DELTA CYG (Delta Cygni). Cygnus, the Swan, flies south along the Milky Way, its tail marked by Deneb (Alpha Cygni), its head by Albireo (Beta), and its outstretched wings by Gienah (Epsilon, to the west) and Delta (to the east), Delta the only major star of the constellation with no proper name, rather odd, since the third magnitude (2.87) star is certainly bright enough to warrant one. Other than its prominence in a great constellation, Delta has no great claim to fame but one, that of being a triple star. This interesting system lies at a fairly healthy distance of 171 light years, and consists of two stars quite close together and one much farther out (this sort of common configuration lending stability). The bright naked- eye star is a class B (B9.5, almost A0 like Vega, the temperature 9800 Kelvin) subgiant near the end of its core hydrogen-fusing lifetime with a luminosity 180 times that of the Sun, a radius 4.7 solar, and a mass between 3 and 3.3 solar. Like many hot stars, it spins fairly rapidly, at least 135 kilometers per second at the equator, 60 times that of the Sun. The close companion, currently only 2.4 seconds of arc away, is a sixth magnitude (6.33) class F (F1) dwarf with a temperature of 7300 Kelvin, a luminosity 6.2 solar, and a mass 1.6 solar. Analysis of the partial orbit so far observed gives a mean separation of 157 Astronomical Units, an eccentricity that causes the distance to vary from 230 to 84 AU, and a period of 780 years. These quantities (from Kepler's third law) conspire to give a total combined mass of 6.4 solar, as opposed to the 4.9 solar given by the luminosities, which is clearly the result of natural errors in distance and orbit. Much farther away is the faint third component, a 12th magnitude star that is moving along with the others, and thus seems attached to them, its brightness telling that it is a class K (K5) dwarf with a mass only about 2/3 of the Sun. From the class B star, the class F component would appear as a bright star with the light of 275 full Moons, while from the F star, the B star would be hust unresolvable with the human eye, and would shine with 7000 full Moons. Off in the distance, the K star would be about as bright as Deneb is from here, while from the K star, the bright pair would appear as two sparkling lights at most 2.5 degrees apart and would respectively shine with the light of 15 and just under one full Moons.
Written by Jim Kaler. Return to STARS.