MIAPLACIDUS (Beta Carinae). In spite of intense study, some stars refuse to yield their true characters. Here is the opposite, one that seems to be about as completely understood as reasonably possible. One of the few stars of the far southern hemisphere to carry a proper name, Miaplacidus -- Beta Carinae in Carina, the Keel of the Ship Argo -- is also one of the brightest of the sky. Almost first magnitude (bright second, 1.68), it ranks 28th in the list of bright stars, and in its constellation is beaten out only by brilliant Canopus, which ranks second. It and Atria (in Triangulum Australe) are in addition the two brightest stars closest to the South Celestial Pole, each offset from it by about 20 degrees, both circumpolar (never setting) from latitudes below about 20 degrees south (and sadly invisible from latitudes above 20 degrees north). While the root of the name means "placid" or "gentle," the word is not well understood, some references citing "gentle waters" (appropriate for Argo), others stating "unknown." In any case, the name is of modern origin, since the ancients had no way of seeing this deep southern star. Physically, Beta Car is a relatively common, white class A (A2) subdwarf, one (from its class alone) that has just given up core hydrogen fusion. At a distance of 111 light years, with a temperature that from various sources averages 9100 Kelvin, Miaplacidus shines with the light of 210 Suns, which yields a radius 5.85 times that of the Sun. A direct measure of angular diameter (1.59 thousandths of a second of arc) together with the distance gives a radius of 5.83 solar, showing that all the parameters must be correct. A rapid rotation velocity of at least 139 kilometers per second gives a rotation period under 2.1 days. The theory of stellar structure and evolution shows the star to have a mass between 3 and 3.1 times solar, and that in accord with the spectral class, at an age of 350 million years, it is indeed giving up core hydrogen fusion and instead has a core of nearly pure helium. Helium fusion to carbon will begin in a mere 2.5 million years, after the star becomes a red giant. While a number of class A stars are surrounded by dusty disks that imply planet formation has taken place, Miaplacidus seems devoid of such. Indeed, there is no evidence for any kind of companion at all. The only mystery we are left with is the name.
Written by Jim Kaler. Return to STARS.