KUMA (Nu Draconis). The faintest star that makes the skewed box of
Draco's (the Dragon's) head is one of
the favorite, and most-easily seen, double stars of the northern
sky. Even steadily-held binoculars will split it into two nearly
identical white stars, the two separated by a large 62 seconds of
arc. The proper and un-traditional name "Kuma" is of obscure
meaning, one source suggesting "at last," but why is anybody's
guess. The star is far better known by its Greek letter name Nu
Draconis, the western one of the pair Nu-1, the eastern one Nu-2
(the numbers, when applied, always going east to west in order of
increasing right ascension, the "longitude" of the sky).
Individually of the fifth magnitude, they combine to produce a
naked-eye star of magnitude four (4.13). The most recent measures
give Nu-1 the slight edge (4.87) over Nu-2 (4.89), a difference of
only two percent. Both are white class A. Nu-1 is classed as an
ordinary (hydrogen-fusing) A6 dwarf that should have a temperature
of 8000 Kelvin, while Nu-2 is classed a bit warmer, as an A4 dwarf.
Nu-2, however, is also a "metallic-line" star (meaning that its
spectrum is rich in metal absorptions), and is actually cooler than
its class would suggest, coming in at a measured 7350 Kelvin.
Small differences aside, both shine with about 9 times the
luminosity of the Sun and have masses 1.7
times solar. The biggest difference is that Nu-2 itself is double,
with a close and probably low-mass companion that circles it every
38.6 days. The metallic-line phenomenon is caused by the diffusion
of chemical elements in a relatively quiet stellar surface, and is
related to relatively slow rotation rates. This kind of
metallicity is also related to binarity, the companion apparently
helping to slow the spin, Nu-2 rotating at a minimum of 50
kilometers per second at the equator. (While 25 times that of the
Sun, that is still not that fast for hotter stars.) Though Nu-1
rotates only 66 kilometers per second, it appears more normal. (It
is suspected of being double, but that is not confirmed.) Nu-1's
little companion orbits at only about 0.2 Astronomical Units from
its brighter host. Nu-1 and Nu-2 on the other hand are separated
by at least 1900 Astronomical Units and take at least 44,000 years
to make a full circuit. From each, the other would shine as three
full Moons, while from Nu-1, the Nu-2 pair would be separated by
only about 20 seconds of arc.