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.
Written by Jim Kaler. Return to STARS.