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ASCELLA

(Zeta Sagittarii). At bright third magnitude (2.63) and beautifully prominent because of its location, Ascella makes the lip of the Little Milk Dipper of Sagittarius. The Latin name -- not so old as most -- also reveals that the star makes the "armpit" of the Archer. The Greek letter names of the stars in this constellation show that Bayer clearly often had something other than brightness in mind when he applied them. In order, the top three are Epsilon (Kaus Australis), Sigma (Nunki), and Zeta, our Ascella. Alpha (Rukbat) is way down the list. Physically, the star is not one, but two, that orbit each other, on the average separated by about half a second of arc (making their separation a great challenge for the eye at a telescope). Because of their proximity to each other, their condition is not clear (the light of one star messing up that of the other). Both are class A, the brighter, at magnitude 3.26, an A2 "giant." The other, at magnitude 3.37 (the two adding to the magnitude 2.63 star that we see) is classified as either an A2 dwarf (which implies full core hydrogen fusion) or as an A4 subgiant (which implies recent stoppage of fusion). We will adopt the latter here. From the distance of 89 light years, we derive respective luminosities for the "A" and "B" components of 31 and 26 times that of the Sun, which from estimated temperatures (they have not been measured) of 9000 and 8500 Kelvin give masses of 2.2 and 2.1 solar, the combination clearly showing that both are REALLY dwarfs and neither giant nor subgiant. (We still cleave to the actual classification.) The two orbit every 21.075 years at an average separation of 13.4 Astronomical Units (40 percent farther than Saturn is from the Sun). The rather elliptical orbit takes them from a maximum separation of 16.1 AU to 10.6 AU. From the orbit and Kepler's orbital laws we find a sum of masses of 5.4 times that of the Sun, 25 percent larger than determined from luminosity and temperature (and the theory of stellar structure and evolution). Errors in distance, temperature, and orbital parameters all account for the discrepancy. At a separation of 75 seconds of arc is an eleventh magnitude (10.63) "C" component. If an actual part of the system, it must be a class K7 dwarf located at least 2000 AU from the inner double, one that orbits with a period of at least 40,000 years. However, slight shifts in separation over the past century are about that expected from the double's motion through space relative to the distant background, so little "C" is probably just a line-of-sight coincidence, adding to Ascella's many uncertainties.