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.