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Observational Data
Designation Eta Tauri
25 Tauri
Right ascension 03h 47m 29.0766s[1]
Declination +24° 06′ 18.4883″[1]
Constellation Taurus
Type of object Giant star
Magnitude Apparent Mag: +2.86[1]
Absolute Mag: -2.61[2]
Distance from Earth 403.17 ly[2]
Radial velocity 5.4±1.2 km/s[1]
Proper motion RA: 19.34 mas/yr[1]
Dec.: -43.67 mas[1]
Parallax 8.09±0.42 mas[1]

Alcyone (Eta Tauri, 25 Tauri) is a giant star situated in the constellation of Taurus, the bull.[2] The star the brightest star in the Pleiades, the fifth brightest in Taurus and can be seen with the unaided eye.[3] Its name means "the central one" in Greek.[4] In Greek mythology, Alcyone was the Atlantid nymph who would later become the mother of Hyrieus. In China, it is often referred to as, “the Sixth Star of the Hairy Head.” [4]

Like many other stars, Alcyone is actually a system of several stars. The primary (designated Alcyone A) is comprised of an eclipsing binary. Three smaller and fainter stars, designated Alcyone B, C and D, orbit Alcyone A. Alcyone B and C are 8th magnitude stars. Alcyone C is a Delta-Scuti variable star.[3] Its magnitude varies slightly with a period of 1.13 hours between +8.25 to +8.30. Alycone D is an F-type star and is a little fainter than all the rest, with a magnitude of +8.7.[3]

The Alcyone system lies 403.17 light years from Earth and approximately 24,500 light years from the centre fo the galaxy.[2] With a spectral type of B7 IIIe, Alycone A i thought to have a radius 8.40 solar radii and a mass equal to 6 solar masses. As a blue star is very hot with a surface temperature of some 13,000 kelvin.[4] It also have an exceptionally quick rotation rate; its equator moves at 200 km/s which is over 100 times larger than the equivalent for the Sun.[3] The centrifugal force generated by this rotation has caused gas to be spun into a light emitting disk surrounding the star's equator.


  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Alcyone from the SIMBAD Astronomical Database
  2. 2.0 2.1 2.2 2.3 Alcyone, Eta Tauri, 25 Tauri, HD23630, HIP17702, HR1165. Retrieved on 2019-01-18.
  3. 3.0 3.1 3.2 3.3 Alcyone (Eta Tauri). Retrieved on 2019-01-18.
  4. 4.0 4.1 4.2 Alcyone – Star Facts. Retrieved on 2019-01-18.