Ain (Epsilon Tauri)

Ain, Epsilon Tauri (ε Tau), is an orange giant star located 146 light-years away in the constellation Taurus. With an apparent magnitude of 3.53, it is the seventh brightest star in the constellation. Ain marks the northern eye of the Bull. It is a member of the Hyades, one of the brightest open clusters in the sky. The star hosts an exoplanet, Epsilon Tauri b, discovered in 2007.

Star type

Ain is a red clump giant of the spectral type K0III. It is currently fusing helium in its core. It has a mass of 2.458 solar masses and a radius 12.46 times that of the Sun. With an effective temperature of 4,880 K, it shines with 79.4 solar luminosities. The star has an estimated age of about 600 million years. It spins at 3.0 km/s, taking 141.1 days to complete a rotation.

Ain has a visual companion, an 11th magnitude star, at a separation of 182 arcseconds. The two stars look like a visual double star but are not physically related. The optical companion lies at a greater distance.

Ain (Epsilon Tauri), image credit: ESO/Digitized Sky Survey 2 (CC BY 4.0)

Planet

Epsilon Tauri hosts an extrasolar planet, Epsilon Tauri b (officially named Amateru). The super-Jupiter was discovered by a team of astronomers at the Okayama Astrophysical Observatory, National Astronomical Observatory of Japan, in 2007. The team used the High Dispersion Echelon Spectrograph at Okayama Astrophysical Observatory (OAO) to search for extrasolar planets around G-type and K-type stars. Epsilon Tauri b was the first planet detected in an open star cluster.

The researchers reported an orbital period of 594.9 ± 5.3 days, a semimajor axis of 1.93 ± 0.03 AU, a temperature of 541 K (268 °C; 514 °F), a radius of 1.18 Jupiter radii, and a minimum mass of 7.6 ± 0.2 Jupiter masses for the planet.

In 2023, a study refined the planet’s parameters. The astronomers found a mass of at least ≥7.190 ± 0.056 Jupiter masses, an orbital period of 585.82 days, and a semimajor axis of 1.868 AU.

The planet was formally named Amateru (pronunciation: /æməˈtɛruː/) during the International Astronomical Union’s NameExoWorlds campaign in 2015. The name comes from Japan. It is a Japanese appellation for shrines dedicated to Amaterasu, the Shinto goddess of the Sun.

Hyades

Ain is one of the brightest members of the Hyades (Melotte 25, Collinder 50, Caldwell 41), the nearest open cluster to the Sun. Located only 153 light-years away, the star cluster has an apparent magnitude of 0.5 and an apparent size of 330 arcminutes. In depictions of the constellation Taurus, it forms the head of the Bull.

The brightest stars in the cluster – Chamukuy (Theta Tauri Aa, mag. 3.35), Ain (Epsilon Tauri, mag. 3.53), Prima Hyadum (Gamma Tauri, mag. 3.65), and Secunda Hyadum (Delta¹ Tauri, mag. 3.90) – have all evolved away from the main sequence despite the relatively young age.

Chamukuy is a white, A-type giant, and Prima Hyadum and Secunda Hyadum are both yellow giants. Like Ain, these stars have over 2.5 times the Sun’s mass and lie 145 – 160 light-years from the Sun.

The Hyades cluster is about 20 light-years across and has an estimated age of 625 million years. It contains a mass of 400 solar masses.

The Hyades stars appear in the same line of sight as Aldebaran, the luminary of Taurus and one of the brightest stars in the sky. The red giant star is not a member—it lies only 65.3 light-years away – but dominates the Hyades field of view.

Ain and the Hyades cluster, image credit: ESO/Digitized Sky Survey 2 (CC BY 4.0)

Facts

Like many other stars in zodiac constellations, Ain lies near the ecliptic – the Sun’s apparent path across the sky – and can occasionally be occulted by the Moon and much more rarely by planets.

Ain forms a V-shaped asterism with Chamukuy (Theta² Tauri), Prima Hyadum (Gamma Tauri), Secunda Hyadum (Delta¹ Tauri) and Aldebaran (Alpha Tauri) that outlines the head of the celestial Bull. Aldebaran and Ain mark the Bull’s eyes.

Name

The name Ain (pronunciation: /ˈeɪn/) comes from the Arabic phrase ain al-thaur, meaning “the Bull’s eye.” The name refers to the position of Epsilon Tauri in Taurus, marking one of the Bull’s eyes.

English astronomer John Flamsteed named the star Oculus Boreus, “the northern eye.” The star known as Oculus Australis, “the southern eye,” was the brighter Aldebaran, the lucida of the Taurus constellation.

The International Astronomical Union’s (IAU) Working Group on Star Names (WGSN) approved the name Ain for Epsilon Tauri on December 15, 2015.

In traditional Chinese astronomy, Ain was known as 畢宿一 (Bì Xiù yī), the First Star of Net. It formed the Net asterism with Delta³ Tauri, Delta¹ Tauri (Secunda Hyadum), Gamma Tauri (Prima Hyadum), Aldebaran (Alpha Tauri), Chamukuy (Theta² Tauri), 71 Tauri, and Lambda Tauri. The asterism was part of the larger Net mansion, which represented the body of the White Tiger.

Location

Ain is easy to find because it is part of the bright Hyades. Aldebaran and the Hyades can be found by extending the imaginary line from the three stars of Orion’s Belt – Alnitak, Alnilam, and Mintaka – to the northwest.

In the V-shaped star pattern of the Hyades, Ain appears at the opposite end to the much brighter Aldebaran.

The location of Ain (Epsilon Tauri), image: Stellarium

Constellation

Ain is located in the constellation of Taurus. The celestial Bull is one of the ancient constellations listed by Claudius Ptolemy in his Almagest in the 2nd century CE. In Greek mythology, it is linked with Zeus, who assumed the form of a white bull in order to seduce Europa.

Taurus is the 17th largest constellation in the sky, occupying 797 square degrees. The southernmost part of the constellation lies on the celestial equator. As one of the zodiac constellations, Taurus also lies on the ecliptic, the Earth’s orbital plane around the Sun.

Taurus is best-known for being home to the Pleiades and Hyades – the two brightest and closest open clusters to the Sun – and to the orange giant Aldebaran (Alpha Tauri), the 14th brightest star in the sky.

Taurus constellation map by IAU and Sky&Telescope magazine (Roger Sinnott & Rick Fienberg) (CC BY 3.0)

Other notable stars in Taurus include the hot blue giants Elnath (Beta Tauri) and Tianguan (Zeta Tauri), which mark the Bull’s horns, the massive blue main sequence star Lambda Tauri at the Bull’s chest, the blue giants Alcyone (Eta Tauri), Atlas (27 Tauri) and Electra (17 Tauri) in the Pleiades cluster, the white and yellow giants Chamukuy (Theta Tauri), Prima Hyadum (Gamma Tauri) and Secunda Hyadum (Delta Tauri) in the Hyades cluster, the hot blue quadruple star system Xi Tauri, the variable class prototypes T Tauri and RV Tauri, and the evolved Sun-like star HD 37124, which hosts three exoplanets.

Other deep sky objects in Taurus include the supernova remnants Messier 1 (the Crab Nebula) and Sh2-240 (the Spaghetti Nebula), the variable reflection nebula NGC 1555 (Hind’s Variable Nebula), the planetary nebula NGC 1514 (the Crystal Ball Nebula), and the interacting pair of galaxies NGC 1410 and NGC 1409.

The best time of the year to observe the stars and deep sky objects in Taurus is during the month of January, when the Bull appears higher in the sky in the early evening. The entire constellation is visible from locations north of the latitude 60° S.

The 10 brightest stars in Taurus are Aldebaran (Alpha Tau, mag. 0.86), Elnath (Beta Tau, mag. 1.65), Alcyone (Eta Tau, mag. 2.87), Tianguan (Zeta Tau, mag. 2.97), Chamukuy (Theta² Tauri, mag. 3.40), Lambda Tauri (mag. 3.47), Ain (Epsilon Tau, mag. 3.53), Omicron Tauri (mag. 3.61), Atlas (27 Tau, mag. 3.63), and Prima Hyadum (Gamma Tau, mag. 3.654).

Ain – Epsilon Tauri