#spiral_galaxy__spiral_nebula  a galaxy having a spiral structure; arms containing younger stars spiral out from old stars at the center
  supertype:  #extragalactic_nebula__extragalacticnebula__galaxy  (astronomy) a collection of star systems; any of the billions of systems each having many stars and nebulae and dust; "`extragalactic nebula' is a former name for `galaxy'"
     supertype:  #aggregation__collection__accumulation__assemblage  several things grouped together
        supertype:  #group__grouping  any number of entities (members) considered as a unit
           supertype:  pm#collection  something gathering separated things (entities/situations)
              supertype:  pm#divisible_entity__divisibleentity  many classifications under this category are application-dependant
                 supertype:  pm#entity  something that can be "involved" in a situation
                    supertype:  pm#thing__something___T__t___3D_or_4D_thing_or_anything_else  any category (type or individual) is instance of this type; any type is also a subtype of this type
                 supertype:  pm#divisible_thing__divisiblething
                    supertype:  pm#thing__something___T__t___3D_or_4D_thing_or_anything_else  any category (type or individual) is instance of this type; any type is also a subtype of this type

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