I think those ARE the 'stars' we are talking about
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I think the stars should represent a person's activity over a course of time but I don't think it's possible with the current software to apply an entire math formula to how stars are calculated, they can only be displayed using a few criteria such as registration date, post count, activity, group member status or set manually.
However to link the stars to a postcount is kind of double since the post count is already displayed. In which case I would recommend giving people one star for each year they have been a member and start using different color logos when that number exceeds every 8 years or so depending on how crowded you want the userbars to get.
EDIT AFTER MAX: Good point, fortunatlly I think it can also factor in if a person has been active within a certain amount of time. So if someone is registered for 6 years but hasn't posted for a year, he could be reset to just 1 star.