DENEB AL OKAB BOREALIS (Epsilon Aquilae). Two stars mark the tail
feathers of Aquila, the celestial
Eagle, Zeta and close-by, but fainter, Epsilon Aquilae, the pair
known collectively by one name, Deneb al Okab, the "tail of the
eagle." Separately we might call them Deneb al Okab Australis (the one to the
south, which Bayer named Zeta) and Deneb al Okab Borealis (the one
to the north), which was given the letter "Epsilon" even though at
mid-fourth magnitude (4.02) it is rather faint. Though paired in
mythology, the two have nothing to do with each other. Zeta, at a
distance of 83 light years, is rather nearby, while Epsilon is
almost twice as far away, 154 light years. Epsilon is an ill-
studied multiple star, at least so it would seem. The bright
naked-eye star is a cool (4720 Kelvin) class K (K1) rather ordinary
giant that shines 66 times more brightly than the Sun. Temperature and luminosity lead to a
radius 12 times solar, not all that large, while direct measure of
angular diameter gives 10 solar, which given the errors involved is
good agreement. "Okab-north" weighs in at 2.5 times the mass of
the Sun, and is a good example of a "clump" helium-fusing giant,
one of many that share its temperature and luminosity. It stands
out somewhat in that it is rich in the cyanogen molecule, and has
a reputation as a very mild barium star, one somewhat enriched in
that element. All such stars are binaries, enriched by the
evolution and mass loss of a companion that is now a white dwarf.
Epsilon Aquilae does seem to have a close companion that orbits
with a 1270-day period, which would (guessing at the companions's
mass) give an orbital size of 3.5 Astronomical Units. No one knows
what kind of star it is. One or two 10th magnitude companions are
much farther out, one 145 seconds of arc away, which translates to
a distance of at least 3700 Astronomical Units and a period of at
least 110,000 years. If a true companion, it is a cool class K
dwarf. The other's separation is not clear. One source gives 66
seconds of arc, another 131 seconds. It might well be a line of
sight coincidence. If nothing else, the system shows that we still
have a long way to go in cataloguing all the characteristics of
even the naked-eye stars.