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.