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.