Turn Right At Orion (11 page)

From 30 light-years out. Crab II produced a ghostly effect. It appeared as a vast, dimly shining panel, taking up as much space on my sky as the Big Dipper does on Earth's. Its total luminescence amounted to only one-hundredth the brightness of the full Moon, spread over an area equal to nearly 1600 full moons. Near its center I caught a glimpse of my destination, the neutron star, appearing as any ordinary star of magnitude zero, just as Arcturus, Vega, or Capella. This neutron star was young and still shone brightly.

The nebula, like the familiar old Crab, had an oblong shape that appeared more articulated the closer I approached. In addition to the texture created by the luminous filaments, Crab II had a distinctive architecture. Two deep, rounded indentations cut into the nebula, giving it the appearance of having a waist. I could clearly make out that the constriction was three-dimensional, pinching the nebula all around. The filaments girdling the indentation had a very slightly “off” color, compared to the other filaments, and I likened them to whalebone stays corseting a satin-clad figure. The metaphor seemed apt. The expanding nebula was being held back, though not stopped, by the constriction, while in the perpendicular directions, where it was unconstrained, it appeared to expand freely. For the most part the
filaments seemed to form a random network, but there were a few places where it was hard not to visualize a greater degree of organization. At one place in particular, filaments were arranged in such a way that they seemed to describe a tubular conduit. Nothing seemed to be flowing through it, yet it was hard not to attach a dynamical significance to this sharply outlined decoration, if only as a symbol of what I now saw was a delicately squeezed and shaped explosion.

I was not long to have the luxury of such a global view. Almost without warning, I found myself immersed in the sea of glowing filaments. As I approached the nebula, what had struck me even more than the shape had been the colors. Now I was overwhelmed by the iridescence of the scene. Filaments shone with the familiar rich green of oxygen and the reds of hydrogen, sulfur, and nitrogen, but many more subtle hues could also be discerned. Of course, each time an electron popped from any one orbit in an atom to a lower one, it emitted a very distinctive color, and there were many, many orbits in each type of atom. Also, many of the atoms had lost one or more of their electrons, and each of these needy atoms—ions—had its own assortment of orbits. Thus the array of colors was staggering, and I also noted the ultraviolets of hydrogen, carbon, and helium, the yellow of helium, and the violets of oxygen and neon. The individual colors were all familiar, the stuff of spectroscopy class in grad school. But something seemed odd: The mix was different from what I had come to expect.

The peculiar combination of hues and their relative intensities—let's call it the “spectrum” of “lines,” now that we are going to do something quantitative with it—depends, more or less, on two factors. One is how well or roughly the atoms are treated. The other is the mixture of different chemical elements, or “composition.” The atoms in Crab II's filaments were being disturbed in a couple of ways. Most important, they were being tickled by that bluish glow that I had remarked on earlier. Atoms see light as chopped up into its constituent particles, or photons, which constantly move around at the speed of light
(naturally) and sometimes hit electrons. When that happens, the photon is absorbed and can either knock the electron into a higher orbit, whence it drops back down and emits new photons with very specific colors, or knock the electron clear out of the atom. In the latter case, the precise hues are emitted when the freed electron finds an atom to attach itself to and drops down through a sequence of orbits as it heads for home. The photons that make up blue light do not pack enough punch to jostle most of the electrons out of their preset orbits; as a result, they do little. But it took more than just blue light to give the nebula its garish cast. My eyes settled on the blues because they filtered this light for its visible content, but the true spectrum was a continuum of colors from the radio (which had even less effect on electrons than the blue photons did), through the infrared and all visible colors, and on past the violet into the ultraviolet, X-rays and gamma rays. Tipped as the spectrum was toward the more energetic rays—on beyond violet—there were plenty of photons capable of ionizing and otherwise disturbing the atoms of the filaments. And as though that weren't enough, there was a second mechanism that also seemed to be operating: The atoms suffered collisions with freely moving electrons, or even with other atoms, that also knocked orbiting electrons out of their appointed rounds. Some of this activity came from the chaotic motion of heat, acquired either from the very fast electrons that (I was soon to learn) produced the bluish glow or as the filaments were warmed by basking in the blue radiance itself. The rest of the motion had its origin in the explosive energy with which, the filaments had been shaped and expelled from their point of common origin.

Considering the harsh environment, the atoms inside the filaments of this reincarnated Crab were not treated too badly. The fact that they were not bashed to pieces and completely dismantled allowed them to produce the rich spectrum of hues that I experienced and enabled me to make a detailed study of their properties. By analyzing, comparing, and keeping track of the strengths of all these spectral lines, I could deduce both the nature
of the filaments' excitation and their chemical composition. Here I had a surprise. The reason why the colors made up such a strange mix was that the filaments were overwhelmingly composed of helium, with small admixtures of oxygen, carbon, and everything else. Hydrogen—normally the dominant species—was only 10 percent by weight in the filaments, and it was even rarer in the gas that composed the “corset stays” binding the nebula's waist. That explained why their colors were even stranger than those of the “normal” filaments.

I had never encountered gas with such a weird composition before. Helium is a rare element on Earth, but that's because it doesn't react with anything and most of it evaporated into space during Earth's formation. What little helium can be scavenged on Earth has been produced as a by-product of radioactive decay and trapped underground in pockets of rock. It is more common elsewhere in the Universe. The element was first discovered in the spectrum of the Sun (hence the name, which is derived from
helios,
the Greek word for “Sun”), It makes up about 27 percent of the sun by weight, with nearly all the rest consisting of hydrogen. These ratios were what I was used to. One found them nearly everywhere—except on Earth, where much of the hydrogen had escaped as well. The near universality of the helium-to-hydrogen ratio is easy to understand. It bespeaks the manner in which most of the helium in the Universe had formed: in the crucible of the Big Bang, just a few minutes after time-zero. There seemed to be only one way for this makeup to have been distorted toward the extreme of nearly pure helium: via nuclear reactions inside the star that had exploded. As it turned out, my first encounter with helium-rich gas proved to be only the tip of an iceberg. In other settings, later, I was to encounter chemical compositions far more bizarre that held even more important clues to the great cycle of matter, as mediated by the deep interiors of stars.

I was curious about the nature of the sickly bluish glow that seemed to be everywhere. At first it had reminded me of the glow that sometimes envelops one when one is walking down a street in a thin mist, except that the glare of a mist is all secondhand light, scattered from some other source. In this case there was no light from a nearby street lamp to scatter. This vapor was intrinsically luminescent. The color (as I perceived it) and the sensation of being surrounded by an irradiant medium reminded me of snorkeling in a South Sea lagoon rich with clouds of bioluminescent creatures—they put out a very similar kind of glow, though more blue-green in tint. The medium also had that mottled look one finds in a bioluminescent sea, with striations and bright patches where (perhaps) the medium had been disturbed. Only this wasn't bioluminescence. As I sampled the medium that occupied the spaces between the filaments, I saw that its active ingredient, or at least one of them, was an extremely hot gas of electrons.

The glow, however, was not at all the radiation of a typical hot gas. No matter how I plotted and parsed its spectrum, I could not get any indication of temperature. It seemed to have all temperatures and no temperature at once. I remembered that I had seen a similar glow near the Galactic Center's black hole. I
measured the speeds of the electrons and found that they were charging around at so close to the speed of light that at first I had trouble telling them from photons. Rather than traveling in straight lines, though, they were executing tight gyrations. I knew immediately that the agent of this motion had to be a magnetic field and that the glow had to be what my colleagues called synchrotron radiation.

A quaint name, “synchrotron radiation.” It referred originally to an antique type of atom smasher. When an electrically charged particle moves through a magnetic field, it is deflected from its straight-line path onto a circular path or helix. Early particle physicists found this circular motion convenient, because it provided a way of keeping fast-moving particles confined to their experimental apparatus and stopped the particles from crashing through the walls and doors of their labs. In the device called a synchrotron, an alternating electrical Impulse was synchronized with the circular gyrations in such a way that the electrons were gradually accelerated to speeds approaching the speed of light. But the experimenters began to encounter problems. A gyrating particle emits light. And when the electrons got really close to light speed, the physicists found that most of the energy they were pumping in through electric fields was coming straight out again, in the form of a glow that they called synchrotron radiation.

Thus synchrotron radiation was originally an investigator's nuisance. But it was a key to understanding Crab II, because when I measured the totality of the bluish light, I found that it, not the sharp hues of the spectral lines, provided the dominant illumination of this nebula. I also saw that the glow got stronger and harsher, the closer I moved toward the neutron star. Now I was weaving my way through the thicket of filaments, trying to avoid colliding with the denser gas. Essentially all of the matter was concentrated in the filaments. This was debris from the exploded star, and it had to have enough bulk to account for several solar masses of matter. In contrast, the glowing medium between the filaments was a far better vacuum than any region
of interstellar space I had sampled so far. I wondered why this super-hot matter didn't just dodge between and around the filaments and escape into the surrounding space, which, after all, seemed to have little means of hemming it in. What I saw instead were bubbles of the hot fluid bulging up against the filaments or wrapping around them, but then being held back as though by some elastic membrane. The strange, springy behavior of magnetic fields came to mind, as it had when I visited Cygnus X-1 and tried to fathom how the swirling gas managed to lower itself toward the black hole. I knew the magnetic field had to be there, because it was a necessary ingredient for the generation of synchrotron radiation. I also knew how to make its structure visible. Through polarized lenses I could map out the lines of force. The magnetic field here was indeed the agent that turned this luminous near-vacuum into a kind of jelly, its springy, membranous quality permitting the network of filaments to act as a cage with few bars.

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?