SIGMA CYG (SIGMA CYGNI), along with nearby V1334 CYG. How can one
resist a star with such a name, "Sig-Cyg," especially since this
fourth magnitude (4.23) class B (B9) white supergiant (a rare class) in
northern Cygnus is a gateway to a somewhat
dimmer rather mysterious odd-ball of a Cepheid pulsator, obscure V1334
Cygni (known in some circles as HR 1334). Not that Sig-Cyg is all
that much better understood. The star's main problem (rather of
ours with the star) lies in its degree of obscuration by
interstellar dust. Different values are available, ranging from
0.38 magnitudes to 0.64, each of which would bring the star to near
third magnitude were the dust not present (which is hardly possible
in the thickness of Cygnus's Milky Way
combined with the star's distance of 2760 light years, give or take
a hefty 260). Sigma Cyg seems to be embedded in the stuff. The
degree of interstellar obscuration is usually estimated from how
the dust reddens the starlight. A complicating factor here is an
especially high factor of dimming-to-reddening, which makes things
even more uncertain. Be that as it may, Sigma Cyg is certainly a
bit of a winner in the mass and luminosity category. Choosing the
lower value of dust absorption, a conservative luminosity from this
11,300 Kelvin star lies around 26,500 Suns,
the warm supergiant's radius coming in at 43 times that of the Sun,
or about 0.2 Astronomical Units, just over half the size of
Mercury's orbit. A projected equatorial rotation speed of 30
kilometers per second leads to a spin period under 72 days.
Luminosity and temperature then tell of a 15-20 million year old
star carrying 10 solar masses that is fusing helium in its core, or
maybe even 12 solar if it has no yet quite fired it up. Given our
conservative view, the star becomes a fine candidate to blow up as
a supernova. There is
some old evidence for a close companion in an 11-day orbit.
Locating Sig-Cyg provides a chance to contemplate a rather odd
sixth magnitude (5.8 or so) Cepheid variable star called
V1334 Cygni (HR 8157) that lies just over a degree to the south.
Direct parallax of this
class F (F1) bright giant
gives a distance of 2160 light years plus or minus 315. It varies
by only a couple tenths of a magnitude over 3.33 day period, and
rather like Polaris is vibrating in its
"first overtone," a harmonic to its natural fundamental period.
It's also a curious triple that no one knows a great deal about.
From best estimates it seemed at first to consist of two stars
roughly 0.1 seconds of arc (65 Astronomical Units) apart with
identical magnitudes of 6.6, one of which is the Cepheid. Spectral
variations then revealed that the Cepheid has another very close
orbiting neighbor in a 5-year orbit. None of the stars, however,
are seen individually. Because of a strict relation between
absolute brightness and pulsation period, Cepheids are crucially
import in establishing the distance scale of the Universe. Further
observation of V1334 and of other similar orbiting systems will
eventually help measure masses of Cepheids to help us understand
them better. In this case, however, we seem to have a long way to
go.
Written by Jim Kaler 9/03/10. Return to STARS.