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ζ Tauri
	Location of ζ Tauri (circled)
Observation data; Epoch J2000.0 Equinox J2000.0 (ICRS)
Constellation	Taurus
Right ascension	05h 37m 38.68542s
Declination	+21° 08′ 33.1588″
Apparent magnitude (V)	3.010 (2.88–3.17)
Characteristics
Spectral type	B2 IIIpe
U−B color index	−0.749
B−V color index	−0.164
Variable type	γ Cas
Astrometry
Radial velocity (Rv)	+20 km/s
Proper motion (μ)	RA: +1.78 mas/yr ; Dec.: −20.07 mas/yr
Parallax (π)	7.33 ± 0.82 mas
Distance	approx. 440 ly ; (approx. 140 pc)
Absolute magnitude (MV)	−2.67
Orbit
Period (P)	132.987 d
Semi-major axis (a)	1.17 AU
Eccentricity (e)	0.0 (assumed)
Inclination (i)	92.8°
Longitude of the node (Ω)	−58.0°
Periastron epoch (T)	2,447,025.6 HJD
Argument of periastron (ω); (secondary)	0.0 (assumed)°
Semi-amplitude (K1); (primary)	7.43 km/s
Details
ζ Tau A
Mass	11.2 M☉
Radius	5.5 R☉
Luminosity	4,169 L☉
Temperature	15,500 K
Rotational velocity (v sin i)	125 km/s
Age	22.5 ± 2.6 Myr
ζ Tau B
Mass	0.94 M☉
Other designations
	Tianguan, 123 Tauri, HR 1910, HD 37202, BD+21°908, FK5 211, HIP 26451, SAO 77336, GC 6985
Database references
SIMBAD	data

Zeta Tauri (ζ Tauri, abbreviated Zeta Tau, ζ Tau) is a binary star in the zodiac constellation of Taurus, the Bull. It has an apparent visual magnitude of about 3.0, which is bright enough to be seen with the naked eye. Parallax measurements place it at a distance of roughly 440 light-years from the Sun.

The two components are designated Zeta Tauri A (officially named Tianguan /tiænˈɡwɑːn/) and Zeta Tauri B.

Nomenclature

ζ Tauri (Latinised to Zeta Tauri) is the star's Bayer designation; it also bears the Flamsteed designation of 123 Tauri. The designations of the two components as Zeta Tauri A and B derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).^[11]

In Chinese astronomy, Zeta Tauri is called 天關, Pinyin: Tiānguān, formerly transliterated Tien Kwan, meaning Celestial [Frontier] Gate, an asterism within the Net (畢宿 Bì Xiù) mansion).^[12] 天關 (Tiānguān) has also been transliterated as Tien Kwan. (Technically, Tiānguān refers not just to Zeta Tauri but to the Celestial Gate asterism of which Zeta Tauri is the main star, alongside 113, 126, 128, 129, 130 and 127 Tauri .^[13]) In 2016, the IAU organized a Working Group on Star Names (WGSN)^[14] to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.^[15] It approved the name Tianguan for the component Zeta Tauri A on 30 June 2017 and it is now so included in the List of IAU-approved Star Names.^[16]

Properties

A visual band light curve for Zeta Tauri, adapted from Ruždjak *et al.* (2009)^[17]

Zeta Tauri is a single-lined spectroscopic binary system, which means the two components are orbiting so close to each other that they can not be resolved with a telescope. Instead, the orbital motion of the primary component is indicated by Doppler effect shifts in the absorption lines in its spectrum. The two components are separated by an estimated distance of about 1.17 astronomical units, or 117% of the distance from the Earth to the Sun. They are following circular orbits with a period of nearly 133 days.^[4]

Compared to the Sun, the primary, Zeta Tauri A, is an enormous star with more than 11 times the mass and 5–6 times the radius.^[4] It is rotating rapidly with a projected rotational velocity of 125 km s⁻¹.^[8] The companion, Zeta Tauri B, has about 94% the mass of the Sun, although it is unknown whether this is a main-sequence star, a neutron star, or a white dwarf. If it is a main sequence star, then the mass indicates it may have a stellar classification of G4.^[4]

The spectrum of the primary component has a stellar classification of B2 IIIpe.^[4] A luminosity class of 'III' indicates this is a giant star that has exhausted the hydrogen at its core and evolved away from the main sequence. The 'p' suffix indicates an unspecified chemical peculiarity in the spectrum, while 'e' is used for stars that display emission lines. For Be stars such as this, the emission lines are produced by a rotating circumstellar disk of gas, made of material that has been ejected from the star's outer envelope. An oscillatory pattern in this spectrum is being caused by a single-armed spiral density wave in the disk. The disk may be precessing from the gravitational influence of the secondary component.^[4]

Zeta Tauri shows variation in its spectrum and brightness. The General Catalogue of Variable Stars lists it as an eclipsing variable and a Gamma Cassiopeiae variable,^[3] but it may not be either.^[18]^[19] Hrvoje Božić and Krešimer Pavlovski, of Hvar Observatory in Croatia, monitored the brightness of Zeta Tauri from 1981 to 1986 and noticed an eclipse like effect in the light curve.^[20] A latter study of all the available photometric data, including from the Hipparcos spacecraft, failed to confirm the presence of eclipses.^[19]

References

^ ^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.
^ ^a ^b ^c Harmanec, P.; et al. (1980). "Photoelectric photometry at the Hvar Observatory. IV – A study of UBV variations of a group of bright northern Be stars". Bulletin of the Astronomical Institutes of Czechoslovakia, Bulletin. 31 (3): 144–159. Bibcode:1980BAICz..31..144H.
^ ^a ^b ^c 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. Bibcode:2009yCat....102025S.
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j Schaefer, G. H.; et al. (December 2010). "Multi-epoch Near-infrared Interferometry of the Spatially Resolved Disk around the Be Star ζ Tau". The Astronomical Journal. 140 (6): 1838–1849. arXiv:1009.5425. Bibcode:2010AJ....140.1838S. doi:10.1088/0004-6256/140/6/1838. S2CID 15939785.
^ 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. Vol. 30. University of Toronto: International Astronomical Union. p. 57. Bibcode:1967IAUS...30...57E. {{cite book}}: |journal= ignored (help)
^ Anderson, E.; Francis, Ch. (2012). "XHIP: An extended hipparcos compilation". Astronomy Letters. 38 (5): 331. arXiv:1108.4971. Bibcode:2012AstL...38..331A. doi:10.1134/S1063773712050015. S2CID 119257644.
^ ^a ^b Balona, L. A.; Dziembowski, W. A. (October 1999). "Excitation and visibility of high-degree modes in stars". Monthly Notices of the Royal Astronomical Society. 309 (1): 221–232. Bibcode:1999MNRAS.309..221B. doi:10.1046/j.1365-8711.1999.02821.x.
^ ^a ^b Abt, Helmut A.; Levato, Hugo; Grosso, Monica (July 2002). "Rotational velocities of B Stars". The Astrophysical Journal. 573 (1): 359–365. Bibcode:2002ApJ...573..359A. doi:10.1086/340590.
^ 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. S2CID 118629873.
^ "zeta Tau". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2012-01-20.
^ 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].
^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 24 日 Archived 2011-07-16 at the Wayback Machine
^ Richard Hinckley Allen. "Star Names – Their Lore and Meaning: Taurus". Retrieved 2022-08-04.
^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
^ "WG Triennial Report (2015–2018) – Star Names" (PDF). p. 5. Retrieved 2018-07-14.
^ "Naming Stars". IAU.org. Retrieved 16 December 2017.
^ Ruždjak, D.; Božić, H.; Harmanec, P.; Fiřt, R.; Chadima, P.; Bjorkman, K.; Gies, D. R.; Kaye, A. B.; Koubský, P.; McDavid, D.; Richardson, N.; Sudar, D.; Šlechta, M.; Wolf, M.; Yang, S. (November 2009). "Properties and nature of Be stars. 26. Long-term and orbital changes of ζ Tauri". Astronomy and Astrophysics. 506 (3): 1319–1333. Bibcode:2009A&A...506.1319R. doi:10.1051/0004-6361/200810526.
^ Pavlovski, K.; Harmanec, P.; Bozic, H.; Koubsky, P.; Hadrava, P.; Kriiz, S.; Ruzic, Z.; Stefl, S. (1997). "UBV photometry of be stars at Hvar: 1972–1990". Astronomy and Astrophysics Supplement Series. 125: 75. Bibcode:1997A&AS..125...75P. doi:10.1051/aas:1997213.
^ ^a ^b Štefl, S.; Rivinius, Th.; Carciofi, A. C.; Le Bouquin, J.-B.; Baade, D.; Bjorkman, K. S.; Hesselbach, E.; Hummel, C. A.; Okazaki, A. T.; Pollmann, E.; Rantakyrö, F.; Wisniewski, J. P. (2009). "Cyclic variability of the circumstellar disk of the Be star ζ Tauri. I. Long-term monitoring observations". Astronomy and Astrophysics. 504 (3): 929. arXiv:0907.2250. Bibcode:2009A&A...504..929S. doi:10.1051/0004-6361/200811573. S2CID 16477132.
^ Bozic, H.; Pavlovski, K. (1988). "Photometry of the Be Star zeta Tauri in 1981–1986: Long-Term, Orbital and Rapid Variations Revealed". Hvar Observatory Bulletin. 12 (1): 15–25. Bibcode:1988HvaOB..12...15B.