GAMMA LUP (Gamma Lupi). Far to the south and rather lost in the glory of giant Scorpius to the northeast, Lupus (the Wolf) garners little notice from northerners. Most of the figure is even lost from view above 50 degrees north latitude. And too bad, because this ancient constellation is loaded with modestly bright blue stars, one of which is the third brightest in the constellation, third magnitude (2.78) Gamma Lupi. Even though in the Milky Way at a substantial distance of 420 light years (give or take 30), the star shines clearly with little more than 10 percent absorption of starlight by interstellar dust. Gamma Lupi, though, is not one star, but two, 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 that glows a bit cooler near 17,000 Kelvin. Less than a second of arc apart (which makes study of the individuals quite difficult), the two orbit each other with a period of 190 years at a mean separation of 84.5 Astronomical Units, a high eccentricity taking them between 128 and 41 AU.
Gamma Lupi Gamma Lupi B, the fainter of the two blue stars, orbits Gamma Lupi A, which is set at the cross, taking 190 years to go around at an average separation of 85 Astronomical Units. In reality, each star mutually orbits a common center of mass that lies roughly a third of the way from A to B. The tick marks on the axes are only 0.5 seconds of arc apart, showing how close the stars are to each other. Though the orbit is nearly edge-on, tilted by a mere 5 degrees to the line of sight, the stars still significantly miss eclipsing each other. (W. I. Hartkopf and B. D. Mason, Sixth Catalog of Orbits of Visual Binary Stars, US Naval Observatory Double Star Catalog, 2006.)
Last closest together in 1885, they were farthest apart in 1980 and will make another rendezvous in 2075. The orbit is nearly edge on, tilted by a mere 5 degrees, yet still too high to allow the two blue stars to eclipse each other. Application of Kepler's laws give a combined mass of 16.7 times that of the Sun. After taking ultraviolet light into account, Gamma Lupi A and B have luminosities estimated at 6700 and 960 times that of the Sun, radii of 5.5 and 3.0 solar, and (from theory) masses of 9 and 5.5 solar. Summing them gives a mass of 14.5 Suns, only 15 percent less than that derived from the orbit, which, given all the uncertainties, is quite good. Rather than being a subgiant, however, a star that is giving (or has just given up) core hydrogen fusion, Gamma Lupi A is actually a real dwarf. With an age of just 17 million years, it's about two-thirds of the way through its hydrogen-fusing lifetime (such classification discrepancies not at all unusual). Spinning with a typically high equatorial velocity of at least 270 kilometers per second, Gamma A makes a rotational turn in under a day. The rotation of Gamma B is unknown. Gamma A is right at the limit at which stars blow up as supernovae, though the chances are good that the system will actually die as a massive double white dwarf. Gamma A may even turn into a rare neon-oxygen version (the other the standard carbon-oxygen kind). Only time, or improved observation and theory, will tell.
Written by Jim Kaler 7/24/09. Return to STARS.