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.