Hi I want to know how astronomers count the number of stars (or any other celestial object for that matter) in the sky. Will they take a photograph and do some signal processing like counting the number of brightest spots? Is it possible to count accurately since there are billions and billions of stars. Or only a rough estimate based on some equation or theory is available to do that job? I am not related to this field but just want to know, whether there is any scope for application of signal processing techniques.