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SIGMA CYG (SIGMA CYGNI), along with nearby V1334 CYG. How can oneresist a star with such a name, "Sig-Cyg," especially since thisfourth magnitude (4.23) class B (B9) whitesupergiant (a rare class) innorthernCygnus is a gateway to a somewhatdimmer rather mysterious odd-ball of aCepheid pulsator, obscure V1334Cygni (known in some circles as HR 1334). Not that Sig-Cyg is allthat much better understood. The star's main problem (rather ofours with the star) lies in its degree of obscuration byinterstellar dust. Different values are available, ranging from0.38 magnitudes to 0.64, each of which would bring the star to nearthird magnitude were the dust not present (which is hardly possiblein the thickness of Cygnus'sMilky Waycombined with the star's distance of 2760 light years, give or takea hefty 260). Sigma Cyg seems to be embedded in the stuff. Thedegree of interstellar obscuration is usually estimated from howthe dust reddens the starlight. A complicating factor here is anespecially high factor of dimming-to-reddening, which makes thingseven more uncertain. Be that as it may, Sigma Cyg is certainly abit of a winner in the mass and luminosity category. Choosing thelower value of dust absorption, a conservative luminosity from this11,300 Kelvin star lies around 26,500Suns,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 ofMercury's orbit. A projected equatorial rotation speed of 30kilometers per second leads to a spin period under 72 days. Luminosity and temperature then tell of a 15-20 million year oldstar carrying 10 solar masses that is fusing helium in its core, ormaybe even 12 solar if it has no yet quite fired it up. Given ourconservative view, the star becomes a fine candidate to blow up asasupernova. There issome old evidence for a closecompanion in an 11-day orbit.
Locating Sig-Cyg provides a chance to contemplate a rather oddsixth magnitude (5.8 or so)Cepheid variable star calledV1334 Cygni (HR 8157) that lies just over a degree to the south. Directparallax of thisclass F (F1) brightgiantgives a distance of 2160 light years plus or minus 315. It variesby only a couple tenths of a magnitude over 3.33 day period, andrather likePolaris 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 starsroughly 0.1 seconds of arc (65 Astronomical Units) apart withidentical magnitudes of 6.6, one of which is the Cepheid. Spectralvariations then revealed that the Cepheid has another very closeorbiting neighbor in a 5-year orbit. None of the stars, however,are seen individually. Because of a strict relation betweenabsolute brightness and pulsation period, Cepheids are cruciallyimport in establishing the distance scale of the Universe. Furtherobservation of V1334 and of other similar orbiting systems willeventually help measure masses of Cepheids to help us understandthem better. In this case, however, we seem to have a long way togo. Written byJim Kaler 9/03/10. Return toSTARS.
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