GAMMA CAS (Gamma Cassiopeiae). High overhead in northern autumn evenings lies the W-shaped constellation of Cassiopeia, in mythology the Queen and mother to Andromeda, to us brilliant against the background of the Milky Way and fully circumpolar from most of the United States and Europe and all of Canada. At the center of the "W" lies our star. It stands out in that, oddly, it is among the brightest stars in the sky that carries no proper "western" name, though known as Tsih (the whip) in old China. We do not know why at second magnitude (variable and now at 2.1) the ancients paid it so little attention. The four eastern stars of the "W" are all about the same brightness and were together known through Arabic as "the stained hand." The shortened version of the term, "Caph," eventually went to the eastern-most (the Beta star) star, so perhaps Gamma is a victim of early collectivization. Making it all even odder, it's a star of superlatives. At the extreme hot end of class B (B0, more likely B0.5, but still almost class O), the star is a rarity. It's usually listed as a subgiant, though theory (see below) tells otherwise. The distance of 549 light years (give or take 11 in the second Hipparcos reduction) is well known. The temperature is not, a rounded-off 30,000 Kelvin (from a range of 25,000 to 34,000) appropriate to the B0.5 class adopted here; as on a slippery road, caution is advised. In the Milky Way, Gamma Cas is dimmed by roughly 0.35 magnitudes by interstellar dust. After further correction for a whopping amount of ultraviolet radiation, the luminosity totals around 65,000 times that of the Sun (as compared to the visual luminosity of "only" about 5000 Suns). Temperature and luminosity then conspire to yield a radius of 9.4 times solar. An equatorial rotation speed of at least 280 kilometers per second gives a rotation period under 1.7 days. Fluctuations in X-ray radiation (again, see below) show the true rotation period to be 1.21 days, which then gives an axial tilt of 45 degrees to the line of sight and reveals a true rotation speed of almost 400 kilometers per second, 200 times that of the Sun. As a result the star is notably oval, which makes temperature even more problematic, as it must be hotter at the rotation poles than it is at the equator. Theory then gives a great mass close to 20 times that of the Sun and shows that the star is really a dwarf about halfway through its eight or so million year hydrogen fusing lifetime. Not surprisingly, the star is associated with various surrounding ionized interstellar clouds. Gamma Cassiopeiae is more, however, than just another luminous star. It's unpredictably variable. In 1937, it brightened almost to first magnitude, and it has been as faint as third. The Bright Star Catalogue gives 2.77, and it is now at about 2.15, making it the constellation's brightest star, in conflict with its Greek letter name. Perhaps lack of a proper name tells of ancient faintness. Then, in 1866 one of the founders of the study of stellar spectra, Father Angelo Secchi, discovered that the star radiated light in specific colors, those associated with hydrogen. Gamma Cas thus has the distinction of being the first known "Be star," the "e" standing for "emission." Be stars are fairly common among the class and odd. All rotate with enormous speed (as we see for Gamma Cas). The rotation, high luminosity, and perhaps atmospheric oscillations in an unknown way drive mass from the star into a surrounding disk that radiates the hydrogen emissions. Mass loss is apparently related to the brightness variations. As noted above, Gamma Cas also radiates X-rays, though no one is quite sure why. Of one thing we are sure, that with a mass far above the limit of 8 to 10 Suns, Gamma Cas will someday explode as a supernova. It will all be watched by a much lower mass companion. Detected spectroscopically, it has an orbital period of 203.5 days and therefore a separation of 1.8 Astronomical Units, or a bit more depending on the companion's unknown mass. Another likely companion, Gamma Cas B, an 11th magnitude F6 dwarf, lies 2.2 seconds of arc (at least 370 AU) away, which gives it a period of more than 1500 years. Thirteenth magnitude Gamma C at 54 seconds separation appears to be just a line of sight coincidence. If any star deserves a name, surely this one does. (Thanks to Monica Shaw, who originally helped research the star.)
Written byJim Kaler 1/8/99; revised 5/3/13. Return to STARS.