BETA LUP (Beta Lupi). Unlike many constellations that violate the brightness rule, the Greek-lettered stars of Lupus, the Wolf, go nicely according to plan, from brighter to fainter, second magnitude (2.30) Alpha Lupi (Kakkab) in the lead, followed by bright third magnitude (2.68) Beta Lup, then right on down to near-fourth- magnitude (3.41) Zeta and Eta Lupi. Glowing with blue-white light, with the exception of Zeta Lupi (a yellowish class G giant), Alpha through Eta Lupi are all in the hot class B range. Beta (a class B2 giant) falls right in among the crowd. Lying between distances of 490 to 570 light years, the top five (plus Eta), including Beta Lupi (524 light years away) and many others, are part of the expanding "Upper Centaurus Lupus" (UCL) OB association, all born at about the same time (not that long ago) and place. After correcting for a lot of ultraviolet light from Beta Lupi's hot (22,650 Kelvin) surface, and a small six percent correction for dimming by interstellar dust, we find a luminosity 13,600 times that of the Sun. These lead to a radius of 7.6 times solar, a rotation period (based on a projected equatorial rotation speed of 110 kilometers per second) of less than 3.4 days, a mass between 10.5 and 11 times solar depending on the exact state of evolution, and an age of roughly 18 million years. Whatever the details, the star is really more a subgiant at or near the end of its hydrogen-fusing life and is about ready to make the transition toward becoming a red supergiant whose most likely fate is to explode as a supernova (though becoming a massive neon white dwarf is still a possibility). Beyond that, little is actually known about it. Typical of its class in our local region of the Galaxy, Beta Lup is low in metals compared with the Sun (having about half the solar content relative to hydrogen). Also like many of its kind, it's also a multi- periodic Beta Cephei-type variable with a principal period of 5.57 hours, but with an uncertain variation range. A broader view shows the star a member of the much more extensive Scorpius-Centaurus-Lupus-Crux complex, of which Upper-Centaurus-Lupus is a part and that gives a wondrous blue sparkle to this part of the sky.
Written by Jim Kaler 7/20/07. Return to STARS.