Hubble Classification

A simple system for classifying galaxies, named for its inventor, Edwin Hubble. The system employs four basic designations: E, Ir, S and SB. 'E' is used to designate elliptical galaxies, which are allocated a number from 0 to 7 according to their degree of elongation. 'Ir' specifies an irregular galaxy, one that cannot be otherwise classified on the system. Finally, the types 'S' and 'SB' are used to described spiral and barred spiral galaxies, respectively. Both of these spiral types are further designated as type 'a', 'b' or 'c' depending on the relative size of their nucleus.

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