THETA LUP (Theta Lupi). Lying in the shadow of Scorpius, sandwiched between the Scorpion and northern Centaurus, Lupus (the Wolf) just skims the southern horizon and is not terribly well known to northerners. Those in the south, however, can admire the sparkle of its blue-white stars, many of which are related to each other. Just 4.5 degrees southeast of the Wolf's northern apex, marked by Chi Lupi (a "Pathfinder Star" for the study of strange chemical abundances), lies one of them, the fourth magnitude (4.23) class B (B2.5) dwarf Theta Lupi, which as seen from 45 degrees north latitude transits the summer meridian 13 degrees above the horizon. Were this star in the northern hemisphere, it would probably be well studied, but it isn't and it hasn't been. Theta Lup's substantial distance of 414 light years (give or take 23) places it in back of a small amount of interstellar dust that dims it by a couple tenths of a magnitude. Adopting a temperature (from one estimate and the average for its class) of 20,000 Kelvin, needed to account for quite a lot of ultraviolet radiation, Theta Lup shines at us with a luminosity of 1820 times that of the Sun, which in turn yields a radius of 3.6 times solar. Blue dwarfs are inherently massive and young. The theory of stellar structure and evolution yields a mass of 6.6 Suns and an age of very roughly half the hydrogen fusing time of 63 million years. The core will eventually die as a fairly massive white dwarf of about a solar mass similar to Sirius B. Though Theta Lup has no known actual companion, it does belong to an extended family, the "Upper Centaurus Lupus" association of massive stars, which is centered 450 light years away and whose stars have a more or less common origin. What really brings Theta Lupi to the fore, however, is its rotation speed, which at a minimum (since we do not know the axial tilt) hovers around 315 kilometers per second, as opposed to two km/s for the Sun (on the high side for the class, though not up to the record heights of 400 or so seen in stars like 48 Librae). Theta thus makes a full rotation in under 0.6 days (the Sun taking nearly a month). Rapid rotators commonly possess radiating disks that have somehow been ejected from them, turning the stars into the special "Be" (for "B- emission") stars. Oddly, Theta Lupi shows no such disk. They can come and go, however (witness the case of Delta Scorpii), so in spite of the star's rather obscure location, we need to keep watching it, as such an outburst may yet be in its future.

Written by Jim Kaler 7/12/13. Return to STARS.