CHI AQR (Chi Aquarii). There is a fascination with stellar limits,
with extremes, with looking at the hottest blue stars of class O or
at the other end, at the reddish stars of class M. Even though the
colors are pale when compared to those of the spectrum, they still
stand out in a dark sky, as we can witness by comparing Betelgeuse with the rest of Orion's family. All constellations harbor
some reddish stars, and Aquarius is no
exception. Though fifth magnitude (5.06) and thus relatively faint
to the eye, the reddish color of Chi Aquarii (a class M3 giant with
a temperature of only 3670 Kelvin) is obvious in binoculars or
through the telescope. The irregular variability of some 0.35
magnitudes (from 4.9 to 5.25) and relatively cool type are the
first clues that Chi Aqr is not just a giant, but one in an advanced state of evolution.
From a goodly distance of 640 light years, the star shines with a
luminosity of 1500 Suns, which leads to a
radius 96 times that of the Sun, or 0.45 Astronomical Units, nearly
half the size of the orbit of the Earth: a giant indeed. Direct
measure of angular diameter via interferometer gives a larger
radius of 0.60 AU. Situated just south of the ecliptic, Chi
Aquarii is periodically occulted by the Moon. From the time it
takes the lunar disk to cover the star, we derive a yet larger
radius of 0.66 AU, the discrepancies probably resulting from a
combination of errors in distance and in the estimation of
temperature and thus of the amount of infrared radiation that
factors into luminosity. Though difficult to estimate, the mass
seems be around twice that of the Sun. Most likely, the star is
brightening as a giant for the second time, now with a dead carbon-
oxygen core (the first brightening done with a dead helium core).
Such stars are commonly unstable and variable as they prepare to
slough off their outer envelope before adding to the company of white dwarfs that flock
around us.