Question

Globular clusters were classified by Shapley and Sawyer as being on a scale from 1 to 12 where 1 is very loosely packed central area and 12 is highly packed central area. As a visual astronomer, it seems difficult to judge the concentration of a globular cluster.

Has anyone tried to classify globular clusters concentration classes for something like the Astronomical League Club for Globular Clusters? http://www.astroleague.org/al/obsclubs/globular/globular1.html

It seems that to judge concentration you first need to know what percentage of the diameter of the globular cluster corresponds to the packed core. Visually, knowing the official stated size seems to be required since without that, it is hard to say how concentrated the core is as a glow percentage.

Can any human eye get anywhere near detecting all twelve steps of concentration? It is easy enough to distinguish 3 or 4 levels, but twelve seems daunting.

How can I attempt to properly judge globular clusters on the Shapley and Sawyer scale?

Answer 1

I don't think this is possible for visual observations, either. Harlow Shapley was a pioneer in the very early days of modern, scientific astronomy, before the transistor revolution, so there was less emphasis on quantitative parameters. If this work had been done more recently, Shapley and Sawyer probably would have taken good, deep, photometric, CCD images of their gc's, then fit a 2-D function to their integrated brightness. Parameters would probably include ellipticity, some sense of the original S-S concentration classification, and other things like squariness (technically speaking, higher moments of the distribution).

You could then do some analysis on the parameter sets of a large group of globular clusters, and see if there actually was parameteric clustering, or if they were all just distributed randomly through phase space. That would give you a logical, natural, absolutely quantitative classification scheme.