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Tianguan

Zeta Tauri, 123 Tauri

Proper NamesTianguan
Bayer DesignationZeta Tauri
Flamsteed Number123 Tauri
HR (BSC)1910
HD37202
Constellation Taurus
Right Ascension5h 38m 44s
Declination+21° 9' 2"
Distance445 light years
136 parsecs
MagnitudeApparent: +3.0
Absolute: -2.7
Spectral ClassB4III Blue Giant
Optimum VisibilityDecember / January
NotesTianguan is a variable of the Gamma Cassiopeiae type, a blue giant whose exceptionally rapid rotation causes matter to be expelled into a disc that extends out from the star for many times its diameter. These expulsion events cause the star itself to dim periodically.

The 'horns' of Taurus the Bull are marked by two stars in the eastward parts of the constellation. At the northern horn-tip is Elnath, a blue giant about 130 light years from the Sun, while the southern horn is marked by Tianguan, another blue giant more than three times further away from the Sun than Elnath. Tianguan lies directly northward from Orion, and is the nearest bright star in the sky to Taurus' famous Crab Nebula (this is a line of sight effect: the Nebula is much more distant from the Sun than either of the two horn-stars).

Tianguan is a binary system: its primary component is a massive blue giant, but that giant star has something - its identity is uncertain - in a very close orbit around it. This companion has a mass only about a tenth that of the primary star, and orbits the blue giant at a distance only marginally greater than that of Earth from the Sun, causing the brightness of Tianguan to vary periodically. There is also evidence of circumstellar disk of matter in this system, emanating outwards from the blue star at its core.

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