Theta Aurigae

Theta Aurigae (θ Aurigae, abbreviated Theta Aur, θ Aur) is a binary star in the constellation of Auriga. Based upon parallax measurements, the distance to this system is about 166 light-years (51 parsecs).[1]

θ Aurigae
Auriga constellation map.svg
Red circle.svg
Location of θ Aurigae (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Auriga
Right ascension 05h 59m 43.27012s[1]
Declination +37° 12′ 45.3047″[1]
Apparent magnitude (V) 2.62 - 2.70[2]
Spectral type A0pSi + F2-5V[3]
U−B color index -0.18[4]
B−V color index -0.08[4]
R−I color index -0.06
Variable type α2 CVn[2]
Radial velocity (Rv)+29.5[5] km/s
Proper motion (μ) RA: +43.63[1] mas/yr
Dec.: -73.79[1] mas/yr
Parallax (π)19.70 ± 0.16 mas
Distance166 ± 1 ly
(50.8 ± 0.4 pc)
Absolute magnitude (MV)–1.05[6]
Mass3.38 ± 0.08[6] M
Radius5.1 ± 0.4[7] R
Luminosity263[6] L
Surface gravity (log g)3.6 ± 0.05[7] cgs
Temperature10,400 ± 300[7] K
Rotation3.6187 days[8]
Rotational velocity (v sin i)55[7] km/s
Other designations
Mahasim, 37 Aurigae, BD+37° 1380, HD 40312, HIP 28380, HR 2095, SAO 58636, WDS 05597+3713.
Database references

The two components are designated Theta Aurigae A (also named Mahasim[9]) and B.


θ Aurigae (Latinised to Theta Aurigae) is the system's Bayer designation. The designations of the two components as Theta Aurigae A and B derives from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).[10]

Some authors state that Theta Aurigae had no traditional name,[11] although Richard Hinckley Allen makes a passing reference about the name Mahasim, as a name also used, with various spellings, for Eta Aurigae and Lambda Herculis,[12] from the Arabic المِعْصَم al-miʽşam "the wrist" (of the charioteer). In 2016, the IAU organized a Working Group on Star Names (WGSN)[13] to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.[14] It approved the name Mahasim for the component Theta Aurigae A on 30 June 2017 and it is now so included in the List of IAU-approved Star Names.[9]

It is known as 五車四 (the Fourth Star of the Five Chariots) in Chinese[15]


The primary component, Theta Aurigae A, is a large star with more than three[6] times the mass of the Sun and over five[7] times the Sun's radius. It is radiating 263[6] times the Sun's luminosity from its outer atmosphere at an effective temperature of 10,400 K,[7] giving it the white hue of an A-type star. The star has a stellar classification of A0pSi,[3] with the 'pSi' suffix indicating it is a chemically peculiar star with an abnormal abundance of silicon.

The primary is classified an Alpha2 Canum Venaticorum type variable star and has a surface magnetic field of about 1 kG.[7] Its projected rotational velocity is 55 km s−1,[7] with the star completing a rotation in only 3.6 days.[8] The axis of rotation is inclined by an angle of 51◦ ± 6 to the line of sight from the Earth.[7]

The secondary, Theta Aurigae B, is a +7.2 magnitude companion, 4.5[3] magnitudes fainter than the primary, located at an angular separation of 3.91 arcseconds along a position angle of 304.9° as of 2002.[16] This is an F-type main sequence star with a stellar classification in the range F2-5 V.[3]

The mean combined apparent magnitude of the system is +2.65 but the variation of the primary causes the system's brightness to range from magnitude +2.62 to +2.70 with a period of 1.37 days. The system is an X-ray source with a luminosity of 9.49 × 1026 erg s−1.[3]


  1. ^ a b c d e 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, S2CID 18759600
  2. ^ a b Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1: B/gcvs. Bibcode:2009yCat....102025S.
  3. ^ a b c d e Schröder, C.; Schmitt, J. H. M. M. (November 2007), "X-ray emission from A-type stars", Astronomy and Astrophysics, 475 (2): 677–684, Bibcode:2007A&A...475..677S, doi:10.1051/0004-6361:20077429
  4. ^ a b Johnson, H. L.; et al. (1966). "UBVRIJKL photometry of the bright stars". Communications of the Lunar and Planetary Laboratory. 4 (99): 99. Bibcode:1966CoLPL...4...99J.
  5. ^ Evans, D. S. (June 20–24, 1966), "The Revision of the General Catalogue of Radial Velocities", in Batten, Alan Henry; Heard, John Frederick (eds.), Determination of Radial Velocities and their Applications, Proceedings from IAU Symposium no. 30, 30, University of Toronto: International Astronomical Union, p. 57, Bibcode:1967IAUS...30...57E
  6. ^ a b c d e North, P. (June 1998), "Do SI stars undergo any rotational braking?", Astronomy and Astrophysics, 334: 181–187, arXiv:astro-ph/9802286, Bibcode:1998A&A...334..181N
  7. ^ a b c d e f g h i Shulyak, D.; et al. (March 2007), "The Lorentz force in atmospheres of CP stars: θ Aurigae", Astronomy and Astrophysics, 464 (3): 1089–1099, arXiv:astro-ph/0612301, Bibcode:2007A&A...464.1089S, doi:10.1051/0004-6361:20064998, S2CID 119333293
  8. ^ a b Rice, J. B.; Holmgren, D. E.; Bohlender, D. A. (September 2004), "The distribution of oxygen on the surface of the Ap star θ Aur. An abundance Doppler image to compare with ɛ UMa", Astronomy and Astrophysics, 424: 237–244, Bibcode:2004A&A...424..237R, doi:10.1051/0004-6361:20035639
  9. ^ a b "Naming Stars". Retrieved 16 December 2017.
  10. ^ Hessman, F. V.; Dhillon, V. S.; Winget, D. E.; Schreiber, M. R.; Horne, K.; Marsh, T. R.; Guenther, E.; Schwope, A.; Heber, U. (2010). "On the naming convention used for multiple star systems and extrasolar planets". arXiv:1012.0707 [astro-ph.SR].
  11. ^ Kaler, Jim. "Theta Aur". Retrieved 2017-01-05.
  12. ^ Allen, Richard Hinckley (1963) [First published in 1899]. Star Names: Their Lore and Meaning. New York: Dover Publications, Inc. p. 245. ISBN 0-486-21079-0.
  13. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  14. ^ "WG Triennial Report (2015-2018) - Star Names" (PDF). p. 5. Retrieved 2018-07-14.
  15. ^ "香港太空 館 – 研究資源 – 亮星中英對照表 (Chinese/English Star names)" (in Chinese). Hong Kong Space Museum. Archived from the original on 29 September 2009. Retrieved 31 December 2008.
  16. ^ Roberts, Lewis C., Jr. (May 2011), "Astrometric and photometric measurements of binary stars with adaptive optics: observations from 2002", Monthly Notices of the Royal Astronomical Society, 413 (2): 1200–1205, arXiv:1012.3383, Bibcode:2011MNRAS.413.1200R, doi:10.1111/j.1365-2966.2011.18205.x, S2CID 118398949

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