SHEDAR (Alpha Cassiopiae). The southernmost star of Cassiopeia's famed Chair, Shedar is also the brightest, though not by much and not all of the time. (The strange variable star Gamma Cassiopeia can on rare occasion vault itself well beyond Shedar's apparent brightness.) The name "Shedar" comes directly from the Arabic word for "breast," and describes the position of the star in the heart of the ancient Queen. At mid-second magnitude (2.23), and hard to distinguish in brightness from Caph (the Beta star), Shedar still properly received the Alpha designation from Bayer.
Alpha Cas Shedar, Alpha Cassiopeiae, lies in a thick portion of the Milky Way. Achird (Eta Cas) is just down and to the right, Mu and Theta are to the far left, and Zeta Cas is near the top center edge. North is down. The pair at the lower right edge are (left-to-right) Upsilon-1 and Upsilon-2. To the immediate right of Shedar is the small fuzzy nebula NGC 281.
Shedar is yet another cool class K (K0) orange giant star, of the kind that so heavily populates the outlines of the constellations. From its substantial distance of 230 light years (and still second magnitude in our sky) we calculate a large luminosity 855 times that of our Sun, the star more in the "bright giant" class. The luminosity, and a temperature of 4530 Kelvin, lead to a notable size, 48 times that of the Sun. Shedar and Hamal in Aries are the only two stars in the sky whose "limb darkening" has been directly observed (by use of a sophisticated telescope that makes use of the interference properties of light). The apparent disks of all stars, including that of the Sun, are a bit dimmer at the edges than they are at the centers because our lines of sight at the edges do not penetrate as deeply. As a result, Shedar has a very accurately determined angular diameter of 0.00525 seconds of arc, leading to an improved radius 42 times that of the Sun. (The difference from the earlier radius probably involves errors in the temperature or in the estimate of the amount of invisible infrared radiation included in the luminosity, showing how much there is yet to learn). Shedar's chemical composition is close to solar, though the star seems notably deficient in particular elements such as zirconium and europium. The large luminosity suggests a fairly high mass four to five times solar. Since high mass stars burn out fast, this dying giant, which has ceased core hydrogen fusion, is only 100 to 200 million years old, having begun life as a star much like Regulus in Leo. For all its seeming mundane character, Shedar has a curious history. Various nineteenth century astronomers claimed it could fade to mid-third magnitude. As a result, the star is still called "variable" in most listings. Since the advent of permanent photographic and electronic recording, however, Shedar has shone steadily. We have little idea of the long-term behavior of many stars, and it is worth keeping an eye on Cassiopeia's chair. Between Gamma and Alpha, who knows what may be seen.
Written by Jim Kaler. Return to STARS.