ABSTRACT
The ionization process increases the number of gas particles, and therefore the pressure, by a further factor of two. As a result, the pressure in the ionized gas is two hundred times greater than that in surrounding neutral material. This ionized gas cannot be confined and will expand. Both the ionized gas and the adjacent neutral gas are set in motion, so that the HII regions are not static configurations. This particular process is not the only way in which interstellar gas is set in motion by interaction with stars. Two other mechanisms are of particular importance. Firstly, there are the effects produced by the very high speed continuous mass loss – a stellar wind – observed to take place from many young hot stars. Secondly, many massive stars terminate their existence in a violent explosive event – a supernova. This chapter discusses all three of these processes using simple models which give good descriptions of the events taking place.
