CELAENO (16 Tauri). Above the "vee" of Taurus's head, more or less pointed to by Orion's Belt, lies one of the most absorbing of celestial sights, from Greek mythology the Seven Sisters, the Pleiades. From our modern view they are collectively a beautiful young open cluster of hundreds of stars topped by 10 bright blue-white class B stars that lie 430 light years away, the pack led by Alcyone, Eta Tauri. While six of the stars are readily visible with the naked eye, there have to be seven, these daughters of Atlas and Pleione. Hence perhaps the various stories of the "Lost Pleiad." Which one depends on which version. Some say Merope (23 Tauri), others say Electra (17 Tau), while still say it must have been Celaeno (16 Tau). The parents of these sisters, Atlas (27 Tau) and Pleione (28 Tau), rank second and seventh in apparent brightness, Atlas confusingly part of the set of visible six. Of the sisters, Electra ranks second and Merope ranks fourth, so neither seems "lost." So, as the faintest of the trio (nearly sixth magnitude, 5.46), and in agreement with the Bright Star Catalogue, we pick Celaeno as our Lost Pleiad. (Oddly nobody chose Sterope, the faintest of them all.) If nearly 0.3 magnitude of dust absorption could be removed from the line of sight, this class B (B7) subdwarf would appear at magnitude 5.19. From the star's distance, its temperature of 13,200 Kelvin (from which we calculate the amount of ultraviolet light), and a correction for the light from a close companion (see below), we calculate a luminosity 240 times that of the Sun, which leads to a radius 3 times solar and a mass 3.7 solar. The minimum rotation speed of 185 km/s in turn leads to a rotation period of less than 19 hours. Though called a subdwarf (which implies that it has given up core hydrogen fusion), Celaeno is actually a well- advanced hydrogen fusing dwarf. At that mass, the hydrogen fusing lifetime is 225 million years, which is nicely consistent with the age of the cluster itself, only 130 million years. Interferometry reveals our "Lost Pleiad" to have a companion a factor of six fainter at a separation of just 0.0062 seconds of arc, which at the distance of the Pleiades corresponds to a class A3 star about just under one Astronomical Unit away, which in turn implies a period of just under half a year. Welcome them both back home.
Written by Jim Kaler. Return to STARS.