ABSTRACT
We showed in Chapter 5 that a massive hot star will photoionize the interstellar gas in its neighbourhood. In doing so it increases the gas temperature from about 102 K to about 104 K, i.e. by a factor of about a hundred. The ionization process itself 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 it and the adjacent neutral gas are set in motion, so that the HII regions described in Chapter 5 are not static configurations. This particular process is not the only way in which interstellar gas is set in motion by means of 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. The effects of these explosions on the interstellar gas are extremely important in determining many of its properties. We will discuss in this chapter all three of these processes using simple models which, although approximate, give quite good descriptions of the events taking place. They serve to show how the ideas of gas dynamics discussed in chapter 6 can be put into practice. More sophisticated treatments must, however, be done numerically because of the inherently complicated interplay of various physical effects.
