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NODUS SECUNDUS

(Delta Draconis). With a rather unromantic name, Nodus Secundus, Draco the Dragon's Delta star, is less-well-known from Arabic as Al Tais, the Goat. Better to use the Latin "Nodus," as it more refers to its host constellation, where it marks the second (Secundus) of four loops in the Dragon's long winding mythological body. Nicely visible at mid-third magnitude (3.07), and one of the brighter stars of the far north, it lies only a bit more than 20 degrees from the sky's north pole (marked by Polaris), and is circumpolar (always visible) from everywhere north of the Tropic of Cancer. Within its daily circle about the North Celestial Pole, it is exceeded in brightness only by Polaris, Kochab, and Pherkad, the Alpha, Beta, and Gamma stars of Ursa Minor. Physically, Nodus number two is a class G (G9, almost class K) giant star, though since it is a bit warm (4830 Kelvin) for such a giant, not quite so large as many. At a distance of almost exactly a light-century (100 light years), the star radiates at a rate of 63 solar luminosities, giving it a radius 11 times that of the Sun. Its luminosity and temperature combine to yield a mass almost exactly 2.5 times solar and an age of 700 million years. Nodus Secundus is a fine example of a helium-fusing giant, one that is now converting its core helium into carbon and oxygen. Sometime in the astronomically near future, the helium will run out and the star will brighten as it prepares to slough its outer envelope and become a mid-mass white dwarf. It is not clear whether Nodus-2 is a single or double star. Lying 88 seconds of arc away is a faint twelfth magnitude "companion" about which nearly nothing is known. The seeming proximity may be just a line of sight coincidence. Yet the two seem to be regarded as a real double. If so, "Delta Draconis B" is at least 2700 Astronomical Units from the giant. Nearly 70 times Pluto's distance from the Sun, the orbital period would be at least 87,000 years. From a hypothetical planet orbiting the giant, the companion (which from its brightness would be a class M1 dwarf) would shine redly at roughly the brightness of Venus, while from the dwarf the giant would shed the light of a half-dozen full Moons.