ASTRO:
     type=galaxy cluster (starscape)
     const=Hercules
     mag=15-13
     dist~500 M ly
     size~6 M ly (image boundary)
IMAGE:
     location=EB Driveway
     BrtlCls=4
     moon=13% WxCr
     exposure=CMOS OSC: 52x180s (2.6h), G100
EQUIPMENT:
​     optics=ES102CF APO / Hyperion 24mmEP (afocal) 1.7x, FL(eff.)=1238mm, f(eff.)/12.1
     camera=ZWO ASI2600MC-Pro
     filter=Optolong L-Pro LPS
     mount=Celestron AVX
     guiding=Orion 60x240mm, ZWO ASi224MC
SOFTWARE:
     acquisition=Stellarium, APT, PHD2,
     processing=PixInsight (RCAstro), Photoshop (APF-R, Lumenzia), LrC

Taking the term “deep sky” to a whole new level, this cluster of galaxies makes-up what is known as the Hercules Cluster, an archipelago of island universes lying an astounding 500 million light-years away.  Also known as ACO 2151, this cluster is rich with gas, dust and star-forming spiral galaxies, some of whom are interacting (merging).  It also contains elliptical galaxies, which lack as much gas and dust and associated newborn stars as their spiral cousins, but tend to show more brightly.  The image makes visible more than 70 individual galaxies.  With larger and more powerful equipment, counts have ranged as high as 500 within the same field of view.

The cluster's official designation of ACO 2151 reflects that it is a member of the catalog of rich northern galaxy clusters initially published by American astronomer George O. Abell, and later updated by his contemporaries, Harold Corwin and Ronald Olowin.  The Abell Team's research stands as an important cornerstone to today's understanding of the structure of the Universe.  Led by this cluster, younger scientists have continued to build on Dr. Abell's legacy, by determining the group is part of the larger Hercules Supercluster, which is itself part of the much larger Great Wall super-structure. 

By switching our thinking away from the space-continuum and toward the time-domain, the wonder of the image begins to take-on extreme significance.  We are looking back through time to a part of the Universe as it existed appx 500 million years ago!  …meaning the light we see today left there and began its trek toward us at about the time when Fossil evidence here on Earth marks time that occurred long before the age of Dinosaurs, at the beginning of the Cambrian explosion of life, when the main species of modern life was established as it began to climb out of the aquatic environs onto dry land.

The colors in this remarkably "deep" image clearly show the star forming galaxies with a blue tint, and galaxies with older stellar populations with a redish cast.  The image spans about 3/4 of an arc-degree across the cluster center, which at the cluster's estimated distance corresponds to over 6 million light-years across.  Many galaxies seem to be colliding or merging while others seem distorted - clear evidence that cluster galaxies commonly interact.  In fact, the Hercules Cluster itself may be seen as the result of ongoing mergers of smaller galaxy clusters and is thought to be similar to young galaxy clusters in the much more distant, early Universe.

IMAGER'S COMMENTS:For observers who own or have access to a large telescope (recommended 12" and larger) and a dark viewing site, the Hercules Cluster is one of the most impressive sights in our nighttime sky.  For imagers, the Cluster is best captured with a larger scope, typically 150mm (appx 6”) or better; however, with longer imaging sessions and good focusing techniques, some of the detail can be captured with more moderately sized equipment.

240411 - ACO 2151 "Hercules" galaxy cluster