One of the most important was done by the astronomer George Abell. Abell completed his survey in 1958.
It added considerably to the map made by Shapley and Ames. In fact, his map had over 2700 clusters of galaxies.
That's 2700 clusters of galaxies! Not just galaxies.
But there’s another aspect of Abell’s work that makes this map so valuable to astronomers.
He introduced a classification scheme for the galaxy clusters.
Now, surveys completed since Abell’s have catalogued additional galaxies and surveyed more outer space, but no one has improved upon Abell’s classification scheme.
In fact, the Abell catalogue is used as a starting point for astronomers who study these objects.
One of the reasons his scheme has been so widely accepted is because of his sample size.
With all the clusters in his sample, he could determine the different characteristics of clusters.
And these characteristics form the basis of his classification scheme.
Now, two of the characteristics crucial to his classification were richness and symmetry.
So what did he mean by richness? Well, basically it refers to the number of galaxies there are within a cluster.