The Interstellar Medium

The interstellar medium (ISM) is the stuff between the stars, basically gas, dust, magnetic fields and cosmic rays. Stars form in regions where the ISM is very dense, such as in molecular clouds. During their lifetimes they return some of this material back to the ISM in the form of stellar winds and Planetary Nebulae, and at their deaths during a super nova explosion. The detailed interplay between stars and the ISM is certainly very complicated and presently 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.