ALCYONE (Eta Tauri). Rising over the eastern horizon, Taurus's Pleiades star cluster announces northern autumn,
telling of the cold months to come, when the group will soar high
through the winter sky. More famed as the "Seven Sisters," in
mythology daughters of Atlas, the cluster
sparkles with a jeweler's collection of blue-white diamonds packed
into a circle a little over a degree across. Nine of the stars
carry ancient Greek proper names, those of the seven maidens plus
that of Atlas himself and of the sisters' mortal mother, Pleione. All cultures have stories of the
group, whose risings and settings began and ended agricultural and
navigational seasons; they have been flying doves, little eyes, and
harvest baskets to hold the autumn bounty. Ranging from third to
sixth magnitude, all are potentially visible to the human eye, but
because of crowding most people see 6 or perhaps 8, though the
acute eye can see yet more. With the telescope, hundreds of lesser
lights come into view. Chief among them is Alcyone (the last "e"
pronounced), the only one to acquire a Greek letter name, Eta
Tauri. The six brightest stars make a small dipper, Alcyone
located where the two-star handle (made of Atlas and Pleione) joins
the bowl, three of the sisters (Merope,
Electra, and Maia) making the bowl itself, Asterope, Taygeta, and Celaeno out in front of it. All nine are
bright and luminous B stars, Alcyone apparently somewhat evolved (a
class B7 giant), and though (at 13,000 Kelvin) not the hottest,
quite markedly the brightest, having a luminosity 2400 times that
of the Sun (much in the ultraviolet), the
430-light year distance of the group reducing Alcyone to bright
third magnitude (2.87). Luminosity and temperature (and correction
for a 10 percent dimming by interstellar dust) give a mass of 6
Suns and a radius nearly 10 solar. Tucked next to it is a smaller
binary companion only a few
astronomical units away. Typical of B stars, all named Pleiades
but Maia are rapid spinners. Alcyone, rotating with an equatorial
speed of some 215 kilometers per second (more than 100 times faster
than the Sun, giving it a rotation period of under 2.3 days), has
spun gas from its equator into a surrounding light-emitting disk to
make a "B-emission star" somewhat like Gamma Cassiopeiae, but one with a disk
thicker than most. Pleione, which rotates even faster, over 300
kilometers per second, is similar. 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.
Updated by Jim Kaler 5/18/07. Return to STARS.