Although the space between stars is empty by terrestrial standards, it’s not devoid of matter. There’s a scattering of cold gas and dust, pocked by areas known as prestellar cores with densities of a few thousand particles per cubic centimeter. This is just enough matter to help gravity eventually win its tug of war with the forces that would drive molecules apart.
When shock waves pass through these regions — whether thrown off a dying star or a newly birthed one — they compress the material, kickstarting the process of stellar formation. Passing shock waves can also shake loose molecules stuck to the dust, providing key tracer elements that astronomers can use to visualize shock waves and the areas they affect. To learn more, see this article over at Physics Today. (Image credit: NASA/ESA/CSA/STSCI/K. Pontoppidan/A. Pagan; see also Physics Today)