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.