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.
Written by Jim Kaler. Return to STARS.