TAU AND (Tau Andromedae). In Andromeda (daughter of Cassiopeia and Cepheus who was rescued by Perseus from being devoured by Cetus, the Sea Monster), about two- thirds of the way on a line from bright Mirach (Beta Andromedae) to Almach (Gamma And), lies dim fifth magnitude (4.94) neglected Tau Andromedae. It serves best as a guide to its brighter (fourth magnitude) famed neighbor about a degree to the northwest, Upsilon Andromedae, a more or less solar type dwarf with not just a planet, but a real system of four of them. If nothing else, the Tau-Upsilon progression shows that Bayer clearly used criteria other than brightness in his system of Greek letter names. Tau And though still holds its own as a class B (B8, some say B5) giant (though see below) with a much fainter solar-type binary companion. If truly a B8 giant, its color speaks of no intervening dimming interstellar dust. From its distance of 712 light years (give or take 39), it shines at us with a temperature of 13,000 Kelvin, which together give it a luminosity of 910 times that of the Sun and a radius of just 6 times solar. A projected equatorial rotation velocity of 91 kilometers per second gives a rotation period of under 3.3 days. The theory of stellar structure and evolution then tell of a rather heavy star that carries the mass of 4.5 to 4.6 Suns depending on the exact state of evolution. Not a giant at all, Tau And is more of an aged dwarf or even subgiant that has or is about to give up core hydrogen fusion. Having evolved from a much hotter B2.5 dwarf just over 100 million years ago, it will someday lose its outer layers to turn into a white dwarf of about 0.8 solar masses, stars always dying with much less mass than they started with. (If really a B5 giant, the color then suggests an interstellar dust dimming of 0.3 magnitudes, which raises the luminosity to 1200 Suns but does not much affect the mass). At a separation of 52 seconds of arc lies a magnitude 11.5 star that appears to be keeping Tau itself steady company and most likely belongs to it. If so, it is a solar type star with an orbital radius of at least 11,500 Astronomical Units that takes more than 530,000 years to make a full circuit. One wonders why the fragile gravitational bond has not been broken. Perhaps the little one has a planet (for which there is no evidence). If so, Tau And A might shine in its sky with the light of a full Moon.

Written by Jim Kaler 11/16/12. Return to STARS.