ELECTRA (17 Tauri). Among the sky's great sights is the compact
group of stars in Taurus known as the
Pleiades, or Seven Sisters, the
heavens' memorial to the seven daughters of Atlas and Pleione.
Seven of the clusters' stars are named after the mythical siblings,
and another two after their parents. One sister, Sterope
(sometimes "Asterope"), actually has two stars named after her, for
a total of 10. Among them all, Alcyone
is notably the brightest, and is followed by the "father," Atlas.
Just barely number three, at magnitude 4 (3.70) is Electra, which
is also known by the
Flamsteed number 17 Tauri. All 10 of the
named Pleiades are brilliant hot class B stars, their blue-white
color and compactness as seen from Earth giving the cluster (which
contains hundreds of lesser lights) its wonderful sparkle. Within
the named set is a small range in color and temperature, Atlas,
Pleione, and Maia spectroscopically the
coolest of them (at class B8),
Taygeta,
Merope, and our Electra the
hottest (class B6). Electra has a measured distance of 370 light
years, as found from a parallax determined by the Hipparcos
satellite. However, the cluster is far, and such a distance is
uncertain. The distance of the heavily-studied Pleiades as a whole
is actually 430 light years, which must be Electra's distance as
well, showing the problem with individually determined distances,
especially when the star is a long ways away. Like the other
Pleiads, especially Merope and Maia,
Electra is enmeshed light reflected from dust grains in an
interstellar cloud through which the cluster is now passing. From
its estimated surface temperature of 14,000 Kelvin (appropriate
class B6, which allows the calculation of invisible ultraviolet
radiation), we find a luminosity of 1225 Suns, a radius 6 times solar, and a mass of 5
times solar. Electra is one of the four Pleiades stars that is
classed as a giant, one that is starting to expand as the internal
hydrogen fuel in the core is exhausted (the leader in this "giant
crowd" Alcyone). Stars burn out from the "top down," high mass
stars dying first. From the maximum luminosities of the cluster
stars that have not yet begun the process toward gianthood, we
determine an accurate age of 130 million years for the cluster and
its members. Like so many class B stars, Electra is spinning
quickly, the lower limit 170 kilometers per second, and certainly
quite a bit higher. That gives the star a rotation period of at
most 1 3/4 days. Related to the high spin is a surrounding
equatorial disk of ejected matter that makes Electra one of the
Pleiades four "B-emission" (Be) stars, as the disk radiates on its
own. As determined from its spectra (and confirmed by a lunar
occultation, in which the Moon passed in front of the star),
Electra is also accompanied by a close-in companion that is
probably a class A star like Vega, and that orbits 0.8 Astronomical
Units away with an orbital period of 100.46 days.