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ALCYONE

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 (considered a "giant"), and though (at nearly 13,000 Kelvin) not the hottest, quite markedly the brightest, having a luminosity 1400 times that of the Sun, the 365-light year distance of the group reducing Alcyone to bright third magnitude. 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 over 200 kilometers per second (more than 100 times faster than the Sun), has spun gas from its equator into a surrounding light-emitting disk to make a "B- emission star" somewhat like Cih, 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 visible to the eye, long photographic exposures 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. (Thanks to Jason Pero, who helped research this star.)