Epsilon Tauri

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Epsilon Tauri
Taurus constellation map.svg
The position of ε Tauri in the Taurus constellation.
Observation data
Epoch J2000      Equinox J2000
Constellation Taurus
Right ascension 04h 28m 37.00s[1]
Declination +19° 10′ 50″[1]
Apparent magnitude (V) +3.53[2]
Spectral type K0III[2]
B−V color index 1.014[1]
Proper motion (μ) RA: 106.19 ± 0.38[1] mas/yr
Dec.: -37.84 ± 0.30[1] mas/yr
Parallax (π) 22.24 ± 0.25[1] mas
Distance 147 ± 2 ly
(45.0 ± 0.5 pc)
Mass 2.7 ± 0.1[2] M
Radius 12.692 ± 0.545 [3] R
Luminosity 97 ± 8[4] L
Temperature 4901 ± 20[2] K
Metallicity +0.17 ± 0.04[2]
Age 625 × 106 [2] years
Other designations

Epsilon Tauri (abbreviated ε Tauri or ε Tau) is an orange giant star located approximately 45 parsecs (147 light-years) from the Sun in the constellation of Taurus.[2] It has the traditional name Ain (Arabic عين for "eye") and was given the name Oculus Boreus (Latin for "Northern eye") by John Flamsteed.[5][6] An extrasolar planet (designated Epsilon Tauri b, later named Amateru) is believed to be orbiting the star.

It is a member of the Hyades open cluster. As such its age is well constrained at 625 million years.[4] It is claimed to be the heaviest among planet-harboring stars with reliable initial masses[4] although the star HD 13189 is potentially more massive.[7] Given its large mass, this star, though presently of spectral type K0 III, was formerly of spectral type A that has now evolved off the main sequence into the giant phase. It is regarded as a red clump giant; that is, a core-helium burning star.[4]

Since Epsilon Tauri lies near the plane of the ecliptic, it is sometimes occulted by the Moon and (very rarely) by planets.

It has an 11th magnitude companion 182 arcseconds from the primary.


Epsilon Tauri is the star's Bayer designation; it also bears the Flamsteed designation of 74 Tauri. On discovery the planet was designated Epsilon Tauri b (or Ain b). In July 2014 the International Astronomical Union launched a process for giving proper names to certain exoplanets.[8] The process involved public nomination and voting for the new names.[9] In December 2015, the IAU announced the winning name was Amateru for this planet.[10]

The winning name was based on that submitted by the Kamagari Astronomical Observatory of Kure, Hiroshima Prefecture, Japan: namely 'Amaterasu', the Shinto goddess of the Sun, born from the left eye of the god Izanagi. The IAU substituted 'Amateru' - which is a common Japanese appellation for shrines when they enshrine Amaterasu - because 'Amaterasu' is already used for an asteroid (10385 Amaterasu).[11]

Planetary system[edit]

In 2007 a massive extrasolar planet was reported orbiting the star with a period of 1.6 years in a somewhat eccentric orbit. Its discoverers claimed it was the first planet ever discovered in an open cluster.[4]

The Epsilon Tauri planetary system[4]
(in order from star)
Mass Semimajor axis
Orbital period
Eccentricity Inclination Radius
b (Amateru) >7.6 (± 0.2) MJ 1.93 (± 0.03) 594.9 (± 5.3) 0.151 (± 0.023)


  1. ^ a b c d e f "Entry for star HIP 20889". Vizier Catalogue Service. Retrieved 2015-09-13. 
  2. ^ a b c d e f g "Notes for planet eps Tau b". The Extrasolar Planets Encyclopaedia. Retrieved 2015-09-13. 
  3. ^ Gerard T. van Belle and Kaspar von Braun (2009). "Directly Determined Linear Radii and Effective Temperatures of Exoplanet Host Stars" (abstract). The Astrophysical Journal. 694 (2): 1085–1098. arXiv:0901.1206free to read. Bibcode:2009ApJ...694.1085V. doi:10.1088/0004-637X/694/2/1085. (web Preprint)
  4. ^ a b c d e f Sato, Bun'ei; et al. (2007). "A Planetary Companion to the Hyades Giant ε Tauri". The Astrophysical Journal. 661 (1): 527–531. Bibcode:2007ApJ...661..527S. doi:10.1086/513503. 
  5. ^ Flamsteed, John (1725). Historia Coelestis Britannica. H. Meere. p. 47. 
  6. ^ Allen, Richard Hickley (1963). Star Names – Their Lore and Meaning. Dover Books. p. 391. 
  7. ^ "Notes for planet HD 13189 b". The Extrasolar Planets Encyclopaedia. Retrieved 2015-09-15. 
  8. ^ NameExoWorlds: An IAU Worldwide Contest to Name Exoplanets and their Host Stars. IAU.org. 9 July 2014
  9. ^ NameExoWorlds The Process
  10. ^ Final Results of NameExoWorlds Public Vote Released, International Astronomical Union, 15 December 2015.
  11. ^ NameExoWorlds The Approved Names

External links[edit]

Coordinates: Sky map 04h 28m 37.0s, +19° 10′ 49″