Gamma Cas and Only Gamma Cas

Since I was young there have been a handful of stars that I have always sought out when looking up in the night sky. I suppose you could call them my favorites. Among these special stars is Gamma Cassiopeia (γ Cas), shining brightly at the center of Cassiopeia's 'W' asterism.

Here on AB there have been scores of images of γ Cas posted in the last couple of months, each reflecting the experience, talent and interest of their imagers. Most, I would say, have actually been focused on IC 59+IC63 (Sh2-185), the so-called "Ghost of Cassiopeia". It is a fine set of nebulae, to be sure, but they would not even exist were it not for the ionizing radiation of Gamma Cas. Gamma Cas is really the 'star of the show' (apologies to pun lovers everywhere). I've written extensively about ionization in my other images, so won't do that again, instead, let's talk about the star and only the star.

Gamma Cas is a Type B star. Type B stars are quite rare, actually, comprising a mere 0.15% of the stellar population, so that fact in itself makes it worth taking a closer look.

Lying relatively close to CAS OB2 (which I wrote about here in a collaboration with @Mark Germani ), Gamma Cas has a stellar classification of B0.5IVe. This is about as hot a Type B star can be without being classed as a Type O star. The 'IV' indicates that it is a subgiant and the 'e' tells us that there are Hydrogen emission lines, which is quite significant. Stars that are "Type B" + "e" are referred to as “Be” stars and Gamma Cas was the first Be star discovered and so designated.

If that were all that this star had to offer, I'd say it was fairly ordinary as Type B stars go, but there's so much more. It is an interesting variable star with quite a large magnitude shift that is visible to terrestrial observers after decades of waiting. There is quite a bit of literature available on the peculiarities of these Gamma Cas-type variables if you're interested in such things.

More recently, data has surfaced that shows that Gamma Cas has a number of very faint companions that seem of great interest. Exoplanet researchers are interested in Gamma Cas and it's Goldilocks zone, too. Again, there is a lot of interesting research if you are interested. What's not to love about this star?

More interesting to me, though, is that Gamma Cas is a strong x-ray emission source. This is rather odd, as most Be stars don't have strong x-ray emissions. Magnetic readings of the star show an odd magnetic field, as well, suggesting that the star is fast-spinning and has an accretion disk that may be contributing to the formation of this odd field. Again, there is ongoing research into this aspect of Gamma Cas.

In terms of its actual physical characteristics, it is what you might expect for a Type B star of its age. It weighs in at 15-20 M☉, with a radius 10 R☉. It's quite large (for a subgiant) and also extremely luminous, at 34000 L☉!

As all of its kind, Gamma Cas is expanding and will continue to do so, depleting its Hydrogen along the way. With its relatively low original mass Gamma Cas' iron core will eventually collapse, leading to a Type II supernova. Boom! It will probably be a small, faint one, however, and we won't be around to see it. All that will then be left is a neutron star.

I will end it here with my own 'vision' of Gamma Cas and leave you with a question: Were you thinking you should minimize this magnificent and rare star while photographing Sh2-185?