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THETA HER (Theta Herculis). Not really a part of the classicalfigure ofHercules (who needs nointroduction), fourth magnitude (3.86) Theta Herculis lies due eastofPi Her (which marks the northeastcorner of the famedKeystone) and justwest ofVega. While seemingly yet justanother orange class K (K1)giant, it's one with adifference. Not just a "giant," but classed as a "bright giant,"the star is clearly more massive than its run-of-the-mill class. It's also a "strong CN" giant, a star rich in the cyanogen (CN)molecule, implying that it has dredged up freshly made elementsfrom its nuclear-burning region far below, which also implies greatmass, which is required to launch the currents that bring up thedregs of nuclear fusion. The strong CN absorptions give us a classthat is probably too warm (molecules forming better at lowtemperatures). A measured temperature of 4320 Kelvin implies aclass more like K3. Whatever it is, the luminosity, found from thetemperature and a distance of 752 light years (with an uncertaintyof about 20), is high, around 2400 times that of theSun, which gives a radius of 87 times solar. Direct measure of angular diameter yields a radius of 78 solar, sosomething is a bit off. An equatorial rotation speed of 5.4kilometers per second then gives a rotation period of at most 780days, more than two years, befitting such a great star. Oddly,even though the Theta Her is carbon-nitrogen rich, the metalcontent is rather low, about 60 percent solar, implying morenuclear enrichment than first expected. The mass derived fromtheory depends on the state of evolution. If the star is juststarting its first brightening as a giant (with a dead heliumcore), it comes in at 6.5 times solar, while if it is already ahelium-burner, it only requires an original mass of 5.5 solar. Ineither case, Theta Herculis started life as a hot class B starbetween 55 and 78 million years ago. Written byJim Kaler 10/02/09. Return toSTARS.
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