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′ 49.5″[1]
Apparent magnitude (V) +3.53
Spectral type K0III
B−V color index 1.014
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 M
Radius 12.692 ± 0.545 [2] R
Luminosity 97 ± 8[3] L
Temperature 4901 ± 20 K
Metallicity +0.17 ± 0.04
Age 625 × 106 years
Other designations
74 Tauri, Ain, BD+18 640, CCDM 04286+1911, Coronis, FK5 164, GC 5430, HD 28305, HIP 20889, HR 1409, Oculus Borealis, SAO 93954

Epsilon Tauri (ε Tau, ε Tauri) is an orange giant star, spectral type of K0 III, located approximately 147 light-years[1] away from the Sun in the constellation of Taurus. It is a member of the Hyades open cluster. It has the traditional names Ain (Arabic عين), or Oculus Borealis, both of which mean "eye".

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

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

As a member of the Hyades cluster, Epsilon Tauri's age is well constrained at 625 million years.[3] Given its large mass, this star was formerly a member of spectral type A that has now evolved off the main sequence into the giant phase. It is regarded as a red clump that burns helium at its core.

Planetary system[edit]

In 2007 a massive extrasolar planet was reported orbiting the star. The planet orbits Epsilon Tauri every 1.6 years in a somewhat eccentric orbit. At the time of the discovery it was the only known planet in an open cluster.[4]

The Epsilon Tauri planetary system[3]
(in order from star)
Mass Semimajor axis
Orbital period
Eccentricity Inclination Radius
b >7.6 (± 0.2) MJ 1.93 (± 0.03) 594.9 (± 5.3) 0.151 (± 0.023)


  1. ^ a b c d e f van Leeuwen, F. (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.  Vizier catalog entry
  2. ^ 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:0901.1206. Bibcode:2009ApJ...694.1085V. doi:10.1088/0004-637X/694/2/1085. (web Preprint)
  3. ^ a b c 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. 
  4. ^ "Notes for star eps Tau". The Extrasolar Planets Encyclopaedia. Retrieved 2007-02-09. 

External links[edit]

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