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.
Written by Jim Kaler. Return to STARS.