BETA COM (Beta Comae Berenices). The naked-eye sky is dominated by
luminous stars, stars that are far brighter than the Sun. Only a few stars like the Sun and fainter
sneak through. They simply do not have enough radiative power to
be visible unless they are quite close to us. ("Selection effects"
like this one, in which Nature shows us "what she wishes," pervade
science: the population of microbes far exceeds that of elephants,
yet only the elephants are visible without some kind of aid.) It
is then quite surprising to find the luminary of a constellation to
be a near-solar clone. Fourth magnitude (4.26) Beta Com, with no
proper name at all, just barely beats out Diadem (Alpha Comae Berenices) for the honor
(such as it is) of being the brightest star within the faint but
glorious constellation Coma Berenices
(Berenices Hair). Though within the formal constellation
boundaries, Beta Com is not a part of the star cluster that makes
the constellation's heart, its distance of only 30 light years
placing it 1/9 as far as the cluster and half as far as Diadem. At
6000 Kelvin, this class G (G0, alternatively classed as F9.5) star
is only slightly warmer than our class G2 (5780 Kelvin)
Sun. A
hydrogen-fusing dwarf like the Sun, Beta Com is only 37 percent
more luminous than is the Sun and but 10 percent larger, the result
of 10 percent greater mass. There is some suggestion that the
star might have a close companion (detectable only via
spectrograph), though such a neighbor is unconfirmed and probably
unlikely. How sunlike is Beta Com? It is a bit metal-rich,
containing perhaps 7 percent more iron (relative to dominant
hydrogen) than the Sun. No planets have yet been spectroscopically
detected as they have for several similar stars. A search for a
residual dusty disk (one left over from planet formation) around
the star has also turned up nothing. Yet the spectrum reveals very
sunlike magnetic activity, implying starspots, flares, and all the
other trappings of the solar magnetic engine. Indeed, with a
rotation period only half that of the Sun, Beta Com is probably
more active than the Sun (rotation and up and down convection in
the outer stellar layers producing the magnetism). Like the Sun,
Beta Com also has a long-term magnetic activity cycle of 16.6
years, about 50 percent longer than the famed 11-year solar cycle.
(It may also have a secondary 9.6 year cycle.) If you want to see
something of what we would look like at a distance, look to Coma
Berenice's brightest star.