EPS DRA (Epsilon Draconis). Well, this one is another fine mess, which is a bit surprising given Epsilon Dra's status of nearly third magnitude (3.83, bright fourth) and its position in the main figure of Draco (the Dragon) four degrees northeast of third magnitude Delta Dra. The name that might be applied to it, Al Tinnen (referring to a Serpent), is properly given to Delta but mistakenly as Altais, which refers to a goat. From Allen the name actually relates to the quartet of Delta, Pi, Rho, and Epsilon. It's best just to use the Greek letter. From the 19th century, Smyth and Chambers call Eps Dra "a fine double...A 5 1/2 light yellow, B 9 1/2 pale blue." While the subjective color of Epsilon Dra's brighter fourth magnitude component (4.01) class G (G7) component may not be far from the mark, that of seventh magnitude (6.71) Epsilon B's color is certainly well off it, the "blueness" a contrast effect, as may be the early estimated faintness. Both stars have been called giants, Eps Dra A a G7 giant, Eps B a K5 giant, such giant pairings somewhat unusual. Eps A's characteristics are pretty well defined. With a distance of 148 light years (give or take just 3) and a temperature of 5000 Kelvin (consistent with its class), which requires the addition of a bit of infrared radiation, "A" shines with the light of 60 Suns, which leads to a radius of 10 times solar. Theory shows that the star is most likely a helium-fusing (to carbon and oxygen) "clump" giant (as on as graph of luminosity vs. temperature such stars gang together) with a mass of about 2.5 times that of the Sun (though it could just be starting to brighten with a dead helium core; it's hard to tell) and an age of some 600 million years. But then there is Eps Dra B. Not only is the subjective color and early magnitude way off, but even modern observers are wildly divergent. While "B" was earlier ranked as a K5 giant, a later look suggested it to be an F5 giant, very different. If we adopt the former, then the temperature should be around 4100 Kelvin and the resulting luminosity ought to be 8 Suns, which cannot be matched by theory. If F5, the temperature should be 6400 K, which works, and which yields a luminosity of 3.4 Suns, a radius of 1.5 times solar and a mass of 1.3 Suns, and shows the star to be not a giant but instead a bit of an elderly dwarf. Eps Dra A is also classed as a metal-poor giant with weakened cyanogen (CN) in its spectrum, consistent with a lowered iron content relative to hydrogen, which runs around 45 percent solar. While we can make out some orbital motion, it's not enough to be able to fit an ellipse and to derive the orbital parameters. The first measurement of separation, 2.5 seconds of arc, was made by none other than William Herschel (the discoverer of orbiting doubles) in 1780. By 2012, the separation had increased to 3.2 seconds. The motion against the background in 129 years, however, is 16 seconds of arc, so the stars are surely true companions. They are now at least 145 Astronomical Units apart (the foreshortening not known) and from Kepler's third law and a summed mass of 3.8 times solar must take at least 900 years to orbit each other. They make a pretty pair, one worth a look. Maybe someday someone will actually make a study of them.
Written byJim Kaler 7/29/16. Return to STARS.