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DENEB AL OKAB BOREALIS (Epsilon Aquilae). Two stars mark the tailfeathers ofAquila, the celestialEagle, Zeta and close-by, but fainter, Epsilon Aquilae, the pairknown collectively by one name, Deneb al Okab, the "tail of theeagle." Separately we might call themDeneb al Okab Australis (the one to thesouth, which Bayer named Zeta) and Deneb al Okab Borealis (the oneto the north), which was given the letter "Epsilon" even though atmid-fourth magnitude (4.02) it is rather faint. Though paired inmythology, the two have nothing to do with each other. Zeta, at adistance of 83 light years, is rather nearby, while Epsilon isalmost twice as far away, 154 light years. Epsilon is an ill-studied multiple star, at least so it would seem. The brightnaked-eye star is a cool (4720 Kelvin) class K (K1) rather ordinarygiant that shines 66 times more brightly than theSun. Temperature and luminosity lead to aradius 12 times solar, not all that large, while direct measure ofangular diameter gives 10 solar, which given the errors involved isgood agreement. "Okab-north" weighs in at 2.5 times the mass ofthe Sun, and is a good example of a "clump" helium-fusing giant,one of many that share its temperature and luminosity. It standsout somewhat in that it is rich in the cyanogen molecule, and hasa reputation as a very mild barium star, one somewhat enriched inthat element. All such stars are binaries, enriched by theevolution and mass loss of a companion that is now a white dwarf. Epsilon Aquilae does seem to have a close companion that orbitswith a 1270-day period, which would (guessing at the companions'smass) give an orbital size of 3.5 Astronomical Units. No one knowswhat kind of star it is. One or two 10th magnitude companions aremuch farther out, one 145 seconds of arc away, which translates toa distance of at least 3700 Astronomical Units and a period of atleast 110,000 years. If a true companion, it is a cool class Kdwarf. The other's separation is not clear. One source gives 66seconds of arc, another 131 seconds. It might well be a line ofsight coincidence. If nothing else, the system shows that we stillhave a long way to go in cataloguing all the characteristics ofeven the naked-eye stars.
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