10 DRA (10 Draconis = CU Draconis). Among the most famed of stars
is fourth magnitude Thuban, Alpha
Draconis (in Draco, the Dragon), which,
compliments of the 26,000-year precession, or wobble, of the Earth's
axis, was the pole star in and around 2700 BC, during the time of
ancient Egypt. Indeed, there were not one, but TWO pole stars,
though the other one, fifth magnitude (4.64) 10 Draconis (Flamsteed 10, no Greek letter), which lies just 1.4 degrees to
the west of Thuban, is never mentioned alongside its far better
known neighbor. And too bad, as of the two it may be the more
interesting. If not, it certainly is the more colorful, a red
class M (M3.5) giant that
through binoculars or a wide field telescope contrasts very nicely
with white Thuban. Cool (3660 Kelvin), from a distance of 390
light years 10 Dra shines with the light of 840 Suns, though the great majority of the
radiation lies in the invisible infrared part of the spectrum. Typical of class M
giants, it's big, with a radius 72 times that of the Sun, 0.34
Astronomical Units, 87 percent the size of Mercury's orbit. Its
mass and exact state of evolution are hard to gauge, as stars like
this one but with a range of masses are so close in
characteristics. It is most likely a 1.5 to 2 solar mass star that
is either just starting to fire up its helium core to fuse to
carbon and oxygen, or it has already used its internal helium core
fuel and is brightening for the second time in a more advanced
state. Again typical of such stars, it's a slight irregular
variable with about a half-magnitude range. As such, it also goes
by the variable star name CU
Draconis (fitting, since the Star of the Week is written from
Champaign-Urbana). At the same time, the star oscillates much more
subtly over a period of two hours. There is no known rotation
period (such stars very slow rotators), nor are there significant
abundance anomalies, though the iron-to-hydrogen ratio is a bit
low, about 75 percent that of the Sun. A minute to a minute and a
half of arc away lie two "companions," 13th magnitude 10-B and 12th
magnitude 10-C, which relative motions show merely lie along the
line of sight, the star, so far as we know, single.
Written by Jim Kaler 5/15/09. Return to STARS.