XI-2 CET (Xi-2 Ceti). Welcome to the Land of Xi's, which sounds
rather romantic when spoken aloud. Without doing any real in-depth
study, this region at the Cetus-Aries-Taurus
border seems to have the greatest concentration of stars named with
the Greek letter "Xi" -- four of them --
than any other place in the sky, and therefore in the whole
Universe! Fourth magnitude (4.28) Xi-2 Ceti, a class B9 giant (but as common, see below)
in Cetus (the Whale or Sea Monster), is
sort of central to the quartet. Its constellation-mate, Xi-
1 (a fourth magnitude class G giant) lies just under 4 degrees
to the west (the "1" and "2" designations almost always going
from west to east), while the faintest of them, the fifth magnitude
B7 subgiant Xi Arietis sits just 2 degrees to the north. The
brightest, the brighter-fourth-magnitude class B9 dwarf Xi Tauri, is the most distant, 14 degrees to
the east. While curious, such a relationship is not really much a
distinction. Perhaps better is Xi-2's prominent (such as it is)
location as the western-most star in Cetus's rather raggedly-
circular head. And that Xi-2 is such a peaceful, ordinary single
star (no companion ever
noted) that it is used as a standard with which to compare other
stars, which in fact makes it rather important. A distance of 193
light years (with an uncertainty of 12) plus a well-determined
temperature of 10,420 Kelvin (to allow for a bit of ultraviolet light) yields a
luminosity of 74 times that of the Sun,
which leads to a radius of 2.6 solar. A projected rotation
velocity of 57 kilometers per second then gives a rotation period
of under 2.6 days. Stellar structure theory shows that far from
being a giant (note the relatively small radius), X-2 Ceti is
actually a middle-aged dwarf roughly mid-way through its 450-
million-year hydrogen-fusing lifetime (spectral classification
being rather difficult for such stars). The iron content, at 45
percent above solar, is somewhat (and uncertainly) high, while
helium is depressed by two-thirds or so and calcium is down by a
factor of two. But for stars like this one, the chemistry is often
wildly different as a result of gravitational settling and
radiative lofting in quiet, unstirred atmospheres. The relative
normalcy implies that the star is not really rotating all that
rapidly and that the rotation period derived above is pretty close
to the mark. Though they share a name, Xi-1 and Xi-2 are quite
uncoupled, Xi-1 twice as far away from us. From each, the other
would shine at but third magnitude, not much of a change from what
we see from here, the Land of Xi's a mere illusion.
Written by Jim Kaler 12/17/10. Return to STARS.