3 VUL (3 Vulpeculae), the "Observer's Nightmare," or to give the star a Latin name, "Spectatori Error Inextricabilis." How's that for a dim fifth magnitude (5.18) star in an equally dim modern constellation, Vulpecula (the Fox), originally Vulpecula cum Ansere (the Fox and Goose), again showing that curious things can come in small stellar packages. Explanations follow later. The formal numeric name is a Flamsteed number from John Flamsteed's famous seventeenth-century catalogue. On the face of it, 3 Vul is a modestly warm class B (B6) giant (but see below) with a temperature of 14,400 Kelvin and a metallicity about half that of the Sun, which is more or less expected for this kind of star. Situated in the Milky Way at a distance of 400 light years, we might expect substantial interstellar dust absorption, but the line of sight is relatively clear, and the dimming is a mere ten percent or so. From these parameters, we calculate a luminosity 315 times that of the Sun, a radius 2.9 times solar and, from an uncertain projected equatorial rotation velocity of 33 kilometers per second, a rotation period of under 4.4 days, none of which is out of line, especially considering that the rotation axis might be pointed more or less at us, which would give the star the faster rotation expected among B stars. The theory of stellar structure and evolution gives a substantial mass of 4.2 times that of the Sun. And now begins the fun. Theory also shows that the star is not a giant at all, but a relatively young 25 million year old dwarf (the "giant" designation a descriptor of the spectrum, not necessarily of the star). Moreover, it's oddly variable by a couple tenths of a magnitude over multiple periods of 1.029, 1.262, and 1.169 days, all of which beat against each other, making 3 Vul a "slowly-pulsating B" (or "SPB") star in the mode of the prototype, 53 Persei. Moreover still, it's a spectroscopic binary (the companion noted through subtle wavelength -- Doppler -- shifts in the spectrum as the companion moves the star we see back and forth) with a period of 376.7 days. Nothing is known about 3 Vul B. Assuming it to be of low mass, the separation (from Kepler's Laws) is 1.7 Astronomical Units, just a bit more than Mars's distance from the Sun, an eccentricity of 15 percent taking it between 1.5 and 2.0 AU. Spectral analysis gives a minimum separation of 0.25 AU, which combined with the separation from Kepler's Laws gives an axial tilt of 8 degrees to the line of sight. If the star's rotation axis is aligned (which seems reasonable), then the true rotation velocity is 240 kilometers per second (and the period under 0.6 days), more in line with its class, but that is a weak assumption at best. What makes the star "an observer's nightmare" is that the oscillation periods are close to one day, while the orbital period is close to one year, which synchronizes the star with the natural observing cycles such that it's in the same oscillation stage at about the same time every night and at the same orbital stage during every observing season, requiring long intervals of observation to get all the variations. Since there is no Latin term for "nightmare," the proper name given here derives from Vergil's Aeneid, and translates to "an insoluble maze for the observer." ("An Observer's Nightmare" as well as variation and orbital data are from an article in the Astrophysical Journal by R. J. Dukes, Jr., W. R. Kubinec, and A. Kubinec. Thanks to Latin scholar David Bright for his reasoned thoughts and the fine proper name.)
Written by Jim Kaler 6/26/09. Return to STARS.