16 and 17 DRA (16 and 17 Draconis), two for one of an unusual
related duo. Naked eye double
stars, like Mizar and Alcor in Ursa Major,
are rather rare. Most that look like pairings are just chance
coincidences on the sky, one star much farther than the other. But
here is a close-to real one, an oddity whose components (like the
one in the Bear's Tail) carry different common names, from west to
east Flamsteed's 16 and 17
Draconis, which lie within Draco about
10 degrees due west of the Dragon's head. Separated by 90 seconds
of arc, they are not quite a naked-eye pair, and require at least
binoculars to split them. Close to the same apparent brightness,
fifth magnitude (5.03) 17 Dra slightly outranks just-barely sixth
magnitude (5.53) 16 Dra, a class B (B9.5) hydrogen-fusing dwarf.
But that is a bit of a cheat, because 17 is ALSO double, made of a
fifth magnitude (5.38) class B9 dwarf that closely matches 16 Dra
and a sixth magnitude (6.42) class A (A1) dwarf just 3.3 seconds of
arc apart (Smythe and Chambers in the nineteenth century oddly
calling them "pale yellow and faint lilac").
Parallax
measures put 16 Dra 427 light years away, 17 Dra 412 light years,
which, given the uncertainties, effectively place the stars at the
same distance. Moreover, the motions are closely similar. Since
they are a true double, we adopt an average distance of 420 light
years. As a true trio, the 17 Dra pair is called "A" and "B,"
while 16 is referred to as the "C" component.
With respective temperatures of 10,500 and (estimated) 9400 Kelvin,
17 Dra A and B shine with the light of 132 and 40 Suns, which lead to radii of 3.4 and 2.4 times
the solar value. Both are fast spinners, A and B with projected
equatorial values of 217 and 231 kilometers per second, which lead
to rotation periods under 0.8 and 0.5 days, the masses (from
structure and evolutionary theory) coming in at 3.1 and 2.4 Suns.
Separated by at least 425 Astronomical Units, from Kepler's Laws, they must take at least 3800 years to orbit
each other. Now to 16 Dra, the outlying "C" member. Pretty much
a clone of 17 Dra A, with slightly less radiance, it shines at 112
times the solar rate, the radius 3.2 times solar. At 77 km/s
rotating somewhat slower, 16 Dra takes under 2.1 days to make a
full turn. A mass of 3.0 Suns leads to a system age of around 250
million years, well short of the 350 million hydrogen-fusing
lifetime.
Next for the "kicker," as 16 Draconis is ALSO a double star, one
quite different from 17. The companion has never actually been
seen. But we know it's there from satellite observations that
reveal a powerful energetic ultraviolet and X-ray signature, one
that cannot be produced by an ordinary class B9-A0 dwarf. The
conclusion is that the companion must be a hot white dwarf, which though
dim to the eye (probably around magnitude 15-16), is radiant at
high energies. It most likely has a temperature in the
neighborhood of 30,000 Kelvin. To have died first, it must have
evolved from a mass greater than 3.1 solar, and to match the age of
the system, more like 3.7, which would have made it a B6 dwarf at
birth. The white dwarf mass (stars vigorously losing mass as they
age) would now be in the neighborhood of 0.7-0.75 times that of the
Sun. All that makes 16-17 Dra a quadruple star, one that at one
time must have appeared like a higher-temperature version of the
famed double-double, Epsilon Lyrae.
About two minutes of arc away is another "companion," 11th
magnitude "D," which, from its motion, is clearly just in the line
of sight. From their separations, 16 and 17 Draconis are at least
11,500 AU apart, which, with total mass now known, gives an orbital
period of at least 38,000 years. From 16 Dra, the 17 Dra pair
would appear a couple degrees apart, the brighter one shining with
the light of a gibbous Moon. (Inspired by "Binocular Showpieces
for Light-Polluted Skies," by Hugh Bartlett, which appeared in the
September 2010 edition of Sky and Telescope.)
Written by Jim Kaler 8/20/10. Return to STARS.