  The clusters of young stars in massive star-forming regions show a wide range
of sizes, morphologies, and numbers of stars. Their highly subclustered
structures are revealed by the MYStIX project's sample of 31,754 young stars in
nearby sites of star formation (regions at distances <3.6 kpc that contain at
least one O-type star.) In 17 of the regions surveyed by MYStIX, we identify
subclusters of young stars using finite mixture models -- collections of
isothermal ellipsoids that model individual subclusters. Maximum likelihood
estimation is used to estimate the model parameters, and the Akaike Information
Criterion is used to determine the number of subclusters. This procedure often
successfully finds famous subclusters, such as the BN/KL complex behind the
Orion Nebula Cluster and the KW-object complex in M17. A catalog of 142
subclusters is presented, with 1 to 20 subclusters per region. The subcluster
core radius distribution for this sample is peaked at 0.17 pc with a standard
deviation of 0.43 dex, and subcluster core radius is negatively correlated with
gas/dust absorption of the stars -- a possible age effect. Based on the
morphological arrangements of subclusters, we identify four classes of spatial
structure: long chains of subclusters, clumpy structures, isolated clusters
with a core-halo structure, and isolated clusters well fit by a single
isothermal ellipsoid.
