Interstellar space (or the interstellar medium, or ISM), as it is sometimes more appropriately called) is the vast volume of space between the stars in our galaxy. Although so incredibly sparse that even its densest portions would constitute an excellent vacuum on Earth, it is a remarkably complex physical system. It is filled with an extremely dilute soup of matter (atoms, ions, electrons, molecules, and microscopic dust grains), radiation (in all bands of the electromagnetic spectrum, from long-wavelength radio waves to very short wavelength gamma rays), and high-energy particles (charged nuclei and electrons that have been accelerated to tremendously high energies). Moreover, as one immediately gathers by viewing the sky, the ISM is highly inhomogeneous, clumpier and denser in some regions than others, and hotter and more turbulent in others.

The properties of the interstellar medium can be deduced by several primary methods. The first...