Movatterモバイル変換
[0]ホーム
KAPPA OPH (Kappa Ophiuchi). As anyone who studies the sky for anytime knows,star names can confuse,even to the point of mistaken identity. A good example is theoccasional confusion betweenX ("ex") andChi Herculis, the two letters, one Romanthe otherGreek, looking very much alike. Here's another. Kappa Ophiuchi is a relatively ordinary thirdmagnitude (3.20) class K (K2)giant in northernOphiuchus to the southwest ofRasalhague (Alpha Oph) that lies 91 anda half (give or take a half) light years away. From that and atemperature of 4620 Kelvin (needed to account for some infraredradiation), we find that the star shines with the light of 53Suns, which gives it a radius of 11.4 timessolar. A slow projected rotation speed of just one kilometer persecond leads to a rotation period that could be as long as 1.6years (though the axial tilt is not known). The mass fromluminosity and temperature is ambiguous, but falls roughly around1.5 solar. A much better mass of 1.2 solar is derived from thecareful observation of minute stellar oscillations, the starperhaps 5 billion or so years old (just a bit older than the lower-mass Sun) and quietly fusing helium into carbon and oxygen in itsdeep core. Now to the heart of the matter. Many decades ago, itwas "discovered" to be an irregular variable (by about a half amagnitude), and it became so listed in various catalogues (thoughskeptically given as "constant?" in the Russian variable-starcatalogue and thus in the Bright Star Catalogue as well). In 1991,however, Tristram Brelstaff in "The Variable Star Observer"suggested that Kappa Ophiuchi had been confused withCHI Oph (aheavily reddened fourth magnitude class B2 subgiant in southernOphiuchus), the twoGreek letters, ifwritten without care, looking somewhat alike. Chi Oph is indeedvariable, with an amplitude that well exceeds half amagnitude. Hardly variable, though, Kappa Oph is listed among the"least variable stars" as observed by the Hipparcos parallaxsatellite. The confusion thus seems rather plausible. So ratherunlike 2 and T Centauri where (forgive it again) it's "T for 2,""Chi for Kappa" does not really work. One might argue that KappaOph was indeed at one time variable, but has stopped pulsating. That seems unlikely, as the pace of evolution in human terms is fartoo slow, and variable giants do not really behave that way. (Yes,Polaris did indeed cease or rest itspulsations, but that is probably a case of aCepheid-type star switchingfrom overtone to fundamental.) Other than its notoriety as aresult of human error (so it is then supposed), the star has goingfor it that it is moving relative to the Sun at a fairly high speedof 68 kilometers per second, more than four times normal,suggesting that it is a visitor from a somewhat different part oftheGalaxy. (Thanks to PhilBagnall for suggesting this star, for providing the backgroundabout the confusion, and for information regarding "The VariableStar Observer.") Written byJim Kaler 6/18/10. Return toSTARS.
[8]ページ先頭