RHO AND (Rho Andromedae). If you look about 5 degrees southwest of
the Andromeda
Galaxy (Andromeda's greatest
feature), roughly between it and the Great Square of Pegasus, you'll find a small triangle of
fifth magnitude stars a couple degrees across. At magnitude 5.18 the
faintest of the three, Rho Andromedae is also the most easterly, while
brighter Theta is at the northern apex and Sigma is at the southern.
Classed as an F3 giant, the
first impression would be that it is well along in its evolutionary
track on the HR diagram (a plot of brightness
vs, temperature, wherein absolute magnitude and spectral class are
used as proxies). Andromeda is rather well off the Milky Way, so dimming by interstellar dust is not much of a
problem and we'll ignore it for now. Rho's temperature is rather
well-defined by a number of observations at around 6720 Kelvin, so most
of the light is in the visual spectrum and we need make little
correction for either infrared or
ultraviolet light. At a distance of 158 light years (give or take
2 and in between Theta and Sigma), the star shines with the light of
16.6 Suns, which with temperature gives it a
radius of only 3.0 times that of the Sun, not much for a so-called
"giant." The projected equatorial rotation velocity of 42 kilometers
per second gives a rotation period of less than 3.6 days. In spite of
the relatively small size, the angular diameter has been measured by
interferometry at 0.600 thousandths of a second of arc to one percent
precision. Given the distance, the star then has a radius of 3.13 times
that of the Sun, just four percent higher than that calculated from
temperature and luminosity, not a bad fit at all. A little juggling
of parameters cold bring the two into exact agreement. While Rho And's
spectrum may be that of an F3 giant, the theory of stellar structure
and evolution shows that the star is really a 1.75 (Or a bit higher)
solar mass subgiant that has
just taken on the role and is more a dwarf that has just given up core
hydrogen fusion and is some 1.8 billion years old. The expected color
of a dwarf is just what we see, so there indeed appears to be no need
for correction by interstellar dimming, not that it makes too much
difference. Rho And appears to be all alone with no companion to witness its
expansion to a real red giant, which will happen before long, though
the pace of evolution is so slow (barring explosions) that we will not
see it. Nowhere nearly massive enough to blow up as a supernova (at least 8 or 9 Suns
is required), Rho And will slough off its outer envelope, maybe produce
an ephemeral planetary nebula, and die as a white dwarf of about 0.6
solar masses. The star teaches a nice lesson in that there is not a
one-to-one relation between spectral class and actual evolutionary
class, the dichotomies more noticeable among the class B stars.
Written byJim Kaler 1/27/17. Return to STARS.