ZETA ARA (Zeta Arae). Directly to the south of Scorpius, within the thick star fields of the Milky Way, lies the deep southern, yet ancient, constellation of Ara, the Altar, its mythology surprisingly rich. Ara's most northerly named and second brightest star (mid-third magnitude Alpha Arae), 50 degrees to the south of the celestial equator, is invisible to anyone living to the north of the 40th parallel. Beta (just a bit brighter and the luminary) and Alpha Arae are followed in brightness not by Gamma, but by Zeta Arae (not surprising as the brightnesses of the top four stars are not all that dissimilar from one another). Another class K (K3) giant, Zeta would not be of much mention were it not also third magnitude (3.13) and readily visible (if one is far enough south). For all its obscurity, it does have a couple other things going for it. Fairly distant, 486 light years away (give or take 14), the star seems fairly well obscured and a bit reddened by intervening interstellar dust, from the calibrations of color against spectral class by a full magnitude. (Interstellar dust absorbs and scatters light better at shorter bluer wavelengths than at longer redder ones. Obscured stars thus look redder than they ought for a given spectral class, from which one can evaluate the degree of total dimming.) That is, were no dust present, we would see a mid-second magnitude (2.11) star. But that presents a puzzle, since Zeta Arae does not seem far enough away for such a significant obscuration. Given it is real, however, the star shines at a hefty 4920 Suns from a 4350 Kelvin surface, which leads to a radius of 123 times solar, Zeta quite luminous for its class. Assuming that there is no dust at all drops the luminosity and radius to 1920 and 77 times that of the Sun. Interferometry apparently decides the issue. The measured angular diameter of the star combined with the accurate distance gives a radius of 128 solar, closely in line with the dust-obscured value. Luminosity and temperature combined with theory then tell of a massive star carrying between 6 and 7 solar masses, most likely six, one quietly fusing its core helium into carbon ad oxygen. Too light to explode as a supernova (which takes at least 8 or 9 solar masses), the star will end its life as a white dwarf (the old nuclear burning core, which will have lost its outer envelope) with a mass close to that of the Sun and similar to Sirius B.
Written by Jim Kaler 7/13/12. Return to STARS.