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.