The Interstellar Medium

The interstellar medium (ISM) is the stuff between the stars: gas, dust, magnetic fields and cosmic rays. Stars form in molecular clouds, regions of the ISM which are dense and cold. During their lifetimes, they return material back to the ISM in the form of stellar winds and Planetary Nebulae, and at their deaths during super nova explosions. The detailed interplay between stars and the ISM is complicated and not well understood. Here we will discuss briefly how the gas and dust can be observed, describe the Jeans criterion for the collapse of gas into stars, and describe the physics of ionised regions of gas called HII regions.