Papers of the Paris meeting in June 1990 on Local Group Galaxies, molecules in early-type galaxies, observations of spiral structure in molecular clouds, a comparison with other gaseous components and IR emission, interacting galaxies and starbursts, gas and star dynamics, galaxy evolution, IRAS ult

Proceedings of the 146th Symposium of the International Astronomical Union, held in Paris, France, June 4-9, 1990

In these lectures, I have discussed a number of basic concepts that provide the necessary background to the current studies of star formation. A ?rst partwas dedicatedto illustrate the conceptofa protostar, discussing con- tions and propertiesof the collapseof a molecular core. A secondpart deals with circumstellardisks. Disks areimportantnot only to the processofstar formation itself, but also because they are in all probability the site where planets form. The age range of pre-main sequence stars coincides with the timescales for the formation of very large planetesimals, the building...