A simple system for classifying galaxies, named for its inventor, Edwin Hubble. The system employs
four basic designations: E, Irr, 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. 'Irr' 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.