The IAU is composed of:

NATIONAL MEMBERS: Organizations representing national professional astronomical communities, desiring to promote their participation in international astronomy and supporting the objective of the Union, may adhere to the Union as National Members.   Organizations desiring to join the Union as National Members while developing professional astronomy in the community they represent may do so on an interim basis.  They may later become National Members on a permanent basis.

INDIVIDUAL MEMBERS: Professional scientists whose research is directly relevant to some branch of astronomy are eligible for election as Individual Members. Individual Members are, normally, admitted by the Executive Committee on the proposal of a National Member.