Abstract
Stars are believed to form from interstellar material through the gravitational collapse of dusty clouds. Interstellar medium is a very dynamical environment in which clouds of atomic gas (and its associated dust counterpart) form in warm medium fragments, perhaps as a result of turbulence or the passage of shock, and subsequently cool down and condense. Although the dust is a tiny fraction (of order 1%) of the total material mass, it plays a major role in the cloud evolution because its opacity can shield the cloud center from the interstellar UV field and dust surfaces act as a catalyst on which molecular hydrogen can form. For high enough column density, the combined effect of dust shielding and self-shielding of H\(_2\) turn the initially predominantly atomic gas into molecular form. H\(_2\) forms first, but more and more complex species such as CO, CN, and HCN, form during cloud evolution.
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