Gamma Cassiopeiae lies at the center of the W-shaped constellation Cassiopeia, the mythological Queen and mother to Andromeda. At magnitude 2.47, the star carries no proper "western" name, though it known as Tsih (the whip) in ancient China.
American astronaut Virgil Ivan "Gus" Grissom nicknamed the star Navi after his own middle name spelled backwards; the star was used as an easily identifiable navigational reference point during space missions. The name endured as a memorial to the astronaut, who perished in the 1967 Apollo 1 fire.
Properties
At a distance of 610 light years, this hot blue class B0 subgiant radiates with a huge luminosity some 40,000 times the Sun's, after accounting for ultraviolet radiation from its 25,000 K surface. Gamma Cassiopeiae is apparently approaching the end of its hydrogen-fusing life. With a mass of around 15 times the Sun's, γ Cas will most likely someday explode as a supernova.
Variability
γ Cas is an eruptive variable, and has given its name to a category of similar objects. In 1937, the star brightened to magnitude +2.2, and was as +3.4 in 1940. Perhaps its lack of a proper name indicates ancient faintness. At maximum intensity, γ Cas outshines both α and β Cas; it is now at magnitude +2.15. γ Cas also radiates X-rays, though no one is quite sure why. Theories include the transfer of lost mass to a compact companion, and magnetic effects similar to those found on the Sun.
In 1866, Father Angelo Secchi, the father of the study of stellar spectra, discovered that the star radiated light in specific colors associated with hydrogen. γ Cas thus has the distinction of being the first known "Be" star, the "e" standing for "emission."
Be stars are now known to be fairly common. All rotate with enormous speed; γ Cas spins at least 300 km/sec at its equator, 150 times faster than the Sun. Its rotation and high luminosity drive mass from the star into a surrounding disk that generates the emission. γ Cas's mass loss is apparently related to its brightness variations.
Companions
Gamma Cassiopeiae is a spectroscopic binary with a period of 203.59 days, an eccentricity of 0.26, and a companion with about the same mass as the Sun. The companion might be a normal star, or it might a white dwarf or neutron star. If it were a collapsed object, this would explain why Gamma Cas is also an X-ray source: gas from the Be star accreting onto a compact companion would release gravitational energy, which would be transformed into thermal energy and then into X-rays.
γ Cas is also a visual double star system, with the designation ADS 782.
Gamma Cassiopeiae Nebulae
The radiation from γ Cas is currently evaporating two nearby nebulae, IC 59 and IC 63. The leading edges of these nebula glow strongly in the red light of hydrogen reemitted after the gas has absorbed ultraviolet from the nearby luminous star. The bluish regions behind the front lines are where light is simply being scattered.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]