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KAPPA PAV (Kappa Pavonis). And now for something different. KappaPavonis is a seemingly innocuous fourth magnitude star (4.4, almostfifth) inPavo (the Peacock) some 15degrees southwest of the second magnitude luminaryAlpha Pavonis. At 67 degrees south of thecelestial equator, it'scircumpolar for anyone south of 23degrees south latitude, near theTropic of Capricorn. Among themost important stars of the sky are theCepheids, mid-temperatureevolvinggiant andsupergiant variable stars thattypically change their brightness by a magnitude or more withpulsation periods of anywhere from 1 to 50 days. An individualCepheid is characterized by a quick rise in brightness followed bya slow fall. The classic examples areDelta Cephei,EtaAquilae, andZeta Geminorum. Asfaint as it appears to the eye, Kappa Pavonis (500 light yearsaway, give or take 85) appears as one of the brightest of Cepheids. Nominally an F5 supergiant-bright giant, it varies betweenmagnitudes 3.94 and 4.75 over a period of 9.0814 days (the periodslowly changing). But it does not really fit among the normalCepheids, which are massive stars that hug the central plane of theGalaxy and are giants andsupergiants that are preparing to, or already are, fusing helium intheir cores. Kappa Pav is instead a bright example of a "WVirginis star." W Vir stars (the prototype only 10th magnitude)mimic true Cepheids but are lower mass stars in a later state ofevolution than standard classical Cepheids like Delta Cep. Moreover, they are distributed out of the central Galactic diskinto a thicker disk and the surrounding halo, giving them thealternative name "Type II Cepheids." They have more in common withRR Lyrae than they do with Delta Cephei.Cepheids are characterized by a strong relation between period andluminosity, which allows their distances to be determined. Theyare crucial in finding the distances ofgalaxies and for understandingtheexpansion of theUniverse. In 1951, it was found that for the same period, theclassical Cepheids were on the average 1.5 magnitudes brighter thanthe W Virginis stars. When the two kinds were separated out, thenew period-luminosity relation for the classical Cepheids of theGalactic disk about doubled the scale of the Universe nearlyovernight. At around 5800 Kelvin, lower than expected for a G5supergiant (probably as a result of lowered metals), the luminosityof Kappa Pav appears to be around 350 times that of theSun. The radius, which is variable, is some 15times solar. The mass, fairly low, is unknown. Consistent with anolder thick-disk or halo population, the star is moving relative tothe Sun about three times faster than normal. But Kappa Pavonis iseven unusual among the W Vir stars, as it is too bright. If it hadbeen in the northern hemisphere, where it would have gotten moreattention, it would be better understood. Nevertheless, unlesssomebody says otherwise, Kappa Pav remains the brightest of itsbreed and deserves far more recognition than it gets.
Written byJim Kaler 10/18/13. Return toSTARS.
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