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) subgiant 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 subgiant (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 just
under one Astronomical Unit away, which in turn implies a period of
just under half a year. Welcome them both back home.