At magnitude 2.30, Alpha Lupi is the brightest star in Lupus, the Wolf. Lupus is one of the most southerly of all the ancient constellations. Shining south of Scorpius, the figure is hardly known in the north, but from southern latitudes it is glorious. α Lup has the traditional Chinese name Men, and the Arabic name Kakkab, which means "Guide Star" or "Star of Fortune". However, according to R. H. Allen, Kakkab's full name is "Kakkab Su-gub Gud-Elim", from the ancient Euphrates Valley, meaning "the Star Left of the Horned Bull" (Centaurus).
Properties and Evolution
Alpha Lupi is bright even though 550 light years away. A hot, class B1.5 III giant, α Lup pours 21,000 solar luminosities into space from a 21,600 K surface, with a radius over 10 times the Sun's. As a giant star, α Lup has probably just ceased its core hydrogen fusion. With 10 to 11 solar masses, the 20 million years old star is just on the dividing line between those that will blow up as supernovae and those that will turn into massive neon-oxygen white dwarfs.
Like a great many hot O and B stars, Kakkab is a member of a loosely organized grouping or association; in this case, the "Upper Centaurus-Lupus" sub-association, or UCL, which in turn is a part of a huge super-collection called the Scorpius-Centaurus Association. The average distance of all the UCL's members is 450 light years, which fits in very nicely with α Lup's individually measured distance of 550.
Variability and Companion
As are many hot class B giants, Alpha Lupi is a β Cephei star. These are all subtle variables that pulsate with multiple periods. With a major oscillation cycle of 0.259847 days, in which it varies by only 0.03 magnitudes (about 3%), α Lup has one of the longest periods of its class. A secondary pulsation takes 0.236798 days.
A 13th magnitude companion 28" away may belong to Kakkab, or may just be in the line of sight.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]