Eta Tauri - Alcyone

At magnitude 2.87, Alcyone, or Eta Tauri, is the brightest of the stars in the Pleiades, and the third brightest star in Taurus. In Greek mythology, the Pleiades represent the daughters of Atlas, and nine of the stars in this cluster carry ancient Greek proper names. Chief among them is Alcyone, the only one to acquire a Greek letter name, η Tauri. It is known as the Sixth Star of the Hairy Head in Chinese.

Properties

All nine of the Pleiades' named "sisters" are hot, luminous, class B stars, at a group distance of 430 light years. Alcyone is a somewhat evolved class B7 III giant, and though at 13,000 K not the hottest, it is quite markedly the brightest, having a luminosity 2400 times the Sun's. It contains a mass of 6 Suns and has a radius nearly 10 times solar. Typical of class B stars, Alcyone spins rapidly, with an equatorial speed of some 215 km/sec, giving it a rotation period under 2.3 days. It has spun gas from its equator into a surrounding light-emitting disk to make a "B-emission star" somewhat like Gamma Cassiopeiae, but with a disk thicker than most. Pleione, which rotates even faster, is similar.

Components

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 arc-seconds (roughly the distance of Jupiter from the Sun). 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", 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" from the primary.

Nebulosity

The Pleiades are now moving through a great dusty cloud of interstellar matter, the dust grains reflecting the light of the blue stars. Though not readily visible to the eye, deep photographic or electronic images show this reflection nebula enmeshing the whole crowd. The nebula is particularly bright around Merope, just to the southwest of Alcyone, though bright Alcyone carries her own blue covering shawl.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]