Cassiopeia is a constellation fulll of astrophotography gems. And for us vikings, conveniantly located close to zenith at this time of year. Light pollution is less prominent there. This cosmic bubble is an extremely faint supernova remnant called CTB1. Abell believed it to be a planetary nebula at first; that's why it is also designated as Abell 85.
The remnant is so faint that a 10 minute exposure through the O-III filter showed absolutely no hint of nebulousity at all. On the other hand, there is signifacntly more signal in S-II, as pointed out by
@Michael J. Mangieri.
The image here is shown in natural colors, like most people seem to prefer for this object. It is combined as R=80%Hydrogen+20%Sulfur, G=100%Oxygen and B=85%Oxygen+15%Hydrogen to compensate otherwise missing H-beta emission. I think the extra S-II data gave an extra punch to the image.
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