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

Epsilon Tauri

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Epsilon Tauri
Taurus constellation map.svg
Red circle.svg
Location of ε Tauri (circled)
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 mas[1]
Distance147 ± 2 ly
(45.0 ± 0.5 pc)
Absolute magnitude (MV)0.145[3]
Mass2.7 ± 0.1[2] M
Radius12.692 ± 0.545[4] R
Luminosity97 ± 8[5] L
Temperature4901 ± 20[2] K
Metallicity [Fe/H]+0.17 ± 0.04[2] dex
Rotational velocity (v sin i)3.0[3] km/s
Age625[2] Myr
Other designations
Ain, Oculus Boreus[6], Epsilon Tau, ε Tau, 74 Tau, BD+18°640, FK5 164, GC 5430, HD 28305, HIP 20889, HR 1409, SAO 93954, CCDM J04286+1911
Database references

Epsilon Tauri or ε Tauri, formally named Ain (/ˈn/),[7] is an orange giant star located approximately 45 parsecs (147 light-years) from the Sun in the constellation of Taurus.[2] 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.[5] It is claimed to be the heaviest among planet-harboring stars with reliable initial masses[5] although the star HD 13189 is potentially more massive.[8] 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.[5]

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.


ε Tauri (Latinised to 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).

The star bore the traditional name Ain (Arabic عين for "eye") and was given the name Oculus Boreus (Latin for "Northern eye") by John Flamsteed.[9][6] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[10] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[11] included a table of the first two batches of names approved by the WGSN; which included Ain for this star.

In July 2014, the International Astronomical Union launched NameExoWorlds, a process for giving proper names to certain exoplanets.[12] The process involved public nomination and voting for the new names.[13] In December 2015, the IAU announced the winning name was Amateru for this planet.[14]

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).[15]

In Chinese, 畢宿 (Bì Xiù), meaning Net, refers to an asterism consisting ε Tauri, δ3 Tauri, δ1 Tauri, γ Tauri, Aldebaran, θ2 Tauri, 71 Tauri and λ Tauri.[16] Consequently, the Chinese name for ε Tauri itself is 畢宿一 (Bì Xiù yī), "the First Star of Net".[17]

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.[5]

The Epsilon Tauri planetary system[5]
(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)

In popular culture[edit]

Epsilon Tauri is a major feature of the book Starsong Chronicles:Exodus, by American author JJ Clayborn. In the book, the star is referenced by the name Ain, but the characters rename the star Coronis.[18]

In Fred Saberhagen's Berserker series in the short story Patron of the Arts (first appeared in Worlds of If, Aug 1965), a human artist and a Berserker machine discuss the value of art and of this specific painting on a starship in the future after a space battle near the Sol system. In this short story, the Earth-Descended peoples place all of humanity's important artworks on an evacuation starship to preserve the works by sending them to Tau Epsilon.[19]


  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. ^ a b Böhm-Vitense, Erika; et al. (December 2000), "Ultraviolet Emission Lines in BA and Non-BA Giants", The Astrophysical Journal, 545 (2): 992–999, Bibcode:2000ApJ...545..992B, doi:10.1086/317850.
  4. ^ Gerard T. van Belle and Kaspar von Braun (2009). "Directly Determined Linear Radii and Effective Temperatures of Exoplanet Host Stars". The Astrophysical Journal. 694 (2): 1085–1098. arXiv:0901.1206. Bibcode:2009ApJ...694.1085V. doi:10.1088/0004-637X/694/2/1085. S2CID 18370219.(web Preprint)
  5. ^ 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.
  6. ^ a b Allen, Richard Hickley (1963). Star Names – Their Lore and Meaning. Dover Books. p. 391.
  7. ^ "IAU Catalog of Star Names". Retrieved 28 July 2016.
  8. ^ "Notes for planet HD 13189 b". The Extrasolar Planets Encyclopaedia. Retrieved 2015-09-15.
  9. ^ Flamsteed, John (1725). Historia Coelestis Britannica. H. Meere. p. 47.
  10. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  11. ^ "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016.
  12. ^ NameExoWorlds: An IAU Worldwide Contest to Name Exoplanets and their Host Stars. 9 July 2014
  13. ^ NameExoWorlds The Process
  14. ^ Final Results of NameExoWorlds Public Vote Released, International Astronomical Union, 15 December 2015.
  15. ^ "NameExoWorlds The Approved Names". Archived from the original on 2018-02-01. Retrieved 2015-12-21.
  16. ^ (in Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
  17. ^ (in Chinese) 香港太空館 – 研究資源 – 亮星中英對照表 Archived 2008-10-25 at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.
  18. ^ Clayborn, JJ (March 2017), Starsong Chronicles: Exodus, ISBN 978-1520611747
  19. ^ Mporcius (2015-03-20). "MPorcius Fiction Log: Berserker by Fred Saberhagen (Part 1)". MPorcius Fiction Log. Retrieved 2019-08-27.

External links[edit]