On the evolution of Barnard globules

The evolution of the dark, dusty globules first reported by Barnard (1919) and generally found in young stellar groups is investigated. A statistical analysis of the data presented by Sim (1968) on the position, orientation, and size of Barnard globules in 66 OB clusters and associations is performed to examine the possible correlation of globule size and the age of the parent star group. Results indicate the existence of a characteristic minimum radius of 10,000 AU for all globules, and the increase of globule size with increasing association or cluster age. A possible scenario of globule evolution is suggested in which a dense cloud slowly contracts to be visible as a dark globule, then rapidly contracts to the minimum characteristic diameter and subsequently expands with age.