KAPPA CMA (Kappa Canis Majoris). Canis
Major has not one, but two "first magnitude" stars, with Sirius (actually minus first magnitude,
nearly minus second) at the top of the list, Adhara
(Epsilon) at the bottom and at the edge of second magnitude. (The
other constellations with two first
magnitude stars are Crux, Centaurus, and
Orion. Oddly, the four come in pairs,
Crux and Centaurus next to one another,
Orion and Canis Major nearly so.)
Adding in the rest of the stars that make Canis Major's fine figure
would seem to leave little room for attention. That is hardly the
case. Near Canis Major's southern boundary (the second-most
southerly star with a Greek letter, just
barely 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 with emissions of hydrogen in its
spectrum that tell of a surrounding disk, one thick enough to the
line of sight to make Kappa CMa into a "shell star.") Between 1963
and 1978, Kappa increased in brightness by some 50 percent, going
from magnitude 3.96 or so to near its current value of 3.52 (making
it nearly third magnitude), the color also becoming a bit redder.
Such behavior is common among Be stars, as witnessed by Gamma Cassiopeiae and Delta Scorpii. From the star's current
visual brightness, its measured distance of 659 light years (give
or take just 16), a 0.8 magnitude dimming by interstellar and
circumstellar dust (as found from the star's color), and a
temperature of 24,200 Kelvin (needed to account for a lot of ultraviolet radiation), we
derive a great luminosity of 22,700 Suns and
a radius of 8.6 times solar. The theory of stellar structure and
evolution then gives Kappa CMa an age of about 15 million years, a
mass of 12 times that of the Sun, and shows that if it is not a
true subgiant (one whose core hydrogen has just run out or will
soon do so), it is close. The problems are that we are dealing
with stellar instability and are observing a combination of light
from the disk and the buried star, so the derived luminosity and
mass would probably be upper limits. If we use the pre-brightening
magnitude and a lower measured inter/circumstellar dimming, the
luminosity drops to 10,700 Suns and the mass to 10.5 times solar.
Be stars seem to be related to high rotation velocity and rapid
stellar oscillation, but no one knows for sure. Kappa CMa surely
fits right in. The projected equatorial rotation velocity may be
as high as 244 kilometers per second. But observation of the disk
shows its axis to be tilted by 35 degrees to the line of sight, so
the true rotation speed could then be as high as 425 kilometers per
second, 80 percent that needed for the star to fling itself apart.
Kappa CMa would then have to be at least ellipsoidal, which is
another factor in compromising any of the stellar parameters,
including the adopted temperature (which would be highest at the
poles, lowest at the equator). Despite all the uncertainties and
the changes in stellar brightness (which of course are what make
the star fun to study), Kappa CMa seems at least to be at or above
the limit at which stars eventually explode as supernovae, though a final
massive neon white dwarf
can't be ruled out. Having no known companions, Kappa will have to
face its fate all alone.
Written by Jim Kaler 3/1/13. Return to STARS.