ABSTRACT
When a star is bom in a molecular cloud, it disturbs the interstellar medium in its vicinity through its wind, jet or radiation pressure, and then neighboring clumps are pushed beyond the brink of gravitational stability, too. The isolated birth of a star is a rare event but it can happen in globules. These are cloudlets that have been stripped off from larger complexes. The presence of very small grains and polycyclic aromatic hydrocarbon (PAHs) in the skin layer has the remarkable effect that the source becomes visible in the mid infrared. The presence of the PAHs has the further interesting consequence that it leads to important heating of the interior by mid infrared photons. This chapter derives the density distribution of a protostellar cloud under the assumption of spherical symmetry. It discusses a protostar where it is possible to verify the theoretical predictions by direct measurement of the dust absorption.
