Nu Draconis

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Nu Draconis (ν Draconis), also sometimes called Kuma, is the faintest star that makes up the skewed box of Draco's (the Dragon's) head. It is a favorite of amateur astronomers because it is one of the most-easily discerned 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.