Gamma Cassiopeiae

With autumn approaching I felt it was about time to turn my spectroscopic attention to Cassiopeia. The first stellar spectrum I ever observed was that of Gamma Cassiopeiae and 15 years later I couldn't wait to see how it would look through my Star Analyser.

Back in 1866, Gamma Cassiopeiae was the first star ever observed with emission lines in its spectrum. This made it the prototype Gamma Cassiopeiae Variable star, the first known Be star (a type B star with emission lines), and one of the most popular targets for spectroscopy ever since.

One of the most common features of a stellar spectrum is to see hydrogen absorption lines. These are caused by hydrogen atoms in the star absorbing light at a wavelength corresponding to the energy required to excite an electron between different energy levels. For example, the spectrum of Sirius that I posted last month shows a clear H-Beta absorption line caused by exciting electrons between the second and fourth energy levels.

Here are three spectra I took of Gamma Cassiopeiae and instead of absorption lines we can see a clear H-Alpha emission line (in the red part of the spectrum).


Clearly some other process must be taking place. Some of the great minds of the early 20th century found that these emission lines must be coming from material around the star rather than the star itself. This material is a disc produced by a combination of very rapid rotation, magnetic fields and stellar pulsing. The cooling of hydrogen atoms in this disk then produce the emission lines seen in the stellar spectrum.

When I observed this star at university (with considerably better equipment!) I could produce a much more detailed spectrum. It was possible to measure the rotation speed of the disc by calculating the Doppler broadening of the emission line. It was also possible to see a narrower absorption line in the middle of the broad emission line caused by the disk absorbing light from the central star.