The formation of interstellar molecules

Abstract

The expanding universe is ${10}^{10}\hspace{0.17em}\mathrm{years}$ old and has a radius of ${10}^{10}$ light years or ${10}^{28}\hspace{0.17em}\mathrm{cm}.$ Matter in the universe, distributed in a highly non‐uniform manner, is concentrated largely in galaxies, which occupy only a minute fraction ${\text{(10}}^{\text{−7}})$ of the volume of the universe. In addition to stars, most galaxies contain diffuse, low density interstellar matter. Because interstellar matter is most easily studied in our own galaxy, the discussion here will focus on it, although this discussion should be pertinent to other galaxies as well.