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.
Written by Jim Kaler. Return to STARS.