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 Gamma Cassiopeia, 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.