ASCELLA (Zeta Sagittarii). At bright third magnitude (2.61) 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 3.47 (the two adding to the magnitude 2.61 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.
Written by Jim Kaler 10/03/03. Return to STARS.