If you read my last post about the Cassiopeia OB 6 Association, then you wouldn't be surprised that this image was next in line. I will say up front that I dislike the common names of astronomical objects very much. Calling something by a name--perhaps what it looks like--is a useful fiction, but we run the risk of getting ensnared by the name, rather than truly seeing what the name is referring to. In the case of this region--which shall go nameless--there is much going on! What follows is my own simplistic explanation.

The Cas OB 6 Association is huge. It extends across ~600ly. It is dominated by the two clusters IC 1805 and IC 1848 and includes numerous smaller, nondescript clusters (all with their own names, of course). There is further evidence of at least 19 young clusters still forming in the Association's nebulosity. Today we're focusing on just the major features of the eastern end of the Cas OB 6 Association. 

Let's start with Westerhout 5. The Westerhout radio catalog contains 'places/areas' that emit 21-cm Hydrogen lines. This is normally interstellar hydrogen. It is not the HII (ionized hydrogen) we are fond of photographing. As it happens, W5 is a massive radio source that is also located in an equally huge HII region. The area encompassed by W5 is huge, a bit more than two degrees across.

The region also has the name Sh2-199, however this refers not to a radio source, but to the ionized hydrogen in the region (nebula). It is object number 199 in the second edition of the Sharpless catalog, a list of 300+ HII regions. 

Although these two 'names' are in the same space, they do not technically refer to the same thing. The Hydrogen detected by Westerhout is being blown away and/or ionized to HII that is then shaped and compressed into the elegant structures of the nebula that we know and love to photograph. The compression can then lead to further star formation...and so it goes! 

And what causes the ionization? It is the all-important IC 1848. Nowadays, unfortunately, IC 1848 is used as a catch-all term, much like "Soul Nebula", but it actually refers to the open cluster towards the apparent right end of the nebula. It has another name, too: Collinder 32, referring to the same cluster. In any case, it is this powerhouse cluster that is largely responsible for the HII ionization at the eastern end of the Cas OB 6 Association.

In particular, HD 17505 is a magnificent class ~O6V beast. It is a multiple star, at least seven visual components have been noted (which I obviously fail to resolve) and the multiple is the main ionizing component in the entire region. Two other much cooler O-class stars are nearby, in the cluster, HD 17520 (Class O9V) and HD 237019 (Class O8V). All three are annotated on Revision B.

Note that HD 17688, is not part of the cluster and is in the foreground, reminding us that our perspective of 3D space is clearly distorted!

A brief note regarding Sh2-198 and Sh2-201, the two small nebulae found in the image. Sh2-198 is being ionized by a somewhat cool class O9.5V star, LS I +59 153, seen at the center of the nebula. I don't believe it is properly associated with the Cas OB6 Association, but I may be wrong! The tiny Sh2-201 is probably part of the Association (due to its distance, if nothing else) and is being ionized by a star cluster at its center visible only in the infrared.

The Cas OB 6 Association (OK, "Heart and Soul", if you must) stars are linked to a common origin and provide profound evidence of a repeating process of star formation dating back billions of years. I never tire of studying these Associations and their magnificent stars, and I hope I've piqued your interest!  (See my four-part Cep OB 2 study for more.)

Thank you for stopping by for a look and a read. I hope you enjoyed yourself as much as I did presenting it to you!