Turn Right At Orion (13 page)

Clearly, the neutron star must be an equilibrium of the second kind. I was not interested so much in the individual motions (or lack thereof) of the particles that made up the neutron star. But, viewed as continuous matter, whatever substance made up the neutron star kept its shape and resisted gravity with remarkable rigidity.

Rigidity—or stability—was another aspect of equilibrium I had to worry about. No equilibrium was worth its salt if it could be overthrown by a sneeze. A playing card standing precisely on edge and a sharpened pencil balanced on its point are both equilibria, surely, but not terribly useful ones. There needed to be some feedback that would oppose any
modest
attempt to upset the balance. I don't think I was asking too much. I would grant that if the Milky Way collided with another large galaxy, its well-ordered stellar disk might not survive the disruption. Not that the stars in the two galaxies would collide physically—I'm certain they wouldn't. The disruption would be more subtle than that. Stars originally belonging to one galaxy would be tempted away by the gravitational lure of the other. Forces changing rapidly in strength and direction, as the remnants of the two galaxies jockeyed for position, would defeat orderly motion, throwing it into disarray. On the other hand, the Milky Way's disk had better be
robust enough to weather a minor disturbance: the intrusion, of my spacecraft, for example; or an errant star flying in from intergalactic space; or even the impact of a moderately massive black hole; or a globular cluster containing a million stars, crossing the disk. The latter might create ripples, modifying the disk slightly, but should leave the basic structure intact.

I pondered the feedback that might lend a disk of stars the kind of stability I was looking for. It would have to be the conservation of angular momentum: the same effect that had spun up the pulsar. If the disk expanded in radius, its stars would no longer be orbiting fast enough to withstand gravity, and the disk would sink back toward its original size. If it were compressed, the stars would speed up in their orbits and the disk would spring back. But then I remembered that a disk of stars, in isolation and orbiting under its own gravity, is
not
stable. I had been through all of this reasoning before, when I had traversed the Galaxy and wondered about the spiral arms. There were other ways to perturb a disk than simply to expand or contract it, and some of these alterations would lead to unruly outcomes. Groups of stars would take sides, gang up on each other, trade angular momentum back and forth, and in so doing disrupt the orderly structure. The disk could slosh, bend, or break. The truly stable entity was the disk plus halo—with emphasis on the halo, which had to contain most of the mass. A halo would act as a moderator, preventing things from getting out of hand.

Like the disk, the halo consisted of moving stars, but they were not marching lockstep in circular orbits; they were moving randomly, every which way. The halo was also a kind of structural equilibrium, and thinking about it gave me a better opportunity to visualize how feedback might lead to rigidity. I imagined that I could somehow take the Milky Way's halo of stars and stuff it inside the toe of a thick wool sock. What resistance would I discern as the stars bumped into the wall of the sock? There would be so many stars hitting the wall and bouncing off that it would feel like the steady pressure against the wall of a balloon. If I grabbed the open end of the sock and squeezed
the trapped stars into a smaller volume, the halo would push back. The pressure against the inside of the compressed sock would be higher, because stars would be hitting the sock more frequently in the confined space
and
because each star would be moving faster. The increase in speed, I realized, followed from a generalized version of the conservation of angular momentum. If I allowed the sock to expand, the opposite would happen: Stars would be moving more slowly and scoring fewer impacts, generating less pressure. So far, this behavior was just like that of any gas—the air inside a balloon, for example. But for the stars of the halo, there was an additional complication. As the same number of stars was confined to a smaller volume, the gravitational attraction inside that volume would increase. The big question was whether the increase in the stars' speed, which made them less susceptible to gravitational collapse, would outstrip the increase in gravitational attraction, which made them more susceptible. There was nothing to do but to put on my theorist's hat and do the calculation. This I did, and the results were clear. Gravity did not prevail against the feedback of increased motion. It was angular momentum again that saved the day.

These thought experiments were getting complicated and threatened to take me too far afield of my principal concern. I had to focus. What was it that opposed the tremendous gravity inside a neutron star? It might be motion of some kind. The chaotic motion of its atoms holds up a star like the Sun, much as the chaotic motion of its stars holds up the halo of a galaxy. In the short term, the feedback in the Sun is exactly the same as that in the Milky Way. After all, the Sun really is made out of gas, very similar to the air in a balloon. But a second level of feedback is needed in the Sun's case. Unlike a system of stars, the atoms in the Sun are always colliding and losing energy, and this energy leaks out in the form of the Sun's radiance. The losses are constantly replaced by nuclear reactions going on in the Sun's center, which keep the Sun shining steadily. If these reactions faltered, the Sun would begin to deflate under its own gravity, but then—like the stars trying to conserve their angular momenta—
the atoms would speed up. The nuclear reactions, being very sensitive to the speeds with which atoms slam together, would immediately pick up their pace, providing the feedback to keep the Sun stable.

The solar analogy was appealing, but it proved to be a dead end. I knew that nuclear reactions don't go on inside neutron stars. Neutron stars are burnt-out entities,
sans
thermonuclear fuel. Perhaps this meant that they had to be able to survive without relying on the motions of their constituent particles. Was that untenable? After all, motion is not the only thing capable of counterbalancing gravity. The universal quality of gravity, in every single one of its manifestations, is simply that of attraction. So any form of repulsion would suffice, if it were strong enough and provided the right kind of feedback.

The Earth resists collapse not because its particles are in lively motion but because it is made of very resilient material. Gravity squeezes the layers of solid and molten rock inside the Earth, but they are so resistant to compression that they can support the weight handily. Even outside the high-pressure environment of a planetary interior, the atoms in these materials—the electron clouds enveloping their tiny nuclei—nearly touch. Any additional squeezing jams the electron clouds together and even makes them interpenetrate slightly. Atoms fiercely resist such overlap. Each atomic nucleus, an infinitesimal BB made of protons and neutrons (only one ten-thousandth the size of its electron cloud!), is electrically charged. Stripped of their electron clouds, they would repel one another with a vehemence so great it is hard to imagine. Compared to their mutual gravitational attraction, the full-blown electrical repulsion would be 10 trillion trillion trillion times larger. But the atoms in ordinary matter do not fly apart violently, because the electron clouds neutralize this repulsion. They possess electrical charges that are exactly equal and opposite to the charges of their nuclei. Thus, in their relaxed states, neighboring atoms coexist benignly, even enjoying a slight degree of attraction brought on by mutually induced distortions in the electron clouds. When atoms are forced together
under such tremendous pressure that the electron clouds begin to overlap, however, some of the perfect screening is sacrificed. A little bit of interpenetration is all it takes for the repulsive force to reappear in just the right amount to resist the pressure.

Thus electrical repulsion is the force that opposes gravity in the Earth and other rocky planets. The feedback in the Earth's equilibrium comes from the fact that the degree of interpenetration—and hence the amount of repulsion—can adjust very precisely to balance the squeezing force of gravity.

But this clearly wouldn't work in a neutron star. In the Earth, the electron clouds overlap slightly. But if the pressure were only 100 times higher than it is near the center of the Earth, the electron clouds would overlap so thoroughly that they would effectively coincide. This is a quantum mechanical no-no called
degeneracy.
If you try to force quiescent electrons to occupy the same space, they will slip out from under your fingers, scurrying away with alacrity to avoid such embarrassment. The effect is remarkable, and it means that motion appears once again as the dominant counter to gravity.

This kind of motion has a very special property. Unlike the motion inside a low-pressure star like the Sun—the motion of heat—the motion associated with degeneracy does not lead to the loss of energy through radiation. Therefore, no nuclear reactions, or any other sources of energy, are necessary to maintain the equilibrium. The speeds simply increase as the matter is squeezed—and that feedback, alone, is sufficient.

It seemed fitting that a neutron star, with all of its other strange properties, should be supported by such. a very strange form of pressure. It isn't the electrons' degeneracy motion that supports the neutron stars in the Crab and Crab II, however. The pressure needed to support the weight of a neutron star is a trillion trillion (1 followed by 24 zeroes) times higher than the pressure at the center of the Earth. The atoms are squeezed 100,000 times closer together than in ordinary matter. At these densities there is no room for electron clouds at all. The atomic nuclei themselves overlap, the electrons having long since been
absorbed into the protons to compose neutrons—hence the popular name of these objects. Neutrons also have the property of resisting degeneracy, and it is their quantum skittishness that provides the main counter to gravity. A more prosaic form of repulsion enhances the motion induced by degeneracy. This is analogous to the electrical repulsion of ordinary matter under pressure, except that this time the force is not electrical but the even stronger nuclear force that binds the chemical elements. A neutron star is a little like the nucleus of a humongous atom, with an atomic weight of 10 followed by 56 zeroes. Compare that to normal matter: Carbon's atomic weight is 12, that of uranium, the heaviest naturally occurring element on Earth, 235. The nuclear force is apparently claustrophobic; when too many nucleons (protons or neutrons) crowd together, it switches from attractive to repulsive. Neutron stars are unique in being the only atomic nuclei held together by gravity.

This visit had given me a lot to think about, but I still felt a nagging frustration. I hadn't been able to plumb the depths of a neutron star in person, so I had had to plumb them in thought. I was not pleased at being forced to adopt such a theoretical perspective when I was so close to my object of consideration and had gone to so much trouble to get there. How much had I really gained by making this long journey? My practical side argued that there was no point in worrying about that now. Equilibrium was such an important aspect of the cosmic constitution that my efforts to understand it really were necessary, whether they took place in comfort on Earth or bucking the harsh, arid wind of a pulsar.

Then I gazed back out through the nebula. In the distance the soft glow of the filaments was dimly visible behind the stark blue synchrotron glare. I could almost sense the cool debris rushing away from me in all directions. I imagined that this was the original Crab Nebula, in its heyday as observed from Earth, not the reincarnation that I had fortuitously discovered at just the right moment. I could appreciate the incredible energy that had been released near this point—first, all of a sudden in the
explosion witnessed in 1054, and then, over the thousand years since, supplemented by the steady braking of the pulsar's spin. It was not the most beautiful of nebulae. But my antipathy toward the Crab Nebula had vanished, to be replaced by respect and a measure of awe. I had explored the concept of equilibrium almost as an afterthought, because there had been so much else to see. The relationship of the familiar-looking filaments to the macabre “bluish glow”; the sensation of crossing through the shock into the inhospitable inner precincts of the rushing wind; the appearance of the pulsar itself. Despite all I had read before about these exotic bodies, no description, had prepared me for their alien character.

I knew that no cosmic object took cognizance of human presence. (I suspected that Earth had only tolerated us and was probably regretting the decision.) But this pulsar, possibly because it had carved for itself such an austere and harsh environment, seemed more indifferent than the rest. Even more indifferent than the black holes of Cygnus X-1 and (perhaps) SS 433, with their warm binary companionship and their sloppy disks of opaque, swirling gas. By comparison to these dog-like attributes, the Crab II pulsar was definitely feline. And in the end I had found this virtually indestructible object—a neutron star—that had emerged out of the violent death of a great star. Maybe it was my own perversity that had led me to explore equilibrium at its extreme outpost, in a body just this side of being a black hole. After all, I could simply have visited the Sun for an example of equilibrium. But seeing equilibrium existing so close to the edge brought home to me how crucial a concept it was. Gravity was a flexible force. It could generate liberating motion, and even shoot jets out into space, or it could clench its teeth and fight the resisting pressure to an immovable draw. It all depended on whether there was the right kind of feedback.

Seeing this endpoint of stellar life also roused in me an understanding that it was not sufficient merely to study how things are in the Universe; we must also investigate how they became what they are. Before there was a Crab Nebula or a Crab II
Nebula with its pulsar, there was an ordinary star that shone, much like the Sun (if considerably more brightly), for at least a few million years and was held up by the heat of its internal gas. Where did that earlier equilibrated globe come from? How did gravity pull its raw materials together in such a way that it staved off final collapse at least for a while? I decided on the spot that I would seek out the opposite end of a stellar life, to find out how material is assembled into a star in the first place. Could I help it if the best place to discover this also turned out to be one of the most charming and picturesque locales in the Milky Way? After thy theorizing I had just been forced to do, I deserved a vacation.