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RUCHBAH

(Delta Cassiopeiae). Among the most loved, most charming, and most visible of constellations is Cassiopeia, the celestial Queen. The five stars that make her familiar "W" (add one more to make her Chair) are circumpolar, always visible, from latitudes above 35 degrees north. And just over the line into third magnitude (2.68), Ruchbah is appropriately the fourth brightest star in the constellation, following right behind Cih. The old Arabian astronomers commonly applied proper names to the stars according their positions within the classical figures, and "Ruchbah" is a fine example, the name a reduction of a longer phrase that refers to Cassiopeia's Knee. Just shy of 100 light years away, its light takes a century to come to Earth. A star with a temperature of 8400 Kelvin, a fair bit warmer than the Sun, it falls into mid-class A. Stars produce radiation whose quality depends on temperature. Relative to what we see with our eyes, cool stars radiate vast amounts of invisible infrared, very hot stars a great deal of invisible ultraviolet, both of which must be factored in to estimate the star's true luminosity. White Ruchbah falls into a narrow temperature range where we simply do not have to worry much about either the ultraviolet or infrared, and little or no correction is needed, Ruchbah's luminosity 63 times solar. Marginally detected as a disk, the star is not quite four times the radius of the Sun and just barely classed as a giant, which means it has begun its death process, core hydrogen fusion shutting down. With a mass 2.5 times solar, this 600 million-year-old star will become a much larger orange giant in a mere 10 million years. Careful observations show that Ruchbah is slightly variable. 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 the star is quite normal, which makes it a good one against which to measure the properties of others. Ruchbah's greatest significance is that it provides a good background against which to study the local complex system of interstellar gas through which the Sun is now passing.