XI-1 CMA (Xi-1 Canis Majoris). In a brilliant constellation like Canis Major, in the Milky
Way with not one but two first magnitude stars (Sirius, actually minus first mag, and Adhara at the end of the list), you run out
of names fast, and to keep going within the Greek alphabet you have to stretch things a
bit, hence the use of attached numbers for stars close together.
Usually listed west to east, the members of such combinations rarely
have anything to do with each other. In the central part of the Big
Dog lies fourth magnitude (4.33) Xi-1 Canis Majoris. A hot blue
class B (B0.5) subgiant
(but see below), it's on the edge of rare class O and as such is fated
to explode. At a distance of 1382 light years (give or take 117),
Xi-1 is three times as far as Xi-2, the latter a class A0 giant just
under a degree to the northeast of Xi-1 but 440 light years away.
Were there any significant interstellar matter in the area,
Xi-1, at a temperature of 27,720 Kelvin, is hot enough to produce
a diffuse nebula. Even along
the whole line of sight there is not very much, as interstellar dust dims Xi-1 by only
0.13 magnitudes. Allowing for a lot of ultraviolet radiation, the star
shines with the light of 35,000 Suns, its
radius 8.1 times solar. For its class, Xi-1 is a slow rotator, the
projected equatorial rotation velocity a mere 27 kilometers per
second, giving it a rotation period that could be as long as 15.2
days. The star could of course be rotating with its rotation pole
more or less pointed toward us, giving a false impression of its spin.
We don't know. But maybe it's enough to produce a magnetic field some 10,000
times that of Earth, if indeed rotation is involved (which seems unlikely),
the field and a shocks in a strong wind somehow making the star
into a powerful pulsing X-ray source. Luminosity and
temperature combined with the theory of stellar structure and
evolution reveal a mass of 15 times that of the Sun, well over the
limit beyond which a stellar core fuses to iron and then collapses,
the resulting energy blowing the rest of the star up as a supernova. But it has a
ways to go. Not a more advanced subgiant as given by its spectral
class, Xi-1 is still a dwarf
that is perhaps three-fourths or so the way through its
hydrogen-fusing lifetime of 12 million years. Such dichotomies in
class are common among the B stars and mean little. Like many stars
of its kind, X-1 is a subtle "Beta Cephei" variable. Such stars
commonly oscillate in brightness by a few hundredths of a magnitude
with multiple periods of under a day. Xi-1 CMa is a rather unusual
monoperiodic variable that jitters by just 0.03 magnitudes (well
under that detectable by eye alone) over a period of 5.03 hours. The
oscillations are somehow related to the X-ray pulses. Xi-1
CMa is accompanied by two "companions." The motion of
14th magnitude Xi-1 B relative to the B0.5 dwarf 27 seconds of arc
away shows "B" to be just a line-of-sight coincidence. A similar
14th magnitude neighbor 28 seconds of arc away but in a different
direction presents a better case, but it too is probably
coincidental. If not, it would be at least 12,000 Astronomical Units
away and take 300,000 years to make a full circuit of the primary
star. Not even a member of an association of other hot stars, it
seems likely that X-1 CMa will have to go through its remaining
evolution and its eventual destruction all alone.
Written byJim Kaler 3/27/15. Return to STARS.