than the Sun with 141% of the Sun's mass and 250% of the Sun's radius. Consequently it is shining brighter and evolving more rapidly than the Sun, with a luminosity nearly eight times the Sun's at an age of 2.2 billion years. This energy is being radiated from the star's outer atmosphere at an effective temperature of 6,300 K. At this heat, the star glows with the yellow-white hue of an F-type star.
The McDonald Observatory team has set limits to the hypothetical presence of one or more planets around the primary with masses between 0.24 and 4.6 Jupiter masses and average separations spanning between 0.05 and 5.2 AU.
Naming and etymology
- With τ, h, υ, φ, e, and f, it composed the Arabic asterism Sarīr Banāt al-Na'sh, the Throne of the daughters of Na'sh, and al-Haud, the Pond,
- In Chinese, 文昌 (Wén Chāng), meaning Administrative Center, refers to an asterism consisting of θ Ursae Majoris, φ Ursae Majoris, υ Ursae Majoris, 15 Ursae Majoris and 18 Ursae Majoris. Consequently, φ Ursae Majoris itself is known as 文昌四 (Wén Chāng sì, English: the Fourth Star of Administrative Center.).
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- Abt, H. A.; Levy, S. G. (March 1976), "Multiplicity among solar-type stars", Astrophysical Journal Supplement Series, 30: 273–306, Bibcode:1976ApJS...30..273A, doi:10.1086/190363
- Morbey, C. L.; Griffin, R. F. (June 1987), "On the reality of certain spectroscopic orbits", Astrophysical Journal, Part 1, 317: 343–352, Bibcode:1987ApJ...317..343M, doi:10.1086/165281
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- Kaler, James B., "THETA UMA (Theta Ursae Majoris)", Stars, University of Illinois, retrieved 2012-02-25
- Abt, Helmut A. (January 2009), "MK Classifications of Spectroscopic Binaries", The Astrophysical Journal Supplement, 180 (1): 117–118, Bibcode:2009ApJS..180..117A, doi:10.1088/0067-0049/180/1/117
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- Allen, Richard Hinckley (1899), Star-Names and Their Meanings, New York: G. E. Stechert, p. 442
- (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 16 日