Formation of Open Clusters

Abstract

A forming star progresses through several evolutionary stages which are marked by different observational signs that can be identified by different techniques. Stellar birth from interstellar matter occurs in dense, cold clouds of gas due to a local increase in the density and interstellar clouds composed of molecules and grains are the preferential place where star formation can be initiated. The observational indications of this prestellar stage are the emission of millimeter radio waves by the cloud's many molecules and the optical obscuration of the background field stars by the grains mixed with the molecular gas. When fragmentation and condensation mechanisms occur in a dense cloud, the gravitationally collapsing fragments cannot be directly observed at visible wavelengths, because their light is blocked by the dust grains which heat up and radiate in the infrared. Thus the various protostellar stages are essentially studied using infrared techniques. Evidently, the spectral energy distribution of the protostars depends on their evolutionary stage and, globally speaking, one can say that the maximum of the infrared emission moves towards the short infrared wavelengths as the protostar evolves. A tentative coherent stellar evolutionary scenario may be given for the radio and infrared sources directly connected with the star and cluster formation: