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DELTA CYG (Delta Cygni).Cygnus, theSwan, flies south along the Milky Way, its tail marked byDeneb (Alpha Cygni), its head byAlbireo (Beta), and its outstretched wingsbyGienah (Epsilon, to the west) andDelta (to the east), Delta the only major star of the constellationwith no proper name, rather odd, since the third magnitude (2.87)star is certainly bright enough to warrant one. Other than itsprominence in a great constellation, Delta has no great claim tofame but one, that of being a triple star. This interesting systemlies at a fairly healthy distance of 171 light years, and consistsof two stars quite close together and one much farther out (thissort of common configuration lending stability). The bright naked-eye star is a class B (B9.5, almost A0 likeVega, the temperature 9800 Kelvin) subgiant near theend of its core hydrogen-fusing lifetime with a luminosity 180times that of theSun, a radius 4.7 solar,and a mass between 3 and 3.3 solar. Like many hot stars, it spinsfairly rapidly, at least 135 kilometers per second at the equator,60 times that of the Sun. The close companion, currently only 2.4seconds of arc away, is a sixth magnitude (6.33) class F (F1) dwarfwith a temperature of 7300 Kelvin, a luminosity 6.2 solar, and amass 1.6 solar. Analysis of the partial orbit so far observedgives a mean separation of 157 Astronomical Units, an eccentricitythat causes the distance to vary from 230 to 84 AU, and a period of780 years. These quantities (from Kepler's third law) conspire togive a total combined mass of 6.4 solar, as opposed to the 4.9solar given by the luminosities, which is clearly the result ofnatural errors in distance and orbit. Much farther away is thefaint third component, a 12th magnitude star that is moving alongwith the others, and thus seems attached to them, its brightnesstelling that it is a class K (K5) dwarf with a mass only about 2/3of the Sun. From the class B star, the class F component wouldappear as a bright star with the light of 275 full Moons, whilefrom the F star, the B star would be hust unresolvable with thehuman eye, and would shine with 7000 full Moons. Off in thedistance, the K star would be about as bright as Deneb is fromhere, while from the K star, the bright pair would appear as twosparkling lights at most 2.5 degrees apart and would respectivelyshine with the light of 15 and just under one full Moons.
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