Gamma Cassiopeiae

Gamma Cassiopeiae is a blue-white Be star in Cassiopeia whose changing brightness and emission lines made it the prototype of the Gamma Cassiopeiae variable-star class.

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Be star: A hot, rapidly rotating star whose spectrum shows emission lines from a surrounding gaseous disk. Gamma Cassiopeiae is the prototype of this emission-line stellar class.
Cassiopeia (constellation): A northern constellation named after the queen Cassiopeia in Greek mythology. Gamma Cassiopeiae is one of the constellation’s brightest stars.
Pleione: A rapidly rotating B-type star in the Pleiades that undergoes changes in its circumstellar disk. Like Gamma Cassiopeiae, it illustrates how Be-star disks can appear and dissipate.
Magnetic reconnection: A process in which magnetic field lines rearrange and release stored magnetic energy. Magnetic activity has been proposed to explain the star’s unusually energetic X-rays.
Circumstellar disk: A ring or disk of gas and dust orbiting a star. Gas expelled by Gamma Cassiopeiae forms the disk responsible for its emission features.
Spectral type: A classification of stars based mainly on the features and temperatures of their spectra. Its spectral classification identifies Gamma Cassiopeiae as a hot blue-white star.
Zeta Tauri: A Be star in Taurus with a circumstellar disk and strong emission lines. It provides a nearby comparison for disk signatures seen in Gamma Cassiopeiae.
Accretion: The accumulation of matter by a celestial body from its surroundings. Accretion onto a companion is one proposed source of Gamma Cassiopeiae’s X-rays.
Stellar rotation: The spinning motion of a star around its axis. Its rapid rotation helps move material into the star’s surrounding disk.
Variable star: A star whose apparent brightness changes over time. Gamma Cassiopeiae’s changing brightness places it among variable stars.