Gamma Cas is the class prototype for a particularly interesting class of eruptive variables. Gamma Cas  was, as its name implies, a bright star in Cassiopeia, and up to 1927 was known only as an emission line star of spectral type B. The star became both spectroscopically and photometrically variable in 1927 until 1935, when the star suddenly brightened by a full magnitude, an enormous change for such a star. Since then, gamma Cas has never had an eruption of similar magnitude, exhibiting smaller but still very noticeable changes over time. Eventually several other emission-line B stars were found to behave the same way, and were dubbed the "gamma Cas" variables. Gamma Cas is only one of several bright variables of this type; delta Scorpii  underwent a similar eruption as recently as the year 2000, proving that even the brightest stars in the sky can do surprising things!
These are bright, blue stars, more massive and hotter than our own Sun, with bright emission lines of hydrogen gas that indicate the presence of circumstellar matter. When stars undergo gamma Cas eruptions, it's related to the formation and destruction of a circumstellar disk of material, and instabilities within this disk. It's unknown whether gamma Cas will exhibit future outbursts like that of the mid-1930s, but long-term monitoring via photometry and spectroscopy is encouraged.
Prepared By: Matthew Templeton