STAR OF THE WEEK: BC CMI (BC Canis Minoris). At the tail end of fifth magnitude (averaging about 6.31), BC CMi, in Canis Minor (Orion's lesser hunting dog), caries neither Greek Letter name nor Flamsteed number, and is best known by its variable-star name. A red class M (M4, though it's been listed as cool as M5-M6) giant, it's also a little different from most that we encounter. BC CMi is listed as an "SRb", or "semi-regular giant" variable star. The SRb's don't vary by all that much and such stars might switch from regular pulsation to irregular. Not well-defined,they may be at the beginning of the process that will make them true Mira (long-period variables that are in the process of ejecting their outer layers eventually to expose their inner now-dead nuclear-burning cores made of carbon and oxygen. BC, which varies roughly between magnitudes 6.14 and 6.42, is faint largely because of its considerable distance of 506 light years from us (give or take 37). Dimming by interstellar dust seems to be of little consequence. Combined with a temperature estimate of 3817 Kelvin, which is needed to assess the amount of infrared radiation, the star radiates with a luminosity of 457 Suns, which leads to a radius of 58 times that of our Sun, or 0.27 Astronomical Units, about 70 percent the size of Mercury's orbit, not all that much for a giant star. Most interesting, BC CMi seems to be multi-periodic. With a variation period originally given as 35 days, one study suggests 123, 143, and 208 days all going on at the same time. The star does not quite belong here, but is more a member of the ancient inner halo of the Galaxy, as shown by its high velocity of 96 kilometers per second relative to the Sun, the motion dominantly toward us. And ancient the star may be, as its mass seems to be only 80 percent of so that of the Sun, which is near the lower limit required to maintain full nuclear fusion in its core. Since stellar aging goes inversely as the mass, the star might have been around for a long time as well. BC CMi radiates a bit of excess infrared light, suggesting a shell or ring of dusty gas that is being ejected as the star evolves. The star is not far enough along its evolutionary path to dredge up freshly-made elements, as the search for unstable technetium, which is the marker of such a phenomenon, must all be made in stars.
Written byJim Kaler 04/21/17. Return to STARS.