OMEGA CAS (Omega Cassiopeiae). Beginnings and endings seem to intrigue, and perhaps stars are not so different. Our focus is often on the brighter stars of a constellation, which usually (but hardly always) are tagged with the first Greek letter, Alpha, followed by the Latin possessive of the constellation name. Cassiopeia follows the tradition, with the luminary, Shedar, as Alpha Cas. (Second magnitude, 2.23, it ranks 69th in the sky, though eruptive Gamma Cas can on rare occasions beat it). About five degrees north of Epsilon Cas (Segin, the easternmost star of Cassiopeia's classic "W") lies the fifth magnitude (at 4.99 almost exactly defining fifth) rather anonymous star at end of the alphabet, Omega Cas. Better than it first appears, Omega is a class B (B8, though some make it as B5) giant: but see below. Even with a pretty good distance of 700 light years (give or take 35), we seem to have a clear look at it with little if any dimming by intrvening interstellar dust. Apparent brightness along with distance, plus a temperature of 12,860 Kelvin (to allow for ultraviolet radiation), yield a luminosity 817 times that of the Sun and a radius of 5.8 solar. Theory then provides a mass of between 4.5 and 4.7 Suns depending on the exact state of evolution. In any case, the star is more of an aged dwarf or more likely a subgiant that just gave up hydrogen fusion than it is a real giant, its age around 100 million years. (If the class is as hot as B5, the color tells of 0.3 magnitude of interstellar dust absorption, which drives the luminosity to 1080 solar and the mass upward by a couple tenths of a Sun: such seems unlikely.) A slow equatorial rotation velocity for the class of just 35 kilometers per second gives a rotation period of under 5.3 days. It's probably close to correct, the star's axis nearly vertical to the line of sight. A more slowly moving unstirred stellar atmosphere allows chemical anomalies, Omega Cas obseved as a "helium weak, strontium enhanced" star in which some elements rise through the effects of radiation, while others fall under the force of gravity. Oddly, our positional reference above, Epsilon Cas, is a helium-weak class B star as well. Our "last star" Omega (not counting all those with Flamsteed and other catalogue numbers) also has the distinction of having a close companion (as discovered via Doppler shifts in the spectrum) in a 69.92 day orbit. Assuming low mass, Omega-B must orbit at an average distance of 0.55 Astronomical Units (1.4 times the size of Mercury's orbit), a fairly large eccentricity taking the companion from as far as 0.7 AU to as close as 0.4 (roughly from the orbit of Venus to that of Mercury). The two seem to be all alone, with no other neighbors in sight. Far below the supernova limit of 8 or 9 solar masses, Omega Cas A will eventually slough off its outer layers to produce an ephemeral "planetary nebula" that will be illuminated by the remaining hot core, which in turn will die as a fairly massive white dwarf of just over 0.8 of a solar mass, all showing that seemingly random stars can be just as interesting as those that shine brightly to the naked eye.

Written by Jim Kaler 10/12/12. Return to STARS.