  As a stellar group forms within its parent molecular cloud, new members first
appear in the deep interior. These overcrowded stars continually diffuse
outward to the cloud boundary, and even beyond. Observations have so far
documented only the interior drift. Those stars that actually leave the cloud
form an expanding envelope that I call the "stellar mantle." Simple fluid
models for the cloud and mantle illustrate their basic structure. The mantle's
expansion speed is subsonic with respect to the cloud's dynamical temperature.
I describe, in qualitative terms, how the expanding mantle and Galactic tidal
radius might together shape the evolution of specific types of stellar groups.
The massive stars in OB associations form in clouds that contract before
extruding a substantial mantle. In contrast, the more slowly evolving clouds
forming open clusters and T associations have extended mantles that encounter a
shrinking tidal radius. These clouds are dispersed by internal stellar
outflows. If the remaining group of stars is gravitationally bound, it appears
as a long-lived open cluster, truncated by the tidal radius. If the group is
unbound, it is a late-stage T association that will soon be torn apart by the
tidal force. The "distributed" populations of pre-main~sequence stars observed
in the outskirts of several star-forming regions are too distant to be stellar
mantles. Rather, they could be the remnants of especially low-mass T
associations.
