Alcyone (Eta Tauri)


Alcyone (Eta Tauri) is the brightest star in the Pleiades and the only one to have a Bayer designation. It is also the third brightest star in the constellation Taurus. The name is that of the daughter of Atlas and Pleione in Greek mythology.


Alycone is actually a multiple star system. The primary, Alcyone A, is an eclipsing binary, consisting of two giant B stars with a separation of about 0.031 arcseconds (roughly the distance of Jupiter from the Sun). The high rotational speed of Alcyone A – over 200 kilometers per second (more than 100 times faster than the Sun) – has caused gas to spin from its equator into a surrounding light-emitting disk. This makes Alcyone A a Be star but with a disk thicker than usual.


The binary Alcyone A is orbited by three companions. Alcyone B and Alcyone C are both 8th magnitude white, main sequence A stars and are separated from A by 117 and 181 arcseconds, respectively. Alcyone C is a Delta Scuti star whose brightness varies from magnitude +8.25 to +8.30 with a period of 1.13 hours. Alcyone D is a yellow-white main sequence F star, with an apparent magnitude of +8.7, separated by 191 arcseconds from the primary.


visual magnitude 2.85
absolute magnitude -2.41
spectral type B7IIIe + A0V + A0V + F2V
luminosity 1,400 Lsun
distance 440 light-years (135)
position RA 03h 47m 29.1s,
Dec +24° 06' 18"
other designations 25 Tauri, HR 1165, HD 23630,
BD+23 541, FK5 139, HIP 17702,
SAO 76199, GC 4541, BDS 1875,
CCDM 03474+2407