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TANIA AUSTRALIS (Mu Ursae Majoris). OurUrsaMajor, the Greater Bear, contains remnants of ancient Arabicconstellations, the best-known example the starAlkaid, which refers to the leader of thedaughters of a funeral bier. Southwest of theDipper's bowl lie three obvious pairs of stars thatrepresent the bear's paws, but to the Arabs were the tracks ofleaping gazelles. The middle pair is the "second leap," from whichcomes the name "Tania" (for "second"). In the multi-cultural mixof constellation lore, the northern one received the Arab-LatinnameTania Borealis, the southern Tania Australis. Bayer assignedthe three "leaps" ordered Greek letters, Tania Borealis receivingLambda, Tania Australis Mu. The two make a lovely contrast, TaniaBorealis a white class A subgiant, Australis a fairly rare (fornaked eye stars) red class M (M0) giant. Tania Australis shines atmid third magnitude (3.05) from a distance of 250 light years(double the distance of Borealis, the two only a line-of-sightcoincidence). When we account for infrared radiation from a 3950Kelvin surface, Tania Australis is found to have a luminosity 850times that of theSun, which leads to a radius 62 times solar (0.28Astronomical Units, three-fourths the size of Mercury's orbit). Having used its core hydrogen, Tania Australis seems to bebrightening along the "red giant branch" with a contracting heliumcore. Before long, the helium will fire up to fuse to carbon, andthe star will dim some and stabilize as a class K giant. TaniaAustralis is an unresolvable binary, the companion (known only fromspectroscopic observations) circling the M giant every 230 days ata distance of at least 1.5 Astronomical Units, suggesting acombined mass over 9 times solar, double that expected on the basisof luminosity and temperature. Tania Australis proper (ignoringthe companion) is also a rare "hybrid star." Magnetically activestars like the Sun blow a relatively fast but thin wind from theirsurfaces. Larger giants blow slower, but much thicker winds. Hybrid stars seem to blow both. Though the star is cooler than thedividing line at which stars seem to lose their X-rays, TaniaAustralis perversely still seems to radiate them. Alas, thesouthern star of the second leap is not very well understood.
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