Beta Draconis - Rastaban

Beta Draconis is the third brightest star in Draco, at magnitude 2.79. It is one of the two leading stars or "eyes" in the head of the Dragon, along with γ Dra. β Dra has the traditional name Rastaban, which is, confusingly, sometimes also used for γ Dra.

The traditional name Rastaban, from the Arabic phrase ra's ath-thu'ban ("head of the serpent") is less commonly written Restaban. Rastaban is also known as Asuia and Alwaid, the latter meaning "who is to be destroyed," though some trace it to Arabic al'awwad ("the lute player"). It is part of the asterism of the Mother Camels (Arabic al'awa'id), which may have influenced this alternate name.

Properties and Evolution

Rastaban is a yellow class G2 Ib supergiant with a color rather to the Sun's. Stars like Rastaban are often in rapid states of evolutionary transition, and as a result there are not many of them. At a distance of 362 light years, Rastaban shines with 950 times the Sun's luminosity from a surface of 5100 K. (Supergiants are cooler than dwarf stars of the same class, so though even the same spectral class as the Sun, Rastaban has a lower temperature.) To be this bright, Rastaban must have a radius 40 times solar.

With a mass of five suns, Rastaban was a class B blue hydrogen-fusing star only half a million years ago. It is now preparing to fuse its internal core helium, and turn quickly into a larger, redder giant, and eventually to become a massive white dwarf. On a graph of luminosity against temperature, the star falls within an strip of instability in which we find the famed Cepheid variables. Yet Rastaban does not vary as it ought to, and no one knows why.

Companion

Rastaban is not alone, but is a binary star, designated ADS 10611. It has a small, cooler, hydrogen-fusing main-sequence companion at a distance of at least 450 AU, which takes at least 4000 years to make an orbit. From a hypothetical planet revolving around the companion, Rastaban would shine with the light of 3000 full moons.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]