SIGMA CAS (Sigma Cassiopeiae). Only a bit over a degree south- southeast of Rho Cassiopeiae, one of the magnificent stars of the local part of the Galaxy, fifth magnitude Sigma Cas (4.86) is pretty much ignored. As a neat visual double, which pumps up its brightness, the star is well worth a look. Moreover, both the components are nicely blue-white, Sigma A a fifth magnitude (4.99) B1 dwarf (but see below), Sigma B, a B3 dwarf at fainter 7th magnitude (7.24). Separated by 3 seconds of arc, the pair well shows the effect that just two subclasses has on luminosity. In the nineteenth century, Admiral Smythe (with G. F. Chambers) waxed enthusiastically about them: "A beautiful double star on the lady's left elbow...'A' 6 (magnitude), flushed white, 'B' 8, smalt blue; the colors are clear and distinct..." ("smalt" an old coloring agent made of blue glass), adding that they are a miniature Epsilon Bootis, though with "less-fine" colors. In fact, "A" is actually bluer than "B," their description the result of a visual illusion caused by magnitude difference and proximity.

Correction for a bit of dimming by interstellar dust lowers the magnitudes to 4.41 and 6.66, for a combined 4.28. Now things get tricky. The new Hipparcos satellite reduction gives a distance of 4200 light years and absolute brightnesses that are way out of line with the spectral classes. We get far better results, consistent with class, with the OLD value of 1520 (give or take 500!), so we will use that. With respective temperatures of 21,500 and 18,500 Kelvin (the latter an estimate), Sigma A and B shine with luminosities of 21,900 and 1835 Suns, giving them radii of 10 and 4 times solar. Spinning with an equatorial speed of at least 182 kilometers per second, Sigma A makes a turn in under 2.7 days, typical for its class; the rotation of Sig-B is not known. Masses are substantial. Sigma Cas A carries between 11 and 12 times that of the Sun, depending on the exact state of evolution. In any case, it is at or near the end of hydrogen fusion and might be called a subgiant (and has in fact been alternatively classed as a B2 giant). The star's age lies between 14 and 17 million years, which is consistent with Sig-B's status as a 6.5 solar mass middle-aged dwarf. Adding a bit more luster to the system, Sigma Cas A (consistent with its class) is suspected of being a rapid oscillator in the form of a Beta Cephei star, varying slightly with simultaneous periods of 8 and 4.5 hours. Given the stars' (albeit uncertain) distance and angular separation, they would have to be at least 1500 Astronomical Units apart, and given their masses (and Kepler's Laws) would take at least 14,000 years to orbit each other (rendering the determination of an actual orbit impossible). At a separation of 106 seconds of arc lies 10th magnitude Sigma Cas C, but it is almost certainly a line-of-sigh coincidence. Adding a bit more, something of a curiosity, Sigma Cas is just half a minute of arc to the west of the equinoctial colure, the great circle in the sky that connects the celestial poles with the equinoxes. Precession -- the 26,000- year wobble in the Earth's axis -- will carry the star across to the other side around the year 2019.
Written by Jim Kaler 9/10/10. Return to STARS.