Zeta Tauri

ζ Tauri

Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Taurus
Right ascension	05h 37m 38.7s
Declination	+21° 08' 33"
Apparent magnitude (V)	+2.97
Distance	417 ± 54 ly (128 ± 17 pc)
Spectral type	B4IIIpe+G8III
Other designations
123 Tauri, HR 1910, HD 37202, BD+21 908, FK5 211, HIP 26451, SAO 77336, GC 6985

Zeta Tauri (ζ Tau, ζ Tauri) is a binary star in the constellation Taurus, the Bull. Known to the ancient Babylonians as Shurnarkabti-sha-shutu, meaning "the star in the bull towards the south," Zeta Tauri is among the most prominent of the stars in well-known constellation figures, representing one of the celestial bull's protruding horns.

In Chinese astronomy, Zeta Tauri is called 天關, Pinyin: Tiānguān, meaning Celestial Gate, because this star is marking itself and stand alone in Celestial Gate asterism, Net (mansion) (see : Chinese constellation)^[1]. 天關 (Tiānguān) is westernized into Tien Kwan, but the name Tien Kwan was designated not only for Zeta Tauri, but also for the stars near by, with the meaning is "the Heavenly Gate" ^[2]

Properties

Defining mid-third magnitude (3.00), Zeta Tauri is a brilliant hot blue-white B-type giant star approximately 417 light years from Earth. It is classified as a Gamma Cassiopeiae type variable star. Its brightness varies from magnitude +2.88 to +3.17 due to both its intrinsic variability, and also because it is an eclipsing binary. The two components of the binary system are separated by approximately one astronomical unit and complete an orbit once every 133 days. Its much lower mass companion star is a yellow G-type star, which has a magnitude of +5.2. A 9 solar mass star around 25 million years old, Zeta Tauri is now evolving, and is close to giving up core hydrogen fusion, if it has not done so already.

It radiates the light of approximately 5,700 of our own sun from a surface heated to 22,000 kelvins. Together, temperature and luminosity yield a radius 5.2 times that of the Sun. What makes the star really special, however, is not its high temperature and luminosity, but its rotation and mass loss. The equatorial rotation speed has been measured as high as 330 kilometers per second, which is 115 times that of the Sun, the star spinning around with a period of only one day (as opposed to the 25-day solar rotation period). The rotation, which is still well short of that needed to break up the star, is somehow related to a thick disk of matter that surrounds it. The disk radiates bright emissions from hydrogen in the red and blue parts of the spectrum, making Zeta Tauri one the sky's best-known "B-emission" stars. The star and disk are both large enough to have had their angular diameters actually measured. The disk is some 64 solar diameters across.

References

1. ^ (Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 24 日
2. ^ Richard Hinckley Allen: Star Names — Their Lore and Meaning: Taurus