Looking out the window once again, I traced the clouds’ silhouettes by the stars that peered around their edges. For the most part, these stars were fairly distant, some even not too far from the Sun. To one side, though, a modest cluster of medium-bright stars stood out. These were nearby, and from their temperatures and luminosities I could tell that they could not be much older than 10 or 12 million years. This was commonly thought to be the first association of bright stars that had condensed out of the Orion molecular clouds; in fact, their brightest members were already gone—burnt out. The present-day fireworks of the nebula surrounded even younger stars, and they were nowhere to be seen. The Trapezium cluster, the other hot and massive stars, the glowing sheets of gas—they all had to be on the far side, the side facing the Solar System. Of course I had modern navigational aids, so I knew where I was headed. The hidden stars were there, all right, their presence manifest in the blotches of infrared luminosity—patches of heated dust—that I now brought up on my screen. There, unmistakably, was the outline of the cavity blasted out by the Trapezium, the illuminated walls of which composed the Orion Nebula itself. A second warm glow, I knew, must be the cluster of newly formed stars that went by the utterly unromantic name BN-KL, after the initials of its discoverers. This grouping was obscured to the eye both from my present direction and from Earth’s point of view, and I imagined it as occupying its own cozy niche completely surrounded by the insulating and opaque molecular gas, a small cabin in dense woods.
Navigating by dead reckoning on the twin glows caused by the Trapezium and BN-KL, I plunged into the molecular cloud. My immediate gut feeling was to wish I hadn’t. There is little so nerve-wracking as flying through a molecular cloud at high speed. As often as my technology performs flawlessly, there is always the slight doubt in the pit of my stomach that Rocinante’s protective shielding will hold. Early airline passengers must have felt the same way. It’s not the speed that bothers me; it’s all that stuff coming at me at some tiny fraction less than the speed of light. I’m always aware how much of a punch it packs. I suppose the feeling is compounded by the fact that it is hardly possible to see anything because of all the dust.
To spare you from sharing my anxiety, let me tell you a little bit about how Rocinante’s shielding works. It should come as no surprise that when I zip through interstellar matter at high speed, it doesn’t see me coming. Of course, I don’t mean “see” in the sense of perceiving my running lights. As fast as I can travel, light always travels faster and can run ahead. What I mean is that there is no mechanical warning of my proximity, no shove that tells the undisturbed gas to get out of the way before I arrive. My craft packs an incredible sonic boom, sweeping up everything in its path with a tremendous shock wave, A sheath of superheated, superpressurized gas is thrown up against Rocinante’s skin, and it is against this that I need protection.
The pressure is not the main problem. It does increase stiffly as I approach the speed of light as measured by my Shangri-La factor, the ratio by which time passes more slowly in my craft than on Earth. For each doubling of this factor, the pressure quadruples. But interstellar matter is so sparse that even with these huge amplifications, the forces are easily parried, provided certain precautions are taken. When I traveled to the Milky Way’s center, for example, I avoided all regions filled with more than one hydrogen atom in every cubic centimeter. At my peak Shangri-La factor of 13,000, I faced maximum pressures of barely 1 Earth atmosphere—easily withstood. My trip to Orion was rather leisurely, by comparison, because I had only to move 5000 light-years or so, and my Shangri-La factor never exceeded a few thousand. By the time I entered the molecular cloud, my craft was well into its deceleration phase and the pressures encountered were truly negligible, even given that hydrogen concentrations as high as 1000 per cubic centimeter, or more, were unavoidable in this comparatively dense environment.
I am more worried about Rocinante’s skin getting too hot. To particles hitting it at speeds within a hair of the speed of light, my vessel’s skin is as porous as a sponge. Oncoming electrons and ions could penetrate to depths of many centimeters (meters, even!), which would not pose a problem (Rocinante has a thick skin) if only they didn’t also deposit all their enormous energies subcutaneously. My main defense is a powerful magnetic barrier that deflects the oncoming particles before they hit. Unfortunately, no magnetic shield is perfect, and on numerous occasions I have watched anxiously as blobs of plasma pierced the force field and struck home. The shield is also helpless to keep out particles of dust, and these have presented a steady, though lighter, onslaught. The surface of my craft, warming until it could radiate away the frictional heat, would reach temperatures of tens of thousands of degrees. No hard material—not even the ceramic of which Rocinante’s shell is constructed—can survive at such temperatures, and I have watched nervously as patches of Rocinante’s skin vaporized. I have had nightmares of my entire craft being eaten away, turning into a metallic/silicate steam and being sloughed off into space. But as you see, I am still here. What has saved me is that the evaporated ceramic forms an insulating layer. Rocinante’s shape, and the play of pressures across it, holds the hot vapor in place; the layer of vaporized spacecraft skin, in turn, bears the brunt of the frictional heating and returns most of it to space with a searing radiance.
Rocinante continued to decelerate as we neared the Trapezium. I knew I was getting close to the cluster’s illuminated cavity because conditions outside my craft had changed. Inside the molecular cloud I had encountered a ubiquitous infrared glow—detectable only with the correct viewing apparatus—that was created by warmed dust. Like any radiation emitted by a solid material, the color of this radiation revealed the dust’s temperature. Now I noticed that, after an interminable stretch of dull sameness, the temperatures were starting to increase, the glow tilting toward shorter wavelengths. I was nearing a source of heat. Off to one side I saw a much hotter area, a few hundred degrees above absolute zero—the temperature of ordinary objects on Earth. I was passing by the BN-KL cluster, still hidden by dust. Straight ahead the gas seemed to be thinning slightly and warming still more. The Trapezium lay there.
Some turbulence, and a few roller-coaster swells, told me that I was crossing into a new zone. This transition was not the sharp shock I had been expecting. I knew that the massive young stars of the Trapezium emitted winds that sped outward at 2000 kilometers per second and carried nearly as much power as the stars emitted in light. When these winds hit the wall of dense gas that lay between me and the star cluster, they pushed on it with uncompensated force, compacting the exposed layers into the cold substratum through which I was now passing. At the same time, the intense ultraviolet rays from the stars fried the cloud wall, destroying the molecules and increasing the temperature 100-fold. The evaporation of this heated layer would have increased the pressure at the cloud surface still further, helping the winds plow their way into the dusty cloud. These were classic conditions for a shock wave: a piston of gas pushed into unsuspecting, cold matter, setting it suddenly into motion with an accompanying increase of temperature, pressure, and density. Such transitions were usually so sharp that as ] had neared the expected location I had said to myself, “Don’t blink”—I didn’t want to miss it. I also gritted my teeth for a single, sharp jolt. But the transition turned out to be gradual, so gradual that I had time to analyze it and find out why.
The seemingly monolithic, gray medium I traversed had already revealed itself to be quite complex. All kinds of particles were present, and all were in motion. The molecules, of course, dominated. They, and the occasional single atoms, danced from collision to collision in straight lines, changing direction at random only as they bumped against one another. Their encounters often seemed amusingly like a square dancer’s do-si-do, as the molecules looped around one another and atoms sometimes exchanged electrons gratuitously as they passed. Grains of dust—a trillion times heavier than a molecule—executed gently curving paths, seemingly obliv
ious to the frenetic small-scale action of the molecules. As they passed, the grains tweaked my electromagnetic sensors ever so slightly, indicating that they carried a slight electric charge. It was the charged grains’ motion in the weak magnetic field, which I also detected, that curved their trajectories.
I puzzled over why the grains should be charged at all. Too few ultraviolet rays penetrated this far into the cloud to tear even a handful of electrons off the grain surface. In any case, that would have given the grains a positive charge, whereas they appeared to be negative. They must have acquired a few electrons, not lost them. Could they have acquired their charge by friction, like the static electricity on a rubber rod stroked by fur? just then I noticed another component in the mix. In addition to molecules and whole atoms, there was a tiny admixture of ionized atoms and freely flying electrons. Curiously, the ions were not primarily those of the ubiquitous element hydrogen but were mainly derived from the much rarer carbon. Focusing on the electrons, I recalled hearing how they could charge up a big obstacle like a dust grain. Because they were lighter than ions, they moved much more quickly and thus hit the grains more frequently. If only a few of them stuck, that’s all it would take to give the grain a negative charge. I smiled at this tortuous chain of reasoning all adding up to the gentle swing of the grains’ trajectories. My instinctive first thoughts of fur, rubber rods, and static electricity no longer seemed so far-fetched, Using friction to remove electrons, after all, was no stranger than using random collisions to acquire them.
The very presence of freely flying electrons and ions, however, posed another mystery. How did they get here? I was still deep in the molecular cloud, well shielded from all nearby sources of ultraviolet radiation. The collisions between atoms were too gentle to knock them apart, but some agent had to be doing it. I gradually began to perceive yet another ingredient in this rich stew of particles. A tiny, tiny fraction of the ions and electrons were whizzing through the cloud with enormous random speeds almost indistinguishable from the speed of light. They were moving so rapidly (like the particles swept up by my craft at high Shangri-La factor) that they could penetrate the entire cloud. My colleagues called these cosmic rays, and I had encountered them before, in open stretches of the Galaxy. I was at first surprised to see them here, but what was to stop them from penetrating into every nook and cranny? These were the culprits that could collide with carbon atoms so forcefully that they knocked off an electron or two. But why carbon, rather than the much more common hydrogen? That was simple. Carbon held on to its outermost electron more loosely than did hydrogen. Easier to ionize, I recalled.
I could now see why this transition, from quiet cloud interior to raucous surface layer, was so gradual. The crushing impulse from the surface of the cloud was not being carried equally by all the particles. Near the cloud’s surface, where everything was ionized, the motions of nearly all components of the gas were heavily regulated by the magnetic field. In the presence of a magnetic field, charged particles—such as ions and electrons—are thrown off their straight-line paths. The magnetism forces them into gyrations, and it is all they can do to spiral up and down the magnetic lines of force, wrapping coils around them like a Slinky. This means that it is the magnetic field that receives any impulse of momentum carried by the ionized particles, and it is the magnetic field that transports this impulse deep into the molecular cloud.
But there’s the rub, literally. Deep inside the cloud, few of the particles are charged. Molecules and atoms abound, but they are not ionized and therefore are not affected by the magnetic field. Therefore, the magnetic field has trouble transmitting its impulse to the cloud’s interior and passing it on to the particles there. The few electrons and ions, gamely tied to the magnetic lines of force, are the keepers of the cloud-crushing impulse. Occasionally, an ion collides with an atom or molecule or merges with a suitable electron and joins the ranks of the whole atoms. Only then does it give up its part of the cloud-crushing force and signal to the cloud’s interior that powerful events are taking place nearby. This is a painstaking, gradual process and hence a gradual transition, collision by collision, from cloud to cavity.
Finally, I had passed into the outer layers of the cloud. There was no question now that my environment was under direct influence of the still-obscured stars. As I scanned my radio and infrared sensors, I could tell that the composition of my surroundings was changing. The largest, most fragile molecules had all but disappeared, leaving mainly robust carbon monoxide and molecular hydrogen intact. Then these gradually vanished, knocked apart by a combination of more violent collisions (the result of steadily increasing temperature) and the gradual increase in penetrating radiation.
The infrared glow ahead of me brightened. Then visible light, at first with a reddish cast and then successively melting into yellow and blues, bathed my craft with ever-increasing intensity. My image of the four bright Trapezium stars grew blinding, as I moved closer through the veils of dusty haze. The cluster’s lopsided quadrangular pattern spread across a larger and larger portion of my visual field. Now the gas around me was visibly fluorescing, with its mix of atomic spectral colors. One final layer, an intense field of the pink light of hydrogen, and then an unbearable ultraviolet glare swept over everything, and I emerged from the cloud. I was in the cavity of the Trapezium.
15
Trapezium
If one tried to draw obvious comparisons between the environment of the Trapezium and that of the cluster at the center of the Milky Way, the former would be found wanting. First, one would have to imagine away the big black hole—there is none in Orion. Only four hot, massive stars made up the bright core of this cluster (I later found out that the nearby BN-KL cluster was richer in this regard), a far cry from the thousands I found in the Galaxy’s center. And the stars here were sauntering about at measly speeds no greater than a few kilometers per second, compared to the hundreds of kilometers per second (influenced, of course, by the black hole’s gravity) at which they move in the Galaxy’s nucleus.
But here in Orion, there were contrasts and stark juxtapositions of structure that, in certain respects, surpassed those in the center of the Milky Way. I emerged through the ionized cloud wall at its closest point to the cluster, less than a light-year from the brightest star in the Trapezium. From this distance, 10,000 times farther than the Earth is from the Sun, the lead star was a pinpoint only 20 times brighter than a full Moon, and the other three Trapezium stars were considerably fainter than that. Yet unlike the Moon (or the direct light from the Sun, for that matter), these stars emitted most of their light in the ultraviolet part of the spectrum. The penetrating glare (even with shielding in place) was hard to take, and I quickly skimmed along the cloud wall to get out from the narrow gap between the star and the molecular cloud.
I now took in the scene from a more comfortable vantage point. The quartet of bright stars seemed to float in front of an endless wall of glowing pink. The hydrogen atoms producing this light were being dismembered by the impacts of ultraviolet photons, only to recover their electrons quickly and then have the process repeat itself almost immediately. Minor shadows in the ultraviolet bath, created by clumps of dust and indentations in the wall, were amplified by the atoms’ sensitive response to light, creating a three-dimensional mottled appearance of curtains and billowing waves. In many places within the cavity and along the wall, the gas was set into motion, with ripples and shock waves creating their own light show of disturbed ions and atoms, in an array of colors. All of this had been visible from Earth. What had not been apparent was that the Orion Nebula was, for the most part, just a thin veneer lying behind the Trapezium. The entire depth of the pink-glowing screen was merely a sixth of a light-year, and behind it lay the vast, dark molecular cloud I had just traversed. I tried to orient myself in order to pick out features of the nebula that were familiar from Earth. I could visualize its appearance in a telescope as resembling a folding fan, its boundaries feathery and indistinct along a th
ird of a circle, where the accordioned paper was unfolded, but angular and almost straight along the two enclosing arms. With considerable difficulty I deduced that one of those arms was a sharp, dark boundary, the silhouette of the foreground molecular cloud. The other, a bright bar that one could pick out by eye with even a small telescope, was apparently an illusion, a fold in the glowing pink sheet that observers on Earth happened to see edge-on.
As I moved farther away from the irradiated wall, in the direction of Earth, I could see that the nebula was nestled in the crook between two dense clouds, both of them cold and heavy with molecules. On the side of the cluster toward the Earth there was little molecular gas, but I was still not in open interstellar space. The cavity surrounding the Trapezium cluster was tenuous, the product of a multiplicity of colliding stellar winds. Only wisps of luminous nebular gas survived in this region—in most of the volume the gas was too hot. But a thin, dense shell, consisting of a mixture of atomic and ionized gas, surrounded the cluster on the sides not bordered by the molecular walls and gradually expanded away from it.
I suddenly realized that the Trapezium cluster was more than just a convenient light source that happened to illuminate the molecular cloud and make it a nice sight for amateur astronomers on Earth. It played an integral part in the fate of the molecular cloud. It owed its existence to the cloud. And, ungratefully, it was doing its best to destroy the cloud. The lid of atomic gas on the Earthward side of the Orion Nebula was being pushed away into space by the radiant heating and fast winds of the Trapezium stars. The same processes were evaporating the sheet of ionized gas overlying the molecular cloud. I ran the movie backwards to visualize what the scene must have looked like in the past—say, a million years ago—and realized that the Trapezium was not nestled into its cloudy nook by chance. Rather, it had created its nest by eroding the molecular gas around it. A few million years ago it would have been surrounded by the molecular cloud, completely embedded in it and invisible except for its infrared signature, much as the BN-KL cluster was now.
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