KAPPA PAV (Kappa Pavonis). And now for something different. Kappa Pavonis is a seemingly innocuous fourth magnitude star (4.4, almost fifth) in Pavo (the Peacock) some 15 degrees southwest of the second magnitude luminary Alpha Pavonis. At 67 degrees south of the celestial equator, it's circumpolar for anyone south of 23 degrees south latitude, near the Tropic of Capricorn. Among the most important stars of the sky are the Cepheids, mid-temperature evolving giant and supergiant variable stars that typically change their brightness by a magnitude or more with pulsation periods of anywhere from 1 to 50 days. An individual Cepheid is characterized by a quick rise in brightness followed by a slow fall. The classic examples are Delta Cephei, Eta Aquilae, and Zeta Geminorum. As faint as it appears to the eye, Kappa Pavonis (500 light years away, give or take 85) appears as one of the brightest of Cepheids. Nominally an F5 supergiant-bright giant, it varies between magnitudes 3.94 and 4.75 over a period of 9.0814 days (the period slowly changing). But it does not really fit among the normal Cepheids, which are massive stars that hug the central plane of the Galaxy and are giants and supergiants that are preparing to, or already are, fusing helium in their cores. Kappa Pav is instead a bright example of a "W Virginis star." W Vir stars (the prototype only 10th magnitude) mimic true Cepheids but are lower mass stars in a later state of evolution than standard classical Cepheids like Delta Cep. Moreover, they are distributed out of the central Galactic disk into a thicker disk and the surrounding halo, giving them the alternative name "Type II Cepheids." They have more in common with RR Lyrae than they do with Delta Cephei. Cepheids are characterized by a strong relation between period and luminosity, which allows their distances to be determined. They are crucial in finding the distances of galaxies and for understanding the expansion of the Universe. In 1951, it was found that for the same period, the classical Cepheids were on the average 1.5 magnitudes brighter than the W Virginis stars. When the two kinds were separated out, the new period-luminosity relation for the classical Cepheids of the Galactic disk about doubled the scale of the Universe nearly overnight. At around 5800 Kelvin, lower than expected for a G5 supergiant (probably as a result of lowered metals), the luminosity of Kappa Pav appears to be around 350 times that of the Sun. The radius, which is variable, is some 15 times solar. The mass, fairly low, is unknown. Consistent with an older thick-disk or halo population, the star is moving relative to the Sun about three times faster than normal. But Kappa Pavonis is even unusual among the W Vir stars, as it is too bright. If it had been in the northern hemisphere, where it would have gotten more attention, it would be better understood. Nevertheless, unless somebody says otherwise, Kappa Pav remains the brightest of its breed and deserves far more recognition than it gets.

Written by Jim Kaler 10/18/13. Return to STARS.