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KAPPA CMA (Kappa Canis Majoris).CanisMajor has not one, but two "first magnitude" stars, withSirius (actually minus first magnitude,nearly minus second) at the top of thelist,Adhara(Epsilon) at the bottom and at the edge of second magnitude. (Theotherconstellations with two firstmagnitude stars areCrux, Centaurus, andOrion. Oddly, the four come in pairs,Crux andCentaurus next to one another,Orion andCanis Major nearly so.) Adding in the rest of the stars that make Canis Major's fine figurewould seem to leave little room for attention. That is hardly thecase. Near Canis Major's southern boundary (the second-mostsoutherly star with aGreek letter, justbarely beaten out for the honor by Lambda CMa), lies a hot, blue,fourth magnitude class B (B1.5) subgiant that is also a classical"Be" or "B-emission" line star (one withemissions of hydrogen in itsspectrum that tell of a surrounding disk, one thick enough to theline of sight to make Kappa CMa into a "shell star.") Between 1963and 1978, Kappa increased in brightness by some 50 percent, goingfrom magnitude 3.96 or so to near its current value of 3.52 (makingit nearly third magnitude), the color also becoming a bit redder. Such behavior is common among Be stars, as witnessed byGamma Cassiopeiae andDelta Scorpii. From the star's currentvisual brightness, its measured distance of 659 light years (giveor take just 16), a 0.8 magnitude dimming by interstellar andcircumstellar dust (as found from the star's color), and atemperature of 24,200 Kelvin (needed to account for a lot ofultraviolet radiation), wederive a great luminosity of 22,700Suns anda radius of 8.6 times solar. The theory of stellar structure andevolution then gives Kappa CMa an age of about 15 million years, amass of 12 times that of the Sun, and shows that if it is not atrue subgiant (one whose core hydrogen has just run out or willsoon do so), it is close. The problems are that we are dealingwith stellar instability and are observing a combination of lightfrom the disk and the buried star, so the derived luminosity andmass would probably be upper limits. If we use the pre-brighteningmagnitude and a lower measured inter/circumstellar dimming, theluminosity drops to 10,700 Suns and the mass to 10.5 times solar. Be stars seem to be related to high rotation velocity and rapidstellar oscillation, but no one knows for sure. Kappa CMa surelyfits right in. The projected equatorial rotation velocity may beas high as 244 kilometers per second. But observation of the diskshows its axis to be tilted by 35 degrees to the line of sight, sothe true rotation speed could then be as high as 425 kilometers persecond, 80 percent that needed for the star to fling itself apart. Kappa CMa would then have to be at least ellipsoidal, which isanother factor in compromising any of the stellar parameters,including the adopted temperature (which would be highest at thepoles, lowest at the equator). Despite all the uncertainties andthe changes in stellar brightness (which of course are what makethe star fun to study), Kappa CMa seems at least to be at or abovethe limit at which stars eventually explode assupernovae, though a finalmassive neonwhite dwarfcan't be ruled out. Having no knowncompanions, Kappa will have toface its fate all alone.
Written byJim Kaler 3/1/13. Return toSTARS.
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