This image shows the central region of a galaxy cluster located only 62 million light years away in the southern constellation of Fornax at a declination of -36 degrees. The cluster is nicknamed the Fornax Cluster for obvious reasons.  The average distance of the cluster is 62 million light years, but the range of distance is surprisingly large (40 to 80 million light years). This is the second richest close galaxy cluster in the sky (the Virgo Cluster is the richest), where close is defined as within 100 million light years.

Abell galaxy clusters encompass massive space and distance and are the largest known gravitationally bound structures in the universe. The Abell catalog consists of 4073 objects - the 2712 clusters of the original George O. Abell Northern Survey of 1958 (numbered from 1 to 2712), and an additional 1361 clusters from the Southern Survey of 1989 (numbered from 2713 to 4076).  Those of you who pay attention to details will see that the 4076 galaxy cluster numbers do not align with the 4073 clusters.  This is because 3 duplicated entries in the Southern Survey were found at a late stage in their work.

So, given that the Abell galaxy clusters are numbered from 1 to 4076, why is this object called Abell S373?  

I have been puzzled for years by this question and I could not find an explanation for it anywhere.  I finally found the answer by digging into the 1958 and 1989 Abell papers.  The 1989 paper was published by co-authors after Dr. Abell's death in 1983.  It turns out that this S373 cluster was not identified by either catalogue above - the Northern Survey or the Southern Survey.  Instead, it was included as part of a 3rd "supplemental" list of 1174 additional southern galaxy clusters.  Each  of these clusters has an "S" preceding their number, so this 3rd list of extra objects runs from S1 to S1174.  According to the 1989 paper, these supplementary southern clusters (including S373) were "not rich enough or too distant for inclusion in the main catalog". 

So, just to be clear, one of the most famous and recognized galaxy clusters was not deemed worthy to be included in the original 4076 member Abell catalog?   Certainly it is not "too distant", since it is practically in our neighborhood.  Apparently, despite being the "second richest galaxy cluster within 100 million light years" (from Wikipedia), it is not rich enough.

The key here is the definition of "richness".  Some define richness as relating to the estimated total cluster mass, which is how this cluster is the "second richest galaxy cluster within 100 million light-years".  But in his 1958 paper, Abell defined richness as "the number of cluster members that are not more than 2 magnitudes fainter than the 3rd brightest member".  To be included in the main catalog, a cluster had to contain 50 such members.  My guess is that Abell S373 does not have 50 such members, and therefore was relegated to the supplementary list. 

Despite being supplemental, this cluster is still a wonderful object.  This cluster image is dominated by the bright elliptical galaxies NGC 1399 and 1404 at the left center of the image, and the magnificent barred spiral galaxy NGC 1365 towards lower right with a diameter of 180 million light years. 

Incredibly, almost 500 galaxies are seen in this image, albeit many very faintly, as shown in the annotated version of the image which is seen in the mouseover.  The majority of these galaxies are between magnitudes 13 and 15,  with typical diameters from 30,000 to 50,000 light years and apparent sizes from 1 to 2 arc-minutes wide.