δ Cas is the fourth brightest star in the constellation Cassiopeia, at magnitude 2.68. It also has the traditional names Ruchbah and Ksora. The old Arabian astronomers commonly applied proper names to the stars according their positions within the classical figures, and Ruchbah is a fine example, derived from an Arabic word "rukbah" meaning "knee".
Properties
Just 99 light years away, Ruchbah's light takes nearly a century to reach the Earth. A class A5 III-IV star with a surface temperature of 8400 K, Ruchbah is a fair bit warmer than the Sun, and its luminosity is 63 times solar. Marginally detectable as a disk, the star is not quite four times the radius of the Sun.
Classed as a giant or subgiant, its core hydrogen fusion is now shutting down. With a mass 2.5 times solar, this 600 million-year-old star has begun its death process, and will become a much larger orange giant in 10 million more years.
Careful observations show that Ruchbah is slightly variable, between magnitudes +2.68 and +2.74, with a period of 759 days. Every 1.26 years it undergoes a partial eclipse when a small orbiting companion star, about which nothing is known, passes in front of it. Otherwise Ruchbah is quite normal, which makes it a good star against which to measure the properties of others, and to study the interstellar gas through which its light is passing.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]