NU PER (Nu Persei). Perseus is so filled with bright blue stars that we might easily miss a modest fourth magnitude (but bright fourth: 3.77) yellow-white one lying amidst the Hero's starry streams. But there it is, Nu Persei, a bit over three degrees northwest of Epsilon Per and roughly a third of the way between Epsilon and the constellation's luminary, Mirfak (Alpha). A class F (F5) bright giants (some say even supergiant), Nu Per is in a rather special stage of life. First, though, with a modest temperature of 6600 Kelvin, a distance of 556 light years (plus or minus 16), and no interstellar dust absorption, the star if found to radiate at a rate of 711 times that of the Sun, well below what we would expect for a supergiant but fine for bright giants. Its rather rapid (for a giant) equatorial rotation speed of at least 47 kilometers per second coupled to a radius of 21 times solar gives it a rotation period of under 22 days, similar to that of the Sun. Theory then tells of a star with a mass of 4.5 Suns that has recently given up core hydrogen fusion and is now making the rapid trek to real gianthood when it will begin to fuse its dead helium core into carbon and oxygen. Some 120 million years ago, Nu was born as a hot, blue class B (around B5) dwarf, which would have made it fit right into its constellation: had Perseus actually existed all that long ago, its stars, like all others, in constant motion. Such an origin nicely explains the current rapid rotation in that class B dwarfs have a well-earned reputation for fast spins, the expansion of Nu Per not yet enough to slow its rotation by all that much.

Watching all this happening is a possible binary companion. At a stable separation of 32 seconds of arc from Nu Per proper lies 12th magnitude Nu Per B, which is clearly tracking Nu Per A through space. Its absolute brightness makes it a G2 dwarf quite like the Sun. And at a separation of 7.7 seconds from Nu-B hovers 14th magnitude Nu Per E, which (were the linkage real) would be a class K8 or so dwarf with a mass just over half solar. Assuming that Nu B and E are a true pair (highly suspect), they are at least 1300 Astronomical Units apart, and from Kepler's laws would take at least 38,000 years to orbit each other. Nu B and A, almost certainly a real pairing, are separated by a minimum of 5400 AU and would (given that B and E are really together) take 160,000 years to make a circuit (rising to 170,000 were E a line-of-sight "optical" coincidence). From Nu A, the B-E binary would then be some 14 degrees apart. For the sake of it, were solar-type Nu B to have a populated planet (and the stars to be at the stated distances apart), E would shine with the light of some dozen Venus's, while more distant Nu A would glow like a dozen full Moons. So what happened to Nu C and D? Both faint, both moving too fast relative to Nu A, they are probably just random stars that happen to be in the way.
Written by Jim Kaler 3/11/11. Return to STARS.