Physical conditions in the interstellar medium

Most modelling of ISM chemistry has concentrated on four broadly different environments for which we have good estimates of the local conditions  

The state of matter within these regions needs to be determined before the balance of energy and chemistry can be understood. Extreme photon fluxes break all chemical bonds, prevent molecule formation and ionise atoms but as the density of species increases the UV and far-UV photons are absorbed and molecules begin to form. Chemical reactions are, however, slow in the gas phase due to the low temperature, and molecules condense out on the surface of dust particles, perhaps forming ice grains. Once on the surface, molecules continue to be photoprocessed by the starlight as well as by the continual bombardment of cosmic rays. 

Dust particles appear to become oriented in interstellar magnetic fields, aligning themselves with the field lines to produce preferential absorption of light polarised 

Most of the long-wavelength light makes it through, but the bluer light is removed. 

Most of the short-wavelength light is scattered away from its original direction 

---