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.