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PLEIONE

(28 Tauri). Mother to Taurus's Pleiades, the famed Seven Sisters, at mid-fifth magnitude (5.09) Pleione (Flamsteed number 28) ranks seventh among the named stars of the cluster, just marginally behind the Pleiades' mythical father, the god Atlas. All nine are hot class B stars, their blue-white similarity giving the cluster so much of its sparkle. Together with Sterope, Pleione is the coolest (class B8) of them, its temperature 12,000 Kelvin. It and Sterope are also the only dwarfs, stars that are fully fusing hydrogen in their cores and have not yet begun to evolve (the others either subgiants like Merope or giants like Alcyone). From its distance of 385 light years, Pleione shines with a luminosity 190 times that of the Sun, its radius 3.2 solar, its mass (from the luminosity and temperature) 3.4 solar. Pleione's glory lies in its spectrum, its array of colors. Along with Cih, Pleione is one of the classic "Be" stars of the sky. The "e" stands for "emission," and refers to emissions of hydrogen that appear at specific wavelengths or colors (particularly one in the red part of the spectrum, hydrogen-alpha). A Be star's emissions come from a surrounding ring of gas that is somehow (though no one is quite sure) related to the star's great rotation speed, Pleione spinning at least as fast as 329 kilometers per second at the equator, 165 times faster than the Sun, giving it a rotation period of under half a day. The emissions of Be stars are split by the Doppler effect as a result of one part of the ring rotating toward us, the other receding. In the extreme "shell star" case, the ring also produces absorptions from both hydrogen and from a variety of elements caused by the ring directly blocking starlight. The difference in Be star styles was once thought to be a matter of orientation, but Pleione puts the lie to the theory by switching among all three phases, normal B star, Be star, Be shell star, the changes taking place at intervals of 17 and 34 years. These changes are related to brightness variations (which gave Pleione the variable star name BU Tauri). As the star enters the shell phase it fades by several tenths of a magnitude, the latest episode occurring in 1970. The switches may be related to the effect of a binary companion (about which nothing is known) that orbits eccentrically with a 35 year period and averages 28 astronomical units from Pleione proper: but no one really knows.