Understanding physical conditions using spectral synthesis code CLOUDY

Most of the astrophysical information about our universe come from spectroscopy. In order to understand the underlying physical processes comprehensively, it is essential to develop tools to analyze these spectra. Interstellar matter is far from equilibrium and the physical state of a non-
equilibrium gas is determined by the underlying microphysics and the external environment, mainly the radiation field striking the gas. As a result, the gas properties and its observed spectrum are sensitive to atomic and molecular rate coefficients, composition and density of the gas, and to the sources of energy that enter the region. Due to complicity of these large number of degrees of freedom, no analytical solution is possible and a numerical approach is the only 
option. The spectral simulation code, CLOUDY, is designed to understand complex physical environments starting from first principles. It determines the physical conditions within a non- equilibrium gas, possibly exposed to an external source of radiation, and predicts the resulting spectrum. It enables one to predict many observed quantities by specifying only the properties of the cloud and the radiation field striking it.
In this talk I will discuss some astrophysical environments where CLOUDY has been used successfully to understand the physical conditions.