What we can gather from telescopes arrives in the form of spectrescopic data from different regions of the electromagnetic spectrum. The molecules present in different astrophysical regions allow to extract very important information of both the physical conditions and the chemistry in those regions.
The excited energy levels of a molecule can be populated by different processes. Collisions between molecules (or atoms) and radiation can excite and de-excite the different energy levels of a molecule (i.e., electronic, rotational or vibrational levels). When an excited level undergoes radiative decay, it emits a photon at the resonance frequency. A molecule decays radiatively if the timescale for this process is smaller than the timescale for collisional de-excitation. It is this radiation which can be observed and analysed to obtain information on the conditions of the medium.
