LAMBDA LUP (Lambda Lupi). For hot stars, look no farther than your nearest Wolf. The one in the sky. Stuck between Scorpius to the east and Centaurus to the west, Lupus's scattered stars are commonly connected in the form of a (as might be expected) loop, within which we find our star, Lambda Lupi, five degrees northeast of the second magnitude luminary, Kakkab (Alpha Lupi). Made of two hot class B (B3) dwarfs only a fraction of a second of arc apart, Lambda Lupi A (magnitude 4.47) and Lambda Lupi B (magnitude 5.27) combine to shine at mid-fourth magnitude (4.05) from a lucky distance of 777 light years, give or take around 120.

Lambda Lup The two hot class B dwarfs that make Lambda Lupi are only a few tenths of a second of arc apart (note the scales) and are difficult to separate without sophisticated equipment. The best fit of an elliptical path to the observed positions of Lambda Lupi B relative to Lambda A (at the cross) shows the two stars to go around each other every 70.8 years. The major axis (the dot-dash line) and the focus (the cross, where we find Lambda A) of the true orbit are offset from the observed because of a 71 degree tilt of the actual orbit to the plane of the sky and by its orientation. From the true orbit, the two stars average 61.8 Astronomical units apart, a high eccentricity taking them between 29 and 100 AU away from each other. Though "B" is charted to go about "A," the two stars really orbit each other around a common center of mass that lies between them and whose location is determined by the mass ratio, which in Lambda Lupi's case has not been found directly through observation. Application of Kepler's laws yields a total mass that is much higher than expected for these class B3 dwarfs, so something must be wrong. The pair needs more observation over a longer period of time. From the Sixth Catalog of Orbits of Visual Binary Stars , W. I. Hartkopf and B. D. Mason, US Naval Observatory Double Star Catalog, 2006.

A sole temperature listing combined with that expected for a B3 dwarf gives 17,900 Kelvin, which is applied to both components. After a 13 percent correction for interstellar dust absorption and a whopping correction for ultraviolet radiation, Lambda Lupi A shines with the light of 3970 Suns, Lambda Lup B with that of 1850, from which we find respective radii of 6.5 and 4.5 Suns. A minimum equatorial rotation speed of 166 kilometers per second probably applies to the brighter alone, and shows "A" to be rotating in under 2.0 days. From theory, Lambda Lupi A is near the end of its hydrogen-fusing lifetime of 33 million years. Depending on the exact stage of evolution, its mass falls between 7.0 and 7.5 Suns. Lambda B then comes in at 6.4 Suns. With a dwarf lifetime of 50 million years, it has roughly 17 million years of core hydrogen fusion left to it, by which time "A" will have completed helium fusion as a giant and will be preparing to eject its outer envelope eventually to become a white dwarf of a solar mass or more ("B" to make one not much lighter). From the observed orbit and distance, they average 62 Astronomical Units apart and take 70.8 years to make a complete circuit of each other. A fairly high eccentricity takes them between 23 and 100 AU apart. Their close approach took place late in 1997, and they'll be farthest apart in 2033. However, there's a problem. Kepler's laws applied to the orbit yield a combined mass of 47 Suns, far above that of 13.5 Suns derived from luminosity and temperature. The results can be reconciled by lowering the size of the orbital ellipse (the semi-major axis) to 41 AU. Much of the difference, though, might be ascribed to error in distance, close doubles often presenting problems. At an angular separation of 6.6 seconds of arc hovers seventeenth magnitude Lambda Lupi C. If gravitationally bound, from its brightness it's an M4 red dwarf that must be at least 1600 AU from the inner pair and take at least 16,000 years to make a full orbit. Unfortunately there is but one measure, and "C" could just as easily be a line of sight coincidence. We can only wait to see whether or not "C" tracks "AB," the pair hustling through space relative to the Sun at 36 kilometers per second, about twice average. Farther afield, Lambda Lupi's motion and distance place it on the far side of the Upper-Centaurus-Lupus association of massive stars, all of which had some kind of common origin and that includes Alpha Lupi, Delta Lupi, Kappa Centauri, and several others of note.
Written byJim Kaler 6/20/14. Return to STARS.