Movatterモバイル変換
[0]ホーム
NU AND (Nu Andromedae). Not all that bright, just over the edgeinto 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 centralAndromeda, the star is the gateway to theAndromeda Nebula, Messier 31, amagnificent galaxy two million light years off that is readilyvisible to the naked eye. Simply go toMirach (Beta Andromedae), the center star ofthe main stream that comes off the northeast corner of theGreat Square of Pegasus, then gonorthwest two stars -- the top one is Nu; M 31 lies just to thenorthwest of it. Nu And is also a very close double star made upof a luminous class B (B5) dwarf and a much fainter star that atclass F8 (as best we can tell) is not that much different from theSun. The spectrograph shows that the twoorbit quickly, taking a mere 4.28 days to make a full circuit. From its rather large distance of 680 light years, the brighterclass B star shines with the light (allowing for considerableultraviolet radiation from its 15,000 Kelvin surface) of nearly1700 Suns, from which we derive a mass 5.8 times that of the Sun. The detailed characteristics of the little one are not known, butit seems to contain around 1.1 times the solar mass. With thesemasses and the orbital period, the two must be only about 20 solarradii 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 starspins with an equatorial speed of 80 kilometers per second, thesmaller 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 staralways pointing at the other, much like the Moon keeps one facepointing to the Earth. Given the closeness of the two, such tidalsynchrony is just what we would expect. At an age of roughly 80million years, the B star is just about to give up its internalhydrogen fusion, if it has not done so already, and is preparing toswell to become a huge giant star. And here is where the mysterycomes in. As it grows, it will encounter the little class Fcompanion. No one is able to predict quite what will happen. Masswill surely transfer to the F star, which will puff up, theencounter severely altering the evolution of the B star. On theother hand, the B star, being by far the dominant, may eventuallyabsorb the F star into itself, leaving one less solar type star inthe Galaxy. Only time, or a better understanding of how closedoubles develop and evolve, will tell. Nu And is also considered tobe a marginal "runaway star," one that has been kicked away fromanother companion at relatively high velocity, suggesting that itmay once have been in a more complicated system.
[8]ページ先頭