p CAR (p Carinae). That's with a lower-case "p;" so as they used to say in the printer's trade, "mind your p's and q's," since they rather look alike. Bayer could not see all of Argo, the Ship, which floats way to the south. In the eighteenth century, Nicolas de Lacaille broke Argo into its three parts of Vela (the Sails), Puppis (the Stern), and Carina (the Keel), in which p and q Carinae are prominent parts of the constellation's outline. After he distributed the Greek letters across them, he continued with lower case Roman letters within each, so Carina has all four, as do Puppis and Vela. At bright third magnitude (3.32), p Car is the brightest all 12, with q Car (magnitude 3.40) right behind it. However, they are quite different and provide a distinct color contrast, p Car a blue-white class B (B4) dwarf, q Car an orange class K giant. Were it not for some intervening interstellar dust, "p" would shine even brighter, at magnitude 2.97. A distance of 483 light years (give or take 29) and a high temperature of 18,100 Kelvin (consistent with the spectral class and needed to account for a lot of ultraviolet radiation) give a total luminosity of 5250 times that of the Sun and a radius of 7.4 times solar. The theory of stellar structure suggests a mass of between 7.1 and 7.6 times that of the Sun, depending on the exact state of evolution. The star is nearing the end of its hydrogen-fusing dwarf lifetime, and as a possible subgiant may have done so already. It's clearly below the limit above which stars explode as supernovae, and will die as a massive white dwarf.

Other than its mass, the outstanding feature of bright, though neglected, p Carinae is its high projected equatorial rotation speed of 303 kilometers per second. Consistently, p Car is a "Be" star, the "e" for emissions (mostly from hydrogen) in its spectrum, which implies a surrounding equatorial disk, the best known of which are Gamma Cassiopeiae and Zeta Tauri. The rotation axis cannot be tilted too much, since p Car is also a "shell star," which implies that the starlight is shining through the disk. A suggested tilt against the line of sight of 67 degrees would increase the rotation speed to 329 kilometers per second, which gives a rotation period of just over a day. But there is nobody there to see the fine sight, as no companion has ever been detected. Nor does the lonely star, seemingly surrounded by so many others in the Milky Way, seem to be a member of any group with a common birthplace.

Written by Jim Kaler 2/21/14. Return to STARS.