NU AND (Nu Andromedae). Not all that bright, just over the edge
into fifth magnitude (4.52), and without any sort of proper name,
Nu Andromedae still has a number of things to recommend it,
including mystery. Located in central Andromeda, the star is the gateway to the Andromeda Nebula, Messier 31, a
magnificent galaxy two million light years off that is readily
visible to the naked eye. Simply go to Mirach (Beta Andromedae), the center star of
the main stream that comes off the northeast corner of the Great Square of Pegasus, then go
northwest two stars -- the top one is Nu; M 31 lies just to the
northwest of it. Nu And is also a very close double star made up
of a luminous class B (B5) dwarf and a much fainter star that at
class F8 (as best we can tell) is not that much different from the
Sun. The spectrograph shows that the two
orbit quickly, taking a mere 4.28 days to make a full circuit.
From its rather large distance of 680 light years, the brighter
class B star shines with the light (allowing for considerable
ultraviolet radiation from its 15,000 Kelvin surface) of nearly
1700 Suns, from which we derive a mass 5.8 times that of the Sun.
The detailed characteristics of the little one are not known, but
it seems to contain around 1.1 times the solar mass. With these
masses and the orbital period, the two must be only about 20 solar
radii apart, which, given the 6-solar-radius of the class B star,
means they are only 3 or so B-star radii apart! The large star
spins with an equatorial speed of 80 kilometers per second, the
smaller with a speed of around 11 km/s. Given the stellar sizes,
these figures give spin periods of around 4 days for both stars,
meaning that they are synchronized by tides, one face of each star
always pointing at the other, much like the Moon keeps one face
pointing to the Earth. Given the closeness of the two, such tidal
synchrony is just what we would expect. At an age of roughly 80
million years, the B star is just about to give up its internal
hydrogen fusion, if it has not done so already, and is preparing to
swell to become a huge giant star. And here is where the mystery
comes in. As it grows, it will encounter the little class F
companion. No one is able to predict quite what will happen. Mass
will surely transfer to the F star, which will puff up, the
encounter severely altering the evolution of the B star. On the
other hand, the B star, being by far the dominant, may eventually
absorb the F star into itself, leaving one less solar type star in
the Galaxy. Only time, or a better understanding of how close
doubles develop and evolve, will tell. Nu And is also considered to
be a marginal "runaway star," one that has been kicked away from
another companion at relatively high velocity, suggesting that it
may once have been in a more complicated system.