The physical conditions of a large variety of structures, such as giant HII regions, holes, shells and rings, present in the interstellar medium are reviewed. Such structures are thought to result from the energy input from OB associations in the form of ionizing radiation, strong stellar winds and supernova explosions.
The study of the effects produced by every single one of the energy sources is proposed here as the way to evaluate, understand, and relate the observed structures with a given energy input. In particular, predictions fron the study of the HII region evolution sround a stellar association are reviewed. Such an evolution , lasting 10^7 yr, accounts for the aging of the exciting stars. The resultant effects are then compared with the large (expecting and static) shells observed in our galaxy, and with alternative theoretical models.
A sequence of events, based on the observed properties and the theoretical models, is proposed here as a likely scenario for the evoluton of the environment of an OB association.