The Crab had to be my next destination, for one reason. Near its center lay a neutron star, the remnant of a star whose explosive demise had been witnessed on Earth in A.D. 1054. In theory, a neutron star resembles black hole as closely as a body can without actually becoming one. Somehow its collapsing precursor knew how to stop gravity in its tracks, to create the tightest possible form of equilibrium. It weighed more than the Sun, but it was only 20 kilometers across. I needed to see what it could teach me.
Despite my misgivings about the visit, I took some consolation from the fact that the Crab Nebula, or its precursor, had once known true glory. Before A.D. 1054, no Earthly astronomer could have seen the Crab Nebula (even if telescopes had been invented) because there was no such object in the sky. But for a few weeks during the summer of 1054, this shattered orb outshone any of its compatriots and would have surpassed Venus as the beacon of the predawn horizon. Unbeknownst to our medieval ancestors, a star had already exploded some 6000 years earlier, in the direction of Taurus. Because the explosion had taken place 6000 light-years away, the evidence of this star’s demise did not burst forth upon earthly skies until the evening of the fourth of July 1054, whereupon the Chinese court astronomers duly took note. The Chinese had provided such detailed records that by 1921, astronomers believed they knew the location of this “guest star” and noted its proximity to the well-known nebula. That same year, it was discovered that the Crab Nebula was expanding and apparently had been doing so for nearly a millennium. It didn’t take long for astronomers to put 2 and 2 together.
I was so fascinated by this rare convergence of historiography with astrophysics that I didn’t notice my entrance into the nebula itself. This surprised me, because I had expected a strong jolt as I crossed from the undisturbed surroundings into the zone occupied by the outward-rushing debris. As I had learned during my very first traversal of the Milky Way, no star lives in a vacuum, and when a star is audacious enough to explode, it must push its surroundings out of the way. At typical explosive speeds (covering 1500 kilometers every second, if not more), the quiet gas that envelops the star cannot anticipate what is about to befall; the blast overruns it without warning. Like the famous horror film in which decent citizens are overcome by the lumbering swarm of zombies and then become zombies themselves, the swept-up atmosphere is shocked into sudden motion and inexorably becomes part of the blast, plowing in turn into its surroundings and infecting them with unstoppable motion. The signature of such a shock wave is a sharp increase in pressure—this is exactly what a sonic boom is—and, often, the radiance that accompanies flash-heated or disturbed gas. I must have crossed the shock, I thought; the fact that I had felt nothing perplexed me.
Not that the existence of a shock had actually been demonstrated by my observer-colleagues on Earth. The environment of the Crab Nebula had been regarded as one of its deeper mysteries. Astronomers were always looking for its “shell,” and not merely for the purpose of justifying its name. Optical, radio, and X-ray observations—any of these should have given evidence of the shock—all came up empty. When the nebula’s mass had been toted up in terms of the gas that one could easily “see,” it fell short by at least four times the mass of the Sun. Only stars more massive than a certain threshold were supposed to explode like this, and the four (some said six) missing solar masses constituted the difference between what was seen and what the theorists expected. Some conjectured that the “shell” was not really part of the explosion but, rather, consisted of slowly moving gas that had wafted off the star’s surface during the eons before it blew up, when it was not a hot star but a cool red giant. They contorted their reasoning to find ways in which this effluence could somehow shield itself from disturbance to the point where it was invisible. Others said that the shell was there, and was actually rushing out at speeds several times greater than the expansion speed of the observable nebula, but that it was too faint to see. Why this should be so was anybody’s guess. To me, the most plausible explanation was that the Crab lived in very sparse surroundings, like the interior of one of those hot stellar bubbles, superbubbles, or chimneys I had seen during my trip to the Galaxy’s center. Such bubbles could be blown, over millions of years, by just the sort of star that the Crab once was. Maybe the Crab-star had evacuated its own neighborhood, and now there was little for its debris to run into.
10
Crab II
I was confident that I would soon encounter the familiar nebula, but as I continued onward without finding a trace of the exploded star, I became worried. The space around me seemed eerily empty. Debris in any form—if compacted into dense enough clouds—would have been hard to detect (assuming I didn’t run into it!), but not so the luminous features that were easily visible from Earth. As I searched in vain for a landmark, it dawned on me that the reason for this desolation was profound. I was not in the Crab Nebula I remembered from Earth; I was in its ruin. I was disheartened, though I should have known. Ninety thousand years of continuous expansion had sapped the nebula’s vitality. The debris from the star was now well mixed with ordinary interstellar matter, and its explosive energy had spread over a volume thousands of times larger than the historical Crab Nebula.
I knew there would still be a neutron star to study, if only I could locate it. I looked around—there were hundreds of faint stellar-looking objects that could have been candidates. At this age, the neutron star should have been very faint at all wavelengths except perhaps the radio band, where it might have shown up as a slowly winking pulsar.
I was debating whether to commence a half-hearted radio search or to give up entirely when I remembered an event that had made an impression on me as Rocinante approached the location of the Crab. I had spotted a new stellar explosions, off to starboard, and had noted in my journal that it exhibited many features in common with the sort of explosion thought to have produced the Crab. As I moved through the Galaxy, I could tell from its changing displacement against the background of more distant stars that it was less than a thousand light-years away. The remarkable coincidence had pleased me. The chance of a second such, explosion, this close in time and space to the first, was miniscule. Now I viewed it as a godsend. What excellent luck! I could be there in less than a thousand years of Galaxy time and, if it did prove to be another Crab, experience the familiar nebula as though it had been reborn. I set course immediately for the nebula I dubbed Crab II.
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 sy
mbol 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.
11
Strange Light
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 glo
w 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.
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