THETA CAR (Theta Carinae). Among the richest of constellations is
great Argo, the Ship. And no wonder,
given its huge size and location in the Milky Way. The big boat begins at 11 degrees south of
the celestial equator and runs all the
way down to 76 degrees south, sprawling over nearly a third of the
sky. And thus no wonder that for convenience' sake, astronomers
had to break it up into parts: Vela (the
Sails), Puppis (the Stern), and Carina (the Keel). With Carina loaded with
bright stars, third magnitude (2.76) Theta Carinae (too far south
to have a proper name) seems like just one more in a treasury of
stellar jewels, yet is outstanding in a variety of ways. Let us
count such ways. First, it is a distant (440 light years), hot,
blue, massive, class B (B0, the hottest kind), young hydrogen-
fusing dwarf. Oddly for such
a star, the temperature is problematic. A scatter of measures
averages 27,900 Kelvin, while a B0 star should be closer to 31,000.
After a 0.22 magnitude correction for dimming by interstellar dust,
and using the lower temperature, we find a luminosity 16,600 times
that of the Sun, a radius 5.5 times solar
("dwarf" simply meaning hydrogen-fuser), a rotation period (from a
projected equatorial velocity of 177 km/sec) under 1.6 days, and a
mass of 13 Suns. Using the higher temperature (which is probably
more reliable), we get 21,900 solar luminosities, 5.1 solar radii,
under 1.4 days, and 15 solar masses. The high luminosity generates
a wind that blows with a mass-loss rate of 55 billionths of a solar
mass per year (a million times that in the solar wind), while
shocks in the wind generate strong X-rays. Second in our list,
Theta Car's spectrum is labelled as "peculiar" (B0p), which in the
jargon of astronomy means a chemical anomaly as result of lofting
upward of some elements, settling down of others, the effect making
Theta a silicon-rich star. The chemical anomalies probably float
in patches. Rotation then brings them in and out of view, giving
a variation with a period of 2.2 days, which is oddly out of
synchrony (too long) for the rotation velocity, so something is
wrong. Third, and of more significance, Theta is a member, indeed
the chief star, of a bright open
cluster called IC (Index Catalogue) 2602. Alignment of course
is not enough: witness Aldebaran,
which merely lies in front of the Hyades. Theta's membership, however, is assured by
common distance and, most importantly, by common motion through
space. Indeed, Theta Car is one of the brightest cluster stars in
the sky (the brightest being Mirfak,
Alpha Persei of the Alpha Per
cluster). Fourth, and of greatest significance, Theta Car is
in fact TOO bright. Clusters, born with an intact mass sequence of
hydrogen-burners, die from the top down, high mass stars going
first. The top of IC 2602's is considerably less massive than
Theta. From the maximum general mass remaining, the cluster's age
is set at 34 million years. Yet Theta is anomalously dated to be
just a couple million years old. Theta Car is far behind in its
evolution, making it the brightest "blue straggler" in the
sky. It should have begun to die a long time ago, and looks as if
it is lingering, falling behind its cluster mates. For years, a
cluster's blue stragglers (most in
massive globular clusters) were a mystery. Some are merely
unrecognized double stars.
Most, however, have masses that are too high as a result of stellar
mergers, either from direct collisions between stars within a dense
cluster's confines, or mergers of the members of what was
once a double star. Even mass transfer from one member to
another might do it. If all these characteristics are not enough
to make an outstanding star, Theta Car's fate will do it, as it
seems to be nicely above the 10 solar mass cutoff, beyond which
stars are destined to blow up as supernovae, the explosion not
doing IC 2602 any good.
Written by Jim Kaler 4/20/07. Return to STARS.