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GAMMA LUP (Gamma Lupi). Far to the south and rather lost in theglory of giantScorpius to the northeast,Lupus (the Wolf) garners little noticefrom northerners. Most of the figure is even lost from view above50 degrees north latitude. And too bad, because this ancientconstellation is loaded with modestly bright blue stars, one ofwhich is the third brightest in theconstellation, third magnitude (2.78) GammaLupi. Even though in theMilky Way at asubstantial distance of 420 light years (give or take 30), the starshines clearly with little more than 10 percent absorption ofstarlight by interstellar dust. Gamma Lupi, though, is not onestar, buttwo, both class B,the dominant one a magnitude 3.0 class B2 subgiant (but see below)with a temperature estimated at 22,400 Kelvin. At magnitude 4.5,the lesser is most likely a hard-to-confirm class B4 dwarf thatglows a bit cooler near 17,000 Kelvin. Less than a second of arcapart (which makes study of the individuals quite difficult), thetwo orbit each other with a period of 190 years at a meanseparation of 84.5 Astronomical Units, a high eccentricity takingthem between 128 and 41 AU.

Gamma Lupi B, the fainter of the two blue stars, orbits Gamma LupiA, which is set at the cross, taking 190 years to go around at anaverage separation of 85 Astronomical Units. In reality, each starmutually orbits a common center of mass that lies roughly a thirdof the way from A to B. The tick marks on the axes are only 0.5seconds of arc apart, showing how close the stars are to eachother. Though the orbit is nearly edge-on, tilted by a mere 5degrees to the line of sight, the stars still significantly misseclipsing each other. (W. I. Hartkopf and B. D. Mason,SixthCatalog of Orbits of Visual Binary Stars, US Naval ObservatoryDouble Star Catalog, 2006.)

Last closest together in 1885, they were farthest apart in 1980 andwill make another rendezvous in 2075. The orbit is nearly edge on,tilted by a mere 5 degrees, yet still too high to allow the twoblue stars toeclipse eachother. Application ofKepler's laws give a combined mass of 16.7 times that of theSun. After taking ultraviolet light into account, Gamma Lupi A andB have luminosities estimated at 6700 and 960 times that of theSun, radii of 5.5 and 3.0 solar, and (from theory) masses of 9 and5.5 solar. Summing them gives a mass of 14.5Suns, only 15 percent less than that derivedfrom the orbit, which, given all the uncertainties, is quite good. Rather than being a subgiant, however, a star that is giving (orhas just given up) core hydrogen fusion, Gamma Lupi A is actuallya real dwarf. With an age of just 17 million years, it's abouttwo-thirds of the way through its hydrogen-fusing lifetime (suchclassification discrepancies not at all unusual). Spinning with atypically high equatorial velocity of at least 270 kilometers persecond, Gamma A makes a rotational turn in under a day. Therotation of Gamma B is unknown. Gamma A is right at the limit atwhich stars blow up assupernovae, though thechances are good that the system will actually die as a massivedoublewhite dwarf. Gamma A may even turn into a rare neon-oxygen version (the otherthe standard carbon-oxygen kind). Only time, or improvedobservation and theory, will tell.

Written byJim Kaler 7/24/09. Return toSTARS.

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