| Hydrogen molecule plays an important role in every astrophysical environment. It is the very first and most abundant molecule to be formed in the universe and most of the baryonic matter is in this form. Despite its huge abundance, it is hard to observe H2 due to its lack of permanent electric dipole moment. The goal of this thesis is to understand the role of H2 in star-forming regions, predict various H2 line intensities, and determine physical conditions using various properties of H2.; The micro-physics of H2 was implemented into the spectral simulation code Cloudy. I have included 1893 levels in detail. Various relevant physical processes are taken into consideration, including state-specific formation rates, direct dissociation, and the two-step Solomon process. The ortho-para conversion by exchange collisions and on grain surfaces and various shielding effects have also been considered.; I studied the physical conditions in the local interstellar medium (ISM) towards the star HD185418 to validate our methods. This line-of-sight has a large number of molecular, atomic, and ionic absorption lines. I reproduced the observed H2 column densities and found a cosmic-ray ionization rate 18 times higher than the background. Similar high ionization rates were observed by another group towards zeta Persei.; Next, high-redshift Damped Lyalpha Absorbers (DLAs) are studied. H 2 observations towards DLAs provide a unique insight into the physical conditions of young galaxies. They have large neutral-hydrogen column densities and are believed to be the progenitors of present-day disk galaxies. I calculated their physical conditions at redshift 2 and found that the observed properties of the ∼15%--20% of the DLAs with detectable H2 absorption require higher densities and in situ star formation.; The validity of various temperature indicators that are used in studies of the ISM was determined. I found that the 21 cm spin temperature and the temperature derived from the ratio of column densities of J=1 and J=0 levels of H2 agree with the kinetic temperature only in molecular regions of dusty environments.; This work also forms a foundation for future studies of environments where H2 is an important constituent. |
