THETA HER (Theta Herculis). Not really a part of the classical figure of Hercules (who needs no introduction), fourth magnitude (3.86) Theta Herculis lies due east of Pi Her (which marks the northeast corner of the famed Keystone) and just west of Vega. While seemingly yet just another orange class K (K1) giant, it's one with a difference. 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 elements from its nuclear-burning region far below, which also implies great mass, which is required to launch the currents that bring up the dregs of nuclear fusion. The strong CN absorptions give us a class that is probably too warm (molecules forming better at low temperatures). A measured temperature of 4320 Kelvin implies a class more like K3. Whatever it is, the luminosity, found from the temperature and a distance of 752 light years (with an uncertainty of about 20), is high, around 2400 times that of the Sun, which gives a radius of 87 times solar. Direct measure of angular diameter yields a radius of 78 solar, so something is a bit off. An equatorial rotation speed of 5.4 kilometers per second then gives a rotation period of at most 780 days, more than two years, befitting such a great star. Oddly, even though the Theta Her is carbon-nitrogen rich, the metal content is rather low, about 60 percent solar, implying more nuclear enrichment than first expected. The mass derived from theory depends on the state of evolution. If the star is just starting its first brightening as a giant (with a dead helium core), it comes in at 6.5 times solar, while if it is already a helium-burner, it only requires an original mass of 5.5 solar. In either case, Theta Herculis started life as a hot class B star between 55 and 78 million years ago.
Written by Jim Kaler 10/02/09. Return to STARS.