The subject of star formation is in reality much more complicated than the preceding discussion suggests. Interstellar space is populated with many kinds of clouds, fragments, protostars, stars, and nebulae. They all interact in a complex fashion, and each type of object affects the behavior of all the others. For example, the presence of an emission nebula in or near a molecular cloud probably influences the evolution of the entire region. We can easily imagine expanding waves of matter driven outward by the high temperatures and pressures in the nebula. As the waves crash into the surrounding molecular cloud, interstellar gas tends to pile up and become compressed. Such a shell of gas, rushing rapidly through space, is known as a shock wave. It can push ordinarily thin matter into dense sheets, just as a plow pushes snow.

Many astronomers regard the passage of a shock wave through interstellar matter as the triggering mechanism needed to initiate star formation in a galaxy. Calculations show that when a shock wave encounters an interstellar cloud, it races around the thinner exterior of the cloud more rapidly than it can penetrate its thicker interior. Thus, shock waves do not blast a cloud from only one direction. They effectively squeeze it from many directions, as illustrated in Figure 19.13. Atomic bomb tests have experimentally demonstrated this squeezing—shock waves created in the blast tend to surround buildings, causing them to be blown together (imploded) rather than apart (exploded).

Figure 19.13 Shock waves tend to wrap around interstellar clouds, compressing them to greater densities and thus possibly triggering star formation.

Once shock waves have begun the compression of an interstellar cloud, natural gravitational instabilities take over, dividing it into the fragments that eventually form stars. Figure 19.14 suggests how this mechanism might be at work near M20. Note the correspondence between the shock-compressed region at lower right and the high-density molecular gas revealed by radio studies (Figure 18.19).

Figure 19.14 An artist's conception of a cloud fragment undergoing compression on the southerly edge of M20 as shock waves from the nebula penetrate the surrounding interstellar cloud.

Emission nebulae are by no means the only generators of interstellar shock waves. At least two other driving forces are available—the deaths of old stars (planetary nebulae and supernovae, to be discussed in Chapters 20 and 21) and the spiral-arm waves that plow through the Milky Way (to be discussed in Chapter 23). Supernovae are by far the most energetic, and probably also the most efficient, way to pile up matter into dense clumps. However, they are relatively few and far between, so the other mechanisms may be more important overall in triggering star formation. Although the evidence is somewhat circumstantial, the presence of young (and thus quick-forming) O- and B-type stars in the vicinity of supernova remnants does suggest that the birth of stars is often initiated by the violent, explosive deaths of others.

This picture of shock-induced star formation is complicated by the fact that O- and B-type stars form quickly, live briefly, and die explosively. These massive stars, themselves perhaps born of a passing shock wave, may in turn create new shock waves, either through the expanding nebular gas produced by their births or by their explosive deaths. These new shock waves can produce "second-generation" stars, which in turn will explode and give rise to still more shock waves, and so on. As depicted in Figure 19.15, star formation resembles a chain reaction. Other, lighter, stars are also formed in the process, of course, but they are largely "along for the ride." It is the O- and B-type stars that drive the star-formation wave through the cloud. Observational evidence lends some support to this chain-reaction model. Groups of stars nearest molecular clouds do indeed appear to be the youngest, whereas those farther away seem to be older.

Figure 19.15 (a) Star birth and (b) shock waves lead to (c) more star births and more shock waves in a continuous cycle of star formation in many areas of our Galaxy. Like a chain reaction, old stars trigger the formation of new stars ever deeper into an interstellar cloud.