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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.