Star of the Month
Gamma Cassiopeiae
Position: 00 hrs 56 min 43 sec 60 degrees 43 min 00 sec
Due south at 21:16 (GMT) on 15 November. It is almost overhead.
Gamma Cassiopeiae
Image: Simbad (http://simbad.u-strasbg.fr/simbad/)
In 1835, the famous positivist philosopher Auguste Comte, seeking an example of the unknowable, declared that we would never be able to tell the chemical composition of the stars. Yet within thirty years he was proved wrong. In 1859, the chemist Robert Bunsen and the physicist Gustav Kirchhoff in Heidelberg showed that each element emitted specific lines of coloured light, their spectral lines, and that if white light passed through the vapour of the element, the spectrum displayed dark lines where the emission lines should be. They were working in the laboratory, but their work provided an explanation for the dark lines in the solar spectrum meticulously recorded by Joseph Frauenhofer in 1814. If we can work out the elements in the sun, we can surely work out the chemical composition of other stars? The Jesuit astronomer Angelo Secchi in Rome invented a stellar spectrograph and from 1863 started to record the spectra of the stars. Soon afterwards, William Huggins in London also started to study the spectra of astronomical objects, such as planetary nebulae. In 1866 Secchi recorded the spectrum of Gamma Cassiopeiae and noticed that in contrast to the dark absorption lines shown by most stars, it also had emission lines. It is now regarded as the archetype of a particular class of stars called Be stars which have the typical B type spectrum, but have emission lines (hence the “e”). Gamma Cassiopeiae is also the archetype of the Gamma Cassiopeiae variables. It is an eruptive variable which varied between magnitude 1.6 and 3.0. It is spinning rapidly which gives it a pronounced equatorial bulge so it is egg-shaped and it has also thrown off material to form a hot circumstellar disc. The emission lines are produced by this hot gas. In the late 1930s, Gamma Cassiopeiae had a shell episode and its brightness shot up to 2.0 then dropped back to 3.4, but it gradually brightened again to about 2.2. It is also a stronger emitter of X-rays than most B or Be stars.
It is easy to find Gamma Cassiopeiae: it is the star in the middle of the W of Cassiopeia. It is also easy to work out its magnitude as two bright stars in the W (Beta at the end and Alpha next to Gamma) have similar magnitudes 2.2 and 2.3 which is close to (or just below) Gamma’s maximum. Epsilon, the fainter star at the other end of the W, is magnitude 3.4 which is close to Gamma’s minimum and Eta between Gamma and Alpha is 3.5. Finally Delta which is between Gamma and Epsilon is a middling 2.7. If this is not enough excitement for you, Gamma Cassiopeiae is also a double. But the separation is narrow (2.1 arc sec) and the secondary is faint, only magnitude 10.9, so you will need a reasonably large telescope, at least 125mm and possibly 150mm.