At less than a light-year from the neutron star, I had left the cage of filaments behind. It was all non-thermal glow here, and the intensity was becoming almost unbearable. Rocinante was being bombarded increasingly by X-rays and gamma rays, which indicated that I was nearing the source of the super-fast electrons. I needed no more confirmation that the ultimate energy source of this glare was the neutron star itself, but I got more confirmation nonetheless. I saw now that this luminous medium was really a wind, emanating from the neutron star and spreading out predominantly along a plane that—I was later to learn—marked the equator of the neutron star’s rotation. Despite the intensely energetic nature of this environment, I saw strangely beautiful and delicate structures. In the exact central plane of the wind there was a very thin sheet that was especially luminous. The sheet was rippling like the surface of a pond, concentric swells coming outward at me one after another. These ripples were traveling at about a third the speed of light, but the distortions of space and time predicted by Einstein’s theory made them seem to approach even faster and produce a whiplash effect as they passed. Well above this sheet, in a direction that I surmise lay above the neutron star’s polar axis, were sprightly little flares, like St. Elmo’s fires at the top of a ship’s mast, dancing amid jets of nearly invisible plasma.
Just a couple of tenths of a light-year from the neutron star, I crossed some sort of boundary and the character of my environment changed completely. I was no longer immersed in the glowing medium, yet I could see it everywhere I looked. I was apparently inside a bubble with a luminous wall. The wind that now surrounded me was no longer radiant, but it was incredibly fast. Its speed was within one part in a trillion of the speed of light, and I saw that it also carried a magnetic field. But the electrons sweeping past me were not emitting much synchrotron radiation, because they were hardly gyrating in this field. Instead they seemed to be moving along with it. It was only after they plowed into the luminous wall—which I now saw was a shock wave—that their orderly motion turned to chaos and the garish light streamed out.
The wind was powerful. It carried 100,000 times the power of the Sun, all of it coming from what still appeared to me as a bright point of light in the distance. This neutron star somehow powered the luminosity of the entire nebula—wind, blue glow, filaments, everything. But how?
From this distance the neutron star appeared about as bright as the full Moon, which meant that it was shining with the luminosity of several hundred suns. It was emitting much less light than it was pumping out in the power of its wind. But if it were only 20 kilometers across, it would have to be very hot all the same: 6 million degrees, if the light leaked out from its interior like a normal star. I searched for the confirming signature, the X-ray “color” that goes with such a temperature, but found that the spectrum of colors was bizarre. In addition to X-rays, I saw far too much ordinary visible light. The radio waves coming from this thing were even more remarkable; if I had measured them alone I would have concluded that the temperature exceeded a trillion trillion degrees!
The pattern of light coming off the neutron star was also very weird. Unlike that from an ordinary star, the light wasn’t coming off evenly in all directions. As I shifted my position away from the plane of the wind, the brightness increased, then decreased, then increased again. Somehow the star’s light was concentrated into parallel bands that circled the globe like bands of latitude. I recalled that when they were first discovered, neutron stars had briefly been known as LGMs for “little green men,” and for a moment I imagined that these bands of light could be artificial. Were they perhaps navigational aids? Then I remembered the real reason for the early wild speculations about these objects: the pulses. Of course! The neutron star of Crab II was a pulsar. It was a relatively fast one, spinning out two oppositely directed, hollow beams of light 30 times a second. When I hadn’t looked too carefully, the bands of light had appeared steady. But with foreknowledge and some imagination, I could tell that I was being strobed. I headed in for a closer look.
This time I knew my limitations in advance. Given that this neutron star’s mass was about 1 ½ times that of the Sun (if neutron stars whose masses had been measured—those in binaries—could be relied on as a guide), I knew that I could get slightly closer than I could to Cygnus X-1 without turning my insides out. (Whether I could have ventured closer to SS 433 I will never know, because cowardice had gotten the better of me there.) Accordingly, I parked 6000 kilometers away from the pulsar, carefully positioning myself out of the blinding glare of its gyrating beams. From this distance the neutron star was half the size of the full Moon as viewed from Earth and could be comfortably observed with binoculars or my small telescope.
I had half expected to see a mirror. Because of the enormous gravity, neutron star surfaces had to be exceedingly smooth. Any “mountain” higher than a few millimeters above the surrounding plain would either melt and drizzle away or crack through the crust beneath it and sink into the mantle. Scaled up a thousand times to Earth size, this would be equivalent to there being no peaks higher than 10 meters above sea level: The whole Earth would be topographically flatter than Florida. Also, because of the extreme densities—thousands of tons per cubic centimeter right up near the surface and rapidly increasing with depth—the matter in the crust of a neutron star would exist in an odd state. The electrons would be quite fluid with respect to the atomic nuclei, never quite being able to decide to which one they belonged. This could be a recipe for a metallic surface, or at least a good electrical conductor, or perhaps a crystalline substance—I could never quite work it out from the treatises I had read. I had visions of seeing the background stars, or the pink and green glowing filaments of the nebula, or (in my more whimsical moments) my own face reflected in a shiny globe, distorted to bizarre proportions by its convex shape and the bending of light rays in its intense gravitational field.
Thus it was a bit of a letdown to discover that any visible characteristics of the neutron star’s surface were washed out by its heat radiation. The ball glowed brightly in X-rays, though with an output not nearly so great as that of the pulsed radiation. I estimated the temperature of the glowing sphere to be a few hundred thousand degrees. I knew that this was not the radiant energy from nuclear reactions; the star was dead from that point of view. It must have been the residual heat from its cataclysmic birth, leaking out from its interior. This made sense—the neutron star had existed for less than two thousand years.
I could see from here that the pulsing radiance had little to do with the stellar surface. The fireworks were all occurring much farther out, most of the action being concentrated about 1600 kilometers away from the neutron star. At 160 times the radius of the star itself, this was really quite far away. If one scaled up the neutron star to Earth size, the activity would be located at three times the distance of the Moon. At the dizzying rate of the pulsar’s spin, the active zones blurred into shimmering bands of light, but with a series of snapshots I attempted to freeze the action and see what was going on. The site of activity was still indistinct, but most of the luminosity appeared to come from a pair of oval rings that lay above two bright spots on opposite sides of the neutron star and rotated with the star as though tethered to it. The rings were not steady and well defined; they danced, flickered, and changed shape, sometimes brightening and at other times nearly fading from view. They reminded me of the aurora borealis, not the ground-based impression one gets of vast luminous curtains of pink, blue, and yellow being ruffled by some heavenly wind, but the view I once had from above, from a satellite orbiting the Earth. Fast particles—cosmic rays—streaming down from the Sun into the upper atmosphere, were channeled along magnetic lines of force and converged in a bull’s-eye encircling the Earth’s magnetic north pole. The aurora formed where the particles hit and disturbed atoms in the upper atmosphere.
This analogy was obviously imperfect. With difficulty I could trace flares and streamers of light connecting the rings to
the spots on the surface of the neutron star, which were a little bit hotter than their surroundings. Frequent discharges—sparks—erupted. In the pulsar the particles seemed to be coming from near the surface, or perhaps they were being created spontaneously by intense electrical disturbances in the space between the star and the luminous rings. And these particles were being flung outward, toward and through the rings, not streaming in from space as in the aurora. In one respect the two systems were similar, though. The pulsar emission was coming from near the neutron star’s magnetic north and south poles.
Where did the magnetic field come from and why was the pulsar spinning so fast? The second part of this question was the easier one to answer. This object was so dense, so compact, that it must have formed through the collapse of a much larger object. I knew the theoretical story, how the core of a massive star must have lost its resistance to gravity and fallen in on itself. But in this case the details hardly mattered. The Sun rotates, every star rotates, and if a rotating object shrinks, it must rotate faster. Conservation of angular momentum. The figure skater drawing in her arms to spin faster. (How many times had I heard that analogy, but was there a better one?) For the magnetic field, the process was more complicated. Stars also have magnetic fields and are good electrical conductors. If a star shrank and the magnetic field failed to keep pace by becoming stronger, then one could calculate that an electrical force would develop and grow. This would drive electric currents, and the currents would strengthen the magnetic field. So the magnetic field would have to grow, whether the star liked it or not.
The analogy that had served me well in my efforts to understand the disk in Cygnus X-1 also worked here. Magnetic lines of force in stellar plasma were like colored stripes in salt-water taffy. They stretched if the star stretched. They twisted if the star twisted. If the star shrank, the stripes would also shrink and cram closer together—voilà, a stronger magnetic field. But that may not be the whole story. I hadn’t actually witnessed the collapse that created the Crab II neutron star. What if it hadn’t been perfectly symmetrical? What if pockets of trapped heat had forced their way to the surface in the form of bubbles and rising blobs? Then, for a brief time before it settled down, the interior of the collapsed star could have been a churning mess of stretching and swirling fluid. The taffy analogy applied once more: The field could have strengthened in this way, too.
Whatever events had led up to the present condition of this neutron star, its extraordinary attributes were undeniable. Its magnetic field was enormous. A trillion times larger than the terrestrial magnetic field that sufficed to channel the particles of the aurora borealis and align the compasses of seafarers. Billions of times larger than the magnets that used to hold notes to my refrigerator back on Earth. And this magnetic field—a gigantic bar magnet embedded in a 20-kilometer-wide bowling ball—was being spun around at 30 times a second. Out 1600 kilometers from the neutron star, the lines of force had to swing around so fast to cover the circumferential distance that their speed approached the speed of light. At this point the difference between magnetic and electrical forces became blurred, fast particles no longer remained under control in the neutron star’s sphere of influence, and—not to put too fine a point on it—all hell broke loose. Particles, electromagnetic energy, and radiation streamed forth, the first two ultimately destined to power the bluish glow, the filaments—the entire nebula.
Still, I wasn’t completely satisfied. What was the ultimate source of this power? Unlike Cygnus X-1, it couldn’t be the strong gravity of the neutron star, at least not directly. No matter was spiraling toward the star, a requirement if gravity were to be relied on to generate energy. The magnetic field, strong as it was, wasn’t strong enough to power this 100,000-fold solar luminosity for 2000 years. It was at best only an enabler, a conduit through which some much more abundant active agent flowed. Only one viable power supply remained: the energy stored in the neutron star’s spin. This was exactly the deduction that the Cornell astronomer Tommy Gold had made in 1968, establishing beyond any reasonable doubt that pulsars were spinning neutron stars. I was able to repeat his experiment on the spot. If the energy were stored as rotation, then as the pulsar used up this energy, expelling it into space, it should spin more and more slowly. And this is exactly what it did. Every year, its spin period increased by 1 part in 2500, so that in a couple of thousand years it would be spinning only 10 or 20 times a second. As it dragged its magnetic field around less and less violently, its expulsion of energy should slow down as well. No wonder the aged Crab Nebula that I had first visited, though bigger, was a dimmer, drabber place.
Thus the Crab 11 pulsar, and the Crab Nebula’s pulsar before it, were huge and powerful flywheels. But a flywheel is just a repository for energy. Some motor must have spun it up. In this case, that motor was no mystery. It was the gravitational pull that had collapsed the star in the first place. The accelerating spin of the skater comes ultimately from the pull of her muscles, as she draws her arms inward. It takes calories to spin up. the spin of the star comes from the pull of gravity. It takes contraction, inflow, or accretion to spin up. I was back to gravity again. Maybe it was the key to everything, after all.
I was nearing exhaustion. I had had no idea that the environment of a neutron star could be so complicated. Working my way inward, I had encountered layer after layer of astonishing phenomena, each one more exotic than the last. The pulsar was in some ways the most extraordinary thing I had seen on any of my excursions. Nothing was quite so bizarre as the horizon of a black hole. But in the case of the black hole, I could at least fathom a connection between the supply of matter and the output of energy, even when its details were obscured, as they had been in SS 433. By contrast, the pulsar was so “clean” that it seemed almost magical. It was nothing like a furnace; no fuel was being consumed to make it shine. Its spin, acquired long ago from energy extracted by gravity and set aside during its sudden collapse, empowered it to shine as a lighthouse beacon for thousands of years. This energy supply was turned into a scorching wind and harsh radiance through mysterious force fields that seemed to act over great distances, their pulleys and levers masterfully concealed from view.
12
The Ends of Equilibrium
My universe seemed to have grown another notch more complex, and still I hadn’t accomplished my mission. Witnessing a pulsar was not the ultimate objective of my quest here. Gravity hadn’t triumphed utterly in the Crab II pulsar, or in the old Crab pulsar, for that matter; there would be a phase beyond the spinning flywheel. Once the pulses slowed down, after the rotational energy had been spent, even if some day the magnetic field died away, the neutron star would still be there. It was a body that had imploded to within a few kilometers of sinking beneath its own gravitational horizon and becoming a black hole but had stopped before it had gone too far. A body that thumbed its nose at gravity despite having no internal source of nuclear or other power to keep it hot. Yet here it was, as stable as a rock, and certain to remain so indefinitely.
I needed to find the origins of a neutron star’s equilibrium. That was why I had come to the Crab Nebula in the first place, although so much had happened since my arrival that I had almost lost sight of this goal. I wasn’t even sure what I meant by equilibrium. An equilibrium involves stasis, persistence, something approaching permanence, but is it necessarily static? Did the well-behaved, nearly circular orbits of stars around the Galaxy’s center—or, for that matter, the orbits of planets around the Sun—constitute equilibria? I supposed so, in the sense that there was a balance of forces—gravity against centrifugal—or, more precisely, a simple and steady kind of motion, an acceleration, that resisted gravity’s efforts to draw each star toward the Galaxy’s center of mass. But in another sense, the answer was not so clear. There could be a structural equilibrium that was static: The shape of the Milky Way’s disk, for example, sketched in by the trajectories of all the stars that shared the generic similarity of executing circular orbits i
n the same flat expanse. The motion, of an individual star on an orbit was not an equilibrium of this kind, because the star’s position, around the circle changed with time.
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.
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