KAPPA DRA (Kappa Draconis). Three stars sit in a little row, the
brightest of them -- Kappa Draconis -- the second star in from the
end of the tail of Draco, the Dragon.
At first the three, also known in Flamsteed terms as 4, 5 (Kappa),
and 6 Draconis, might look like some sort of odd triple system.
The alignment, however, is coincidental, as they have nothing to do
with each other. Yet the contrast is nice, as fourth magnitude
(3.88) Kappa is a blue-white class B (B6) giant, whereas the closer
of the two, 6 Dra, is a fifth magnitude (4.95) red class M (M4)
giant. (4 Dra is a bit warmer fifth magnitude K3 giant). Kappa
Dra, which dominates the trio, has a complex spectral class given
as B6 IIIpe. The "B6" places it in the middle of class B, the
"III" means that it is a giant star, the "p" stands for "peculiar,"
and the "e" means there are bright emissions at certain wavelengths
in the spectrum (in this case from hydrogen). In short terms,
Kappa is a "Be star" much like much brighter Gamma Cas, and as such is surrounded by a
rotating bright disk of radiating matter of its own making. No one
quite knows how such disks are created, but they are always
associated with rapidly rotating stars. Moreover, the star is
classed as a "shell star," which implies a thick absorbing disk,
one set more or less along the line of sight. The disk, or shell,
seems to have a long-term variation of 23 years in addition to
other short periods (the disks rather unstable). Kappa Dra shines
to us from a great distance of 500 light years, radiating the light
of 1400 Suns from a 14,000 Kelvin surface.
The combination tells of a radius 6.4 times that of the Sun. The
minimum (since we do not know the axial tilt) equatorial rotation
speed is argued, one source giving as low as 160 kilometers per
second (80 times solar), another 250. If the former, the star
spins in under 2 days (as opposed to 25 days for the Sun), if the
latter in under 1.3 days. The existence of a companion is also
arguable. From spectral variations, there is a suggestion of one
with a period of 0.89 days, which implies an orbital separation of
only 0.03 or so Astronomical Units (50 million km). We might be
fooled, however, as the variations might be the result of rotation,
implying a rotation speed of 360 km/sec. On the other hand, there
is a suggestion that the star varies like Sheliak (Beta Lyrae),
again implying a companion. Clearly, Kappa Dra is under-studied.
Kappa Dra is a hefty 5 solar mass star that has either just ended
its core hydrogen fusing stage, or will do so almost immediately.
Within only half a million years it will make the transition to
becoming a true red giant that is twice as bright as it is now.
While unrelated, the trio near the end of the Dragon's tail are
still pretty close to each other. Ignoring the inevitable errors
of measurement, the red giant 4 Dra is only 50 light years farther
away, 6 Dra only another 30. From 4 Draconis, our Kappa would be
brilliant, and would shine in a planet's sky (were there one) at
magnitude -1.2, only a little dimmer than we see Sirius.