POLARIS GALACTICUS BOREALIS (31 Comae Berenices). Three great circles divide the sky: the celestial equator, the ecliptic (the apparent solar path), and the Galactic equator (the center line of the Milky Way). Each has north and south poles that lie perpendicular to the circle. The celestial poles (those of the celestial equator) are marked by famed Polaris in the north and by much fainter Sigma Octantis in the south (known occasionally as Polaris Australis). So why not mark the others too, such that we can find them easily in the sky? Sixth magnitude is too faint. Set the criterion as the closest star at fifth or brighter, consistent with Sigma Oct. The north pole of the Galaxy then lies only 0.4 degree from 31 Comae Berenices, in Coma Berenices (a modern constellation that holds the lacy star cluster of the same name, which since ancient times has been called "Berenices Hair"). Though known mostly by its Flamsteed designation, 31 Comae Berenices, it thus clearly deserves a new name, one made up here from Latin, "Polaris Galacticus Borealis," the "Polaris of the North Galactic Pole." The south Galactic pole star is then Alpha Sculptoris ("Polaris Galacticus Australis," which, nearly 3 degrees from that pole, is nowhere near as good). The beauty of these two markers is that they show where the Galactic view is clearest, the direction with the least amount of Milky Way dust, where the lack of obscuration reveals a myriad of myriads of external galaxies. Easy to find, 31 Com is a mid- fifth magnitude (4.94) rather unusual class G (G0) giant with an uncertain surface temperature of 5600 Kelvin. From its measured distance of 307 light years (uncertain to about 8 percent), we derive a modest luminosity (for a giant) of 82 times that of the Sun, a radius 9.7 times solar, and (from the theory of stellar structure and evolution) a mass of 2.55 solar. The star appears to be in a state of rapid transition, swelling and cooling with a dead helium core toward red gianthood, which will ultimately make it nearly 1000 times brighter than the Sun. (It is crossing the "Hertzsprung gap" of temperature and luminosity.) Starting life at the cool end of class B, where stars tend to rotate quickly, "Pol Gal Bor" is still rotating unusually fast (rotation slows with expansion), with an equatorial speed of at least 67 kilometers per second, giving a rotation period under 7.3 days. As a result (rotation and atmospheric convection producing magnetic fields), 31 Comae is surrounded by an extended magnetically heated corona that radiates X-rays. Although set off rather well to the side of the main Coma Berenices cluster and just a bit farther than the average distance of 293 light years, 31 Comae is still considered to be part of the assembly, its age of about 500 million years that of the cluster. The star thus gives a fine chance to admire not just a striking stellar gathering, but to examine the structure of the Galaxy itself. (Thanks to Latin scholar David Bright.)
Written by Jim Kaler. Return to STARS.