Revision 3/2023:
I just needed to redo this one.  The first one is "ok", but not having been at this hobby very long, the extreme stretch I used did not help the background nor the stars.  And the stars had significant artifacts due to my early poor cable management with my RASA.  As it turned out, I also added a huge amount of subs to the mix.  Yes, these were not perfect, but I am learning that one can be too selective when Blinking.  This should have been simple.  But it required 4 turns with WBPP to get it right.  In certain circumstances, certain functions during preprocessing can cause issues and for this type of object I ran into a real issue with Cosmetic Correction.  Upon reading some on the PI forum, I learned some things.  First, the developers of PI apparently believe that WBPP should not be used to generate usable integrations, but as a tool to see just what all is happening.  Hmm...  In the thread I read, I certainly detected some hubris in the responses to genuine questions.  I think the developers of PI really should understand where their butter comes from...  First, the problem I was getting was donuts for stars in the GC.  After reading the thread, I decided to look carefully at the images generated through the process, and in doing so clearly identified Cosmetic Correction as the culprit.  Likely the dense starfield within the GC is just too much to handle at my rig's sampling.  Considering my typical experience with CC is that it ditches not that many pixels within the process, I decided to ditch CC!  Problem solved!  Also, considering my sampling, I felt that this was a clear example where drizzling at 2X was going to be beneficial.  Too many stars were squares or two pixel dimensionally within the GC.  This turned out to be a benefit throughout processing.

No star-reduction used at all with this image.  Only star correction function on BXT.  And a very slight application of halo reduction on BXT.  Sharpening was limited to a masked galaxy, the primary one in the image.  I wanted to minimally effect the background galaxies, since I have come to enjoy their presence!  Liberal use of Clone Stamp to deal with star tails from my poor cabling.


Old Description:
My attempt at a globular cluster with OSC. I struggled with this one. I am not used to doing this sort of object and I have a lot to learn. I was a bit hamstrung also with the data that I collected. Tracking was not great and I had to chuck probably 35% of the subs to get better star symmetry in the center of the field. And at 90 sec, I think my sub exposure time was too long, making it difficult to resolve stars well in the center. Certainly a challenge. This data is part of the data I collected where I believe my offset was set too stingy. You may notice some horizontal lines and I can't get rid of them by calibrating with bias or darks, etc. QHY268, mode 3, gain 0, offset 8. Don't magnify this image too much!

Messier 13 is considered the Northern Hemisphere's greatest globular cluster. It is likely visible to most people with good eyes at a dark site, "if" they know where to look and what they are looking for. I can see it easily in my binoculars at my Bortle 4-5 site. And with my 5 inch Mak Newt I can certainly resolve it clearly as a tight ball of pinpoint stars. Though the colors of the stars cannot be seen.

M13 is 25,000 light years distant. It is said that it contains ~300,000 stars. Globular clusters are typically found in halos around galaxies. Typical spiral galaxies can have hundreds of such clusters. The supergiant ellipticals can have thousands! There are multiple theories as to the origin of globular clusters. Unlike open clusters, where many of our deep sky targets are the breeding grounds of open clusters, there really are no examples of globular cluster-generating nebulae. Even still, some astronomers believe that GCs form within dense molecular clouds. There is one nebula example (sorry I cannot recall) that is believed to be active enough to do so. But this is speculation at this time. I am not a fan of this theory. One simple question of this theory is: Why then are GCs not found in regions where they are generated, instead of being relegated to areas of the galaxy well removed from the action? Open clusters on the other hand appear well within the active spiral arms and star-forming regions of galaxies. I favor the theory that GCs actually are the cores of the minor galaxies that went into the building of their associated large galaxy. As the theory goes, these small galaxies were stripped of most of their loosely bound stars and gas as they passed through or nearby the larger, newly forming galaxy and the gravitationally more tightly bound cores survived the interactions intact before heading out into a more stable position. For those that remained in the maelstrom of the main galaxy, these would eventually be dismantled structurally by many gravitational interactions as they passed through the denser star fields and gas clouds.

GCs are much more dense than open clusters. Near the center of a GC such as M13, the distance between these stars is roughly the distance from our sun to the outer reaches of our solar system. This is close enough that star mergers likely occur. It is also a lousy place for stable planetary system, and therefore likely a poor choice to look for life. Planets were likely stripped from their parent star soon after formation and flung into the soup of stars and planets that make up the cluster. In support of the theory of GCs being the cores of small galaxies, recent work over the last 8 years has found GCs with black holes near their centers. In one, 47 Tucanae, the black hole is an intermediate version of roughly 1500 to 2500 solar masses. In another, it is thought that there is a cluster of neutron stars and black holes at its center. This may help explain how these objects are structurally more robust and hold together. They may also provide insight as to how galaxies and their central massive black holes evolve and grow over time. I'll bet astronomers find more and more of these black holes in GCs.

I do think the galaxy core theory also may have some holes in it as well. When we typically review our photos of the many galaxies, the cores appear to be rather dull, with older stars and less gas and active nebulae than what would typically be found in the active spiral arms or other structures. (OK, ellipticals are boring from center to edge!) It is my understanding that the population of stars in galaxy cores is comprised of older long-lived stars and they tend to be redder. Any very hot blue stars or giants would have burned out long ago. But many clusters appear to have diverse star types, such as red supergiants, and blue stars, with blue stragglers being common. Do these stars align with what the expected age of a GC is if it is indeed a galaxy core?

In any case, we should count ourselves lucky that we do not reside within the inner reaches of a globular cluster. Astrophotography would not be a popular pastime.