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.)