Epsilon Tauri

Epsilon Tauri / Ain

Location of ε Tauri (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Taurus
Right ascension	04h 28m 37.0003s
Declination	+19° 10′ 49.563″
Apparent magnitude (V)	+3.53
Characteristics
Spectral type	K0III
B−V color index	1.014
Astrometry
Radial velocity (Rv)	38.420±0.0004 km/s
Proper motion (μ)	RA: 107.526±0.193 mas/yr Dec.: −36.200±0.126 mas/yr
Parallax (π)	22.3654 ± 0.1721 mas
Distance	146 ± 1 ly (44.7 ± 0.3 pc)
Absolute magnitude (MV)	0.145
Details
Mass	2.57+0.17 −0.25 M☉
Radius	12.35+0.41 −0.36 R☉
Luminosity	78.11+5.68 −5.82 L☉
Surface gravity (log g)	2.66+0.03 −0.05 cgs
Temperature	4,950±22 K
Metallicity [Fe/H]	+0.15±0.02 dex
Rotation	141.1 days
Rotational velocity (v sin i)	3.0 km/s
Age	625 Myr
Other designations
Ain, Oculus Boreus, Epsilon Tau, ε Tau, 74 Tau, BD+18°640, FK5 164, GC 5430, HD 28305, HIP 20889, HR 1409, SAO 93954, CCDM J04286+1911, WDS J04286+1911A
Database references
SIMBAD	data

Epsilon Tauri or ε Tauri, formally named Ain (), is an orange giant star located approximately 146 light-years (45 parsecs) from the Sun in the constellation of Taurus. An exoplanet (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. It is claimed to be the heaviest among planet-harboring stars with reliable initial masses. 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.

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, although this is an unrelated background star.

Nomenclature

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. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 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. The process involved public nomination and voting for the new names. In December 2015, the IAU announced the winning name was Amateru for this planet.

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).

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. Consequently, the Chinese name for ε Tauri itself is 畢宿一 (Bì Xiù yī), "the First Star of Net".

Planetary system

In 2007, a massive exoplanet was reported orbiting the star with a period of 1.6 years in a somewhat eccentric orbit. It was the first planet ever discovered in an open cluster. A 2023 study updated this planet's parameters, and detected additional radial velocity variations that are likely caused by stellar activity.

See also

• Epsilon Tauri in fiction