XI TAU (Xi Tauri). Among the most prominent of stellar
configurations is the vee-shaped Hyades
cluster that, with foreground Aldebaran, makes the head of Taurus, the Bull. The "vee" points
southwest toward Lambda Tauri, and
then if the line is continued, it goes to an unrelated pair of
stars, Xi and Omicron Tau, that are a gateway to the head of Cetus, the Whale. At a distance of 222
light years, fourth magnitude (3.74) Xi Tauri is listed as a
seemingly ordinary white B9 hydrogen-fusing dwarf, which would make
it just a bit warmer than Vega. To the
contrary, the star is surprisingly complex, at first glance triple
(as determined from the spectrum)
and with a closer look, clearly quadruple. The triple most likely
consists of a close pair of
B9 dwarfs in a 7.15-day orbit that together are orbited by a hotter
B8 dwarf that takes 145 days to make a mutual circuit around the
closer pair. The light from the three has never been separated
out, so it is impossible to give measured parameters for each of
them. The best we can do is to look at averages and see where they
take us. B8 and B9 dwarfs typically have respective masses of 2.9
and 2.5 Suns, luminosities that are 63 and
45 times solar, and temperatures of 12,000 and 11,000 Kelvin, which
give radii of 2.1 and 1.8 solar. Correction for their ultraviolet
light (based on temperature) then gives visual luminosities of 36
and 29 Suns. Then there is an 8th magnitude fourth component found
by interferometry that lies several tenths of a second of arc away,
and goes around the inner triple. From its brightness it's an F5
dwarf with a luminosity double that of the Sun. Adding up all the
visual luminosities gives exactly the luminosity found from the
apparent magnitude of Xi Tau (viewed as a single star) and the
distance! So the stellar parameters must all be close to correct.
At least one of the stars is whipping around with an equatorial
rotation speed of 195 kilometers per second. Given their masses
and orbital periods, the inner B8 dwarfs orbit 0.13 Astronomical
Units apart, the outer B8 star goes around the inner B8 pair at a
distance of 1.1 AU, and the outer D component (with a mass of 1.25
Suns) goes around the inner triple with a period of about a century
at a distance of roughly 50 AU. From the outer F dwarf, the inner
B8 pair would appear up to around 10 minutes of arc apart, and the
outer B9 star would at most be just over a degree away from them.
All this from one starry dot just to the northeast of Cetus.
Written by Jim Kaler 3/06/09. Return to STARS.