Hierarchy of Interstellar and Stellar Structures and the Case of the   Orion Star-Forming Region
  Hierarchical structure from stellar clusters, to subgroups, to associations and star complexes is discussed in the context of the Orion stellar grouping and its origin. The analogous structure in gas clouds is also reviewed, with an emphasis on general fractal properties and mass distribution functions. The distinction between the cloud, cluster, and stellar mass functions is discussed in terms of different sampling statistics for hierarchically structured clumps in clouds with an interclump medium. 
Bruce G. Elmegreen (1), Yuri N. Efremov (2) ((1) IBM Watson Research   Center, (2) Sternberg Astronomical Institute, Moscow)