An OB stellar association is a very young, loose collection of O and B class stars. These Associations generally contain no more than 100 OB stars and often significantly fewer. Other, non-OB stars are often found nearby, but contribute little to the activity of the OB Association that we talk about here. These OB stars are born, densely packed, in molecular clouds and blow and burn their way out, gradually drifting apart over a few millions of years before dying as supernovae. Many of the beautiful nebulae that we image are the handiwork of the ionizing properties of the OB Association action. That is certainly the case for the Cassiopeia OB6 Association (Cas OB6).

The Cas OB6 Stellar Association, located about 7600ly away, is what created the iconic "Heart and Soul" Nebulae. Throwing off huge quantities of ionizing radiation, the OB stars burn off the surrounding molecular cloud, creating solar winds that form the nooks and crannies of the nebulae that we image. Cas OB6 is composed of two large, ionizing clusters of OB stars: IC1805 and IC1848. This image features IC1805.

IC1805 (also known as Melotte 15, Collinder 26, and a few other names) is composed of over 10 O class stars, plus scores of smaller "young stellar objects", brown dwarfs, B class stars and other stellar objects of interest.

There are three O Class stars particularly worthy of note:

(1) HD 15570, an emission-line Class O4 supergiant, one of the most massive stars known in our galaxy. Minimum estimates of its mass put it at about 100 M⊙ !  This particular star was, at one time, the object of much research, being, perhaps an evolutionary hybrid between an O star and a Wolf-Rayet (WR) star. 

(2) HD 15629, is a 'young stellar object'. These are essentially new stars and this one is a massive O4.5V supergiant. Minimum mass estimates for this newborn approach that of HD 15570, above.

(3) HD 15558, appears to be a massive, multiple Class O4.5III star--at least a double-O and possibly a triple with numerous other possible companions. Improbable mass estimates of >150M⊙ suggest that HD1558A is actually a binary.

The HII region being immediately affected by these stellar newcomers is designated is Sh2-190, also called Westerhout 4. The W4 designation is important as the area is a strong radio source. Interestingly, the Westerhout Catalog contains HII regions detected by radio telescopes that other catalogs (like the Sharpless and RCW) failed to find.

To the west of Sh2-190 is what is popularly referred to as the 'fish-head nebula'. I don't particularly care for popular names, though, so I'll use Westerhout 3, since it is a very strong radio source. It also carries two other, separate designations: IC 1795 + NGC 896 (both still considered part of the larger designation, Sh2-190). 

What is interesting about this structure is what lies behind it. I love looking closely at images and reminding myself that it is at least 3D space, not 2D. Is this structure in front of or behind the larger portion of Sh2-190? Are there young stellar objects forming behind W3? What is the effective 'range' of IC1805? Can it reach as far as W3 to ionize that region, as well? So many questions. Such a complex region!

Other structures are worthy of mention in this image that relate to Cas OB6:

(1) The microquasar LSI +61 303, a binary system composed of a Be (emisson line) star and presumably a small black hole. This interesting binary is believed to have been blown out of IC 1805 by a supernova explosion about 1.7 million years ago.

(2) There are numerous and unlabelled numerous dark nebulae in the area: LDN1356 and LDN1359 through LDN1373.

Finally, a word about the Open Cluster NGC 1027 on the left side of the image. This cluster, although interesting in its own right, is not associated with CAS OB6. It is 'only' 3000 ly away, again reminding us of the 3D nature of our images.

I hope you've found this interesting and learned something more about this iconic target and one-half of the Cas OB6 Association. Stay-tuned for Part II. Thanks for stopping by and taking the time to read this far!