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.