I recently moved from a Bortle 8-9 backyard with a half-decent view of the sky, to a Bortle 7 backyard with very restricted views - just a small window to the Northeast. This is my first image produced from the new location, and the first using my new CEM26 mount, which I'm still getting the hang of.

When I completed the image, it was clear there was a LOT going on, so I asked my friend Daniel Erickson, an accomplished astrophotographer himself, to provide one of his famous write-ups to help both demystify and astonish, and he very kindly agreed. I'll let him take it from here


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CAS OB2 Stellar Association
by Daniel Erickson

Those that know me know that I study images. I try and understand how the components of each image relate to each other in space and time so I can understand what it is I am actually looking at. Many images are straightforward -- there is one element to dissect. Others, like this one, require a bit more work; they are complex regions that defy simple understanding. That said, let's begin our deep-dive into this fascinating area of space!

What you may not realize is that this image captures in a single frame the entire CAS OB2 Stellar Association. Don't let the two-dimensionality of the image deceive you. The association extends nearly two parsecs 'into the past'. The front is bounded by NGC7654 (M52), an Open Cluster, which is not part of the Association. At the back end is NGC 7510, another Open Cluster, also not part of the Association. In between are a number of fascinating structures that I could spend a week talking about them! But I'll try and limit myself to the highlights.


NGC 7635

Closest to us at about less than 3000 parsecs, is NGC 7635 (Sh2-162), the Bubble Nebula. The big feature in this HII region is, obviously, the Bubble. The Bubble is a shell around a Blue Hypergiant named BD+60°2522, an emission line star with spectral characteristics of O6.5(n)fp. The star is entering its old age (hence the shell creation around the star). The next evolutionary phase may be to a Red Giant or a direct jump to a Wolf-Rayet star (see some of my images from Summer 2022 - Cygnus Wall, Bow Shockwaves and Microquasars, and Wolf-Rayet Stars - for discussions of these evolutionary processes).


NGC 7538

We could be forgiven for being focused on the Bubble Nebula or becoming entranced by the shape of the Lobster Nebula, but for me, the most interesting aspect of this image is NGC 7538 (Sh2-158), an apparently nondescript small blob of HII emission at around 3000 parsecs, which is defined as an open cluster, but this cluster is not composed of stars in the traditional sense. Instead, there are 78 'objects' associated with the cluster, 73 of which are what are called 'dense cores'. The five Type O stars in the Cluster are illuminating the HII region referred to as Sh2-158), but the story is in these largely invisible dense cores.

Dense cores are the smallest and densest of molecular clouds. These cores are very, very cold: 10 K (10 degrees above absolute zero) and are in a state of collapse that may eventually lead to the birth of single stars. The dense cores in the cluster are surrounding a much larger dense object that is particularly massive, with a radius of 0.5 pc and a mass of ∼2,000 M⊙ (our solar system is 1.939255e-4 parsecs in size and <2 M⊙) . This is the largest cold core discovered to date and has at its center a huge protostar that likely could evolve to be the largest star that we've ever seen. This protostar currently has the designation NGC 7538 S and if you're into the deep science of things like this, you can read about it here. Immediately behind NGC 7538/Sh2-158 is the large HII region Sh2-161 - the thinning area near the center suggests some ionization taking place.


Sh2-157

Finally, at the furthest extreme of the Association is Sh2-157, the Lobster Claw Nebula. This large HII region has two remarkable features. First is the bright knot near the junction of the 'claws'. Named LBN 537, this is a cluster of densely packed Young Stellar Objects (YSOs) surrounding a Type B Hypergiant doublet called TYC 4279-1192-1. The second feature (more interesting to me) is the area between the claws. This 'empty' area is the result of a hot-burning O-star that has ionized all of the surround gases. All that is left at this point is the causal star: Wolf-Rayet 157 (at optical coordinates (23 15 12.5454190406 +60 27 01.000751310). This star will one day (soon in astronomical time) collapse into a neutron star or (more likely) erupt in a Type IIa Supernova...the usual end of most Type O stars.


Planetary Nebulae

As a postscript for advanced imagers with larger scopes, you may want to investigate the three Planetary Nebulae in the image (which are not plate-solved):

• Planetary Nebula PK 110-00.1 (actually NOT a PN and is V* V807 Cas, a B3 Ellipsoidal Variable)
• Planetary Nebula PK 110-01.1 (PN WeSb 6)
• Planetary Nebula PK 112-00.1 (PN KjPn 8 -- this one is a unique and lovely, bipolar PN not far from the Bubble)

These are likely extinguished members of this very Association, tiny, tiny remnants of once great stars providing you a challenge to study. Have fun!


When we image an area it is often the case we don't really know what we're looking at. I think that even more than the image itself, the real beauty and significance of our images is that we can take the time to study and learn and understand the scope and magnificence of what we're witnessing. Pause for a moment and look again at the image, imagining a 3D perspective of this slice of space and try to place these objects in their proper order and perspective. It boggles the mind.

Thank you for stopping by to take a look at our collaboration.


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Thank you Daniel!