RASTABAN (Beta Draconis). The great northern serpent's neck points
southward, Draco the Dragon's two
leading stars looking like two eyes staring at Hercules. The names of both come from the same Arabic
root, which means "the serpent." Eltanin (Gamma, the eastern star) means
just that, "the serpent," while Rastaban (Beta, the western one)
comes from a longer phrase that means "the serpent's head," and in
fact was once applied to the star now known as Eltanin. Shining at
the bright end of third magnitude (2.79), Rastaban is the just
barely the third brightest star in the constellation, beat out by
Eltanin and by Eta Draconis,
though it still masters Thuban, the fainter Alpha star (whose
name shares the same root). Rastaban, with a color rather similar
to that of the Sun, is one of the sky's more unusual denizens, a
yellow class G (G2)
supergiant (or possibly bright
giant) that is
similar to the Alpha and Beta stars of Aquarius (Sadalmelik
and Sadalsuud). Stars like
Rastaban are often in rapid states of evolutionary transition, and
as a result there are not many of them, though their brilliance
makes it seem like there are more than there actually are. At a
distance of 362 light years, Rastaban shines with 950 times the
luminosity of the Sun from a surface
with a temperature of 5100 Kelvin (for the same class, yellow
supergiants are cooler than smaller dwarf stars, so though even the
same class as the Sun, Rastaban has a lower temperature). To be
this bright, Rastaban must have a radius 40 times solar, half the
size of Mercury's orbit. The star is not alone, but carries along
a small, cooler dwarf (hydrogen-fusing) companion at a distance of
at least 450 astronomical units and that takes at least 4000 years
to make an orbital circuit. From a hypothetical earth revolving
around the companion, Rastaban would shine with the light of 3000
full moons. Of much greater interest is the star's odd behavior,
or rather lack of behavior. On a graph of luminosity against
temperature, the star falls within an strip of instability in which
we find the famed Cepheid variables, of which Mekbuda (Zeta Geminorum) and even Polaris are examples. Yet Rastaban
(along with its colleagues Sadalmelik and Sadalsuud) does NOT vary
as it ought to. No one knows why the star does not vary. With a
mass five times that of the Sun, Rastaban was a class B blue
hydrogen-fusing dwarf only half a million years ago. It is now
preparing to begin fusing its internal core helium as it quickly
turns into a larger, redder giant, its fate to become a massive
white dwarf.