How do stars form? What factors determine the masses, luminosities, and distribution of stars in our Galaxy? In short, what basic processes are responsible for the appearance of our nighttime sky? Simply stated, star formation begins when part of the interstellar medium—one of the cold, dark clouds discussed in Chapter 18—starts to collapse under its own weight. 
(Sec. 18.3) The cloud fragment heats up as it shrinks, and eventually its center becomes hot enough for nuclear burning to begin. At that point, the contraction stops, and a star is born. But what determines which interstellar clouds collapse? For that matter, since all clouds exert a gravitational pull, why didn't they all collapse long ago? To begin to answer these questions, let us consider a small portion of a large cloud of interstellar gas. We concentrate first on just a few atoms, as shown in Figure 19.1.
Figure 19.1 Motions of a few atoms within an interstellar cloud are influenced by gravity so slightly that their paths are hardly changed (a) before, (b) during, and (c) after an accidental, random encounter.
Even though the cloud's temperature is very low, each atom still has some random motion because of the cloud's heat. (More Precisely 3-1 ) Each atom is also influenced by the gravitational attraction of all its neighbors. The gravitational force is not large, however, because the mass of each atom is so small. When a few atoms accidentally cluster for an instant, as shown in Figure 19.1(b), their combined gravity is insufficient to bind them into a lasting, distinct clump of matter. This accidental cluster will disperse as quickly as it forms. The effect of heat—the random motion of the atoms—is much stronger than the effect of gravity.
Now let's concentrate on a larger group of atoms. Imagine, for example, 50, 100, 1000, even a million atoms, each gravitationally pulling on all the others. The force of gravity is now stronger than before. Will this many atoms exert a combined gravitational attraction strong enough to prevent the clump from dispersing again? The answer—at least under the conditions found in interstellar space—is still no. The gravitational attraction of this mass of atoms is still far too weak to overcome the effect of heat.
We have already seen numerous instances of the competition between heat and gravity (see, for example, More Precisely 8-1). Since the temperature of a gas is simply a measure of the average speed of the atoms or molecules in it, the higher the temperature, the greater the average speed, and thus the higher the pressure of the gas. This is the main reason that the Sun and other stars don't collapse. The outward pressure of their heated gases exactly balances gravity's inward pull.
How many atoms must be accumulated in order for their collective pull of gravity to prevent them from dispersing back into interstellar space? The answer, even for a typical cool (100 K) cloud, is a truly huge number. Nearly 1057 atoms are required—much larger than the 1025 grains of sand on all the beaches of the world, even larger than the 1051 elementary particles that constitute all the atomic nuclei in our entire planet. There is simply nothing on Earth comparable to a star.
Heat is not the only factor that tends to oppose gravitational contraction. Rotation—that is, spin—can also compete with gravity's inward pull. As we saw in Chapter 15, a contracting cloud having even a small spin tends to develop a bulge around its midsection. (Sec. 15.2) As the cloud contracts, it must spin faster (to conserve its angular momentum), and the bulge grows—material on the edge tends to fly off into space. (Consider as an analogy mud flung from a rapidly rotating bicycle wheel.) Eventually, just as in Figure 15.1, the cloud forms a flattened, rotating disk.
For material to remain part of the cloud and not be spun off into space, a force must be applied—in this case, the force of gravity. The more rapid the rotation, the greater the tendency for the gas to escape, and the greater the gravitational force needed to retain it. It is in this sense that we can regard rotation as opposing the inward pull of gravity. Should the rotation of a contracting gas cloud overpower gravity, the cloud would simply disperse. Thus, more mass is needed for a rapidly rotating interstellar cloud to contract to form a star than is needed for a cloud having no rotation at all.
Magnetism can also hinder a cloud's contraction. Just as Earth, the outer planets, and the Sun all have some magnetism, magnetic fields permeate most interstellar clouds. As a cloud contracts, it heats up, and atomic encounters become violent enough to (partly) ionize the gas. As we noted in Chapter 7 when discussing Earth's Van Allen belts and in Chapter 16 when discussing activity on the Sun, magnetic fields can exert electromagnetic control over charged particles. (Secs. 7.4 16.4) In effect, the particles tend to become "tied" to the magnetic field—free to move along the field lines but inhibited from moving perpendicular to them.
As a result (Figure 19.2), interstellar clouds may contract in distorted ways. Because the charged particles and the magnetic field are linked, the field itself follows the contraction of a cloud. The charged particles literally pull the magnetic field toward the cloud's center in the direction perpendicular to the field lines. As the field lines are compressed, the magnetic field strength increases. In this way, the strength of magnetism in a cloud can become much larger than that normally permeating general interstellar space. The primitive solar nebula may have contained a strong magnetic field created in just this way.
Figure 19.2 Magnetism can hinder the contraction of a gas cloud, especially in directions perpendicular to the magnetic field (solid lines). Frames (a), (b), and (c) trace the evolution of a slowly contracting interstellar cloud having some magnetism.
Theory suggests that even small quantities of rotation or magnetism can compete quite effectively with gravity and can greatly alter the evolution of a typical gas cloud. Unfortunately, the interplay of these factors is not very well understood—both can lead to very complex behavior as a cloud contracts, and the combination of the two is extremely difficult to study theoretically. In this chapter we will gain an appreciation for the broad outlines of the star-formation process by neglecting these two complicating factors. Bear in mind, however, that both are probably important in determining the details.
