We have seen how a portion of an interstellar cloud can become unstable, collapse, and fragment into stars. Let us take a moment to ask what happens next—not to the newborn stars themselves (that will be the subject of the next three chapters), but to the Galactic environment in which they have just formed.
The end result of cloud collapse is a group of stars, all formed from the same parent cloud and lying in the same region of space—in other words, a star cluster. 
(Sec. 17.10) As a by-product of cluster formation, a certain amount of unused gas and dust remains. How many stars form, and of what type? How much gas is left over? What does the collapsed cloud look like once star formation has run its course? At present, although the main stages in the formation of an individual star (stages 3—7) are becoming clearer, the answers to these more general questions (involving stages 1 and 2) are still very sketchy; they await a more thorough understanding of the star-formation process.
In general, the more massive the collapsing region, the more stars are likely to form there. In addition, we know from observed H—R diagrams that low-mass stars are much more common than high-mass ones. (Sec. 17.7) The precise number of stars of any given mass or spectral type likely depends in a complex (and poorly understood) way on conditions within the parent cloud. The same is true of the efficiency of star formation—that is, the fraction of the total mass that actually finds its way into stars—which determines the amount of leftover material. However, if, as is usually the case, one or more O- or B-type stars form, their intense radiation and winds will cause the surrounding gas to disperse, leaving behind a young star cluster.
Figure 19.16(a) illustrates this process. They are contrast-enhanced views of part of the Eagle Nebula (Figure 18.9b), showing several narrow "fingers" of gas protruding from the end of the largest (rightmost) pillar in the earlier figure. The fingers remain behind as the rest of the cloud evaporates and disperses because their dense tips, which are thought to contain solar system—sized stage 3 fragments, shield the rest of the gas from ultraviolet radiation (Figure 19.16b). Eventually the dense fragments too are destroyed, and the stage 4 protostar emerges from its protective shell.
Figure 19.16 (a) This enlargement of Figure 18.9(b) shows evidence of "evaporating gaseous globules" (known as EGGs)—star-forming fragments dense enough to survive the onslaught of ultraviolet radiation from newly formed hot, young stars in their vicinity. The EGGs lie at the ends of narrow fingers of gas projecting from the tip of the largest pillar. (b) The fingers survive because they lie in the EGG's shadows and so are shielded from the radiation dispersing the rest of the cloud. From left to right, the sequence shows (1) a dense pocket of gas near the surface of the evaporating cloud, (2) the formation of a finger of gas in the EGG's shadow as the surrounding cloud evaporates, and (3) the finger beginning to detach from the cloud, and the central stellar or prestellar core of the EGG beginning to emerge as the EGG itself is slowly destroyed.
These observations clearly illustrate the important role played by environment in the star-formation process. The first massive stars to form tend to prevent the formation of additional high-mass stars by disrupting the environment in which stars are growing. This is one reason why low-mass stars are so much more common than high-mass stars; it also helps explain the existence of brown dwarfs, by providing a natural way in which star formation can stop before nuclear fusion begins in the growing stellar core. Interlude 19-2 describes another system in which the gas-dispersal process is almost complete.
Until recently, the existence of star clusters within emission nebulae was largely conjecture—the stars cannot be seen optically because they are obscured by dust. However, infrared observations have now clearly demonstrated that stars really are found within star-forming regions! Figure 19.17 compares optical and infrared views of the central regions of the Orion Nebula. The optical image in Figure 19.17(a) shows the Trapezium, the group of four bright stars responsible for ionizing the nebula; the false-color infrared image in Figure 19.17(c) reveals an extensive cluster of stars within and behind the visible nebula. The Becklin—Neugebauer object (Section 19.4) can be seen as the central yellow spot within this region; it is thought to be a dust-shrouded B star just beginning to form its own emission nebula. These remarkable images show many stages of star formation.
Figure 19.17 Some views of the central regions of the Orion Nebula. (a) A short-exposure visible-light image (using a filter that is transparent only to certain emission lines of oxygen) shows the nebula itself and four bright O stars known as the Trapezium. (b) A magnified view of a smaller part of the nebula shows much irregular gas and dust but few obvious stars, which are hidden in the dust. (c) This short-exposure infrared image, acquired by the Hubble telescope in 1997, shows several faint red stars emerging from the nebular gas; the brightest star is known as the Becklin—Neugebauer object.
For every O or B giant, tens or even hundreds of G, K, and M dwarfs may form. Thus, even a modest emission nebula can give rise to a fairly extensive collection of stars. A typical open star cluster, like that shown in Figure 19.18, may measure 10 pc across and contain 1000 or more stars. Less massive, but more extended, clusters are usually known as associations. These typically contain no more than 100 bright stars but may span many tens of parsecs. Associations tend to be rich in very young stars. Those containing many pre—main-sequence T Tauri stars are known as T associations, whereas those with prominent O and B stars, such as the Trapezium in Orion, are called OB associations. It is quite likely that the main difference between associations and open clusters is simply the efficiency with which stars formed from the parent cloud.
Figure 19.18 The Jewel Box cluster is a relatively young open cluster in the southern sky. Many bright stars appear in this image, but the cluster contains many more low-mass, less luminous stars. Because some red giants appear among the cluster's blue main-sequence stars, we can estimate the age of the cluster to be about 10 million years.
In some cases, the ejection of unused gas reduces a cluster's mass so much that it becomes gravitationally unbound and rapidly dissolves. In clusters that survive the early gas-loss phase, stellar encounters tend to eject the lightest stars from the cluster, just as the gravitational slingshot effect can propel spacecraft around the solar system. (Interlude 6-2 ) At the same time, the tidal gravitational field of the Milky Way Galaxy slowly strips outlying stars from the cluster. Occasional distant encounters with giant molecular clouds also tend to remove cluster stars; a near miss may even disrupt the cluster entirely.
As a result of all these influences, most open clusters break up in a few hundred million years, although the actual lifetime depends on the cluster's mass. Loosely bound associations may survive for only a few tens of millions of years, whereas some very massive open clusters such as M67, shown in Figure 19.19, are known from their H—R diagrams to be almost 5 billion years old. In a sense, only when a star's parent cluster has completely dissolved is the star-formation process really complete. The road from a gas cloud to a single, isolated star like the Sun is long and tortuous indeed!
Figure 19.19 M67, one of the oldest known open clusters, has survived for almost 5 billion years—an unusually long time for a star system near the plane of the Milky Way Galaxy.
Take another look at the nighttime sky. Ponder all that cosmic activity while gazing upward one clear, dark evening. After studying this chapter, you may find you have to modify your view of the night sky. Even the seemingly quiet nighttime darkness is dominated by continual change.
