Young stellar objects and their circumstellar dust - An overview

The basic observational classes of young stellar objects are briefly reviewed and some of the theoretical and observational problems in premain sequence stellar evolution are discussed. The evidence which indicates that the infrared excesses in recently formed stars are primarily due to thermal emission from circumstellar dust is summarized. Various indirect lines of evidence on the spatial distribution of the circumstellar dust are examined, and it is concluded that the concentration of the dust in a large disk structure is the most viable model at this time.