Turn Right At Orion (15 page)

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.

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 third 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.

So entranced had I been by the nebular tracery that I had paid little attention to the star cluster and its role in shaping its environment. Now I had a closer, more critical look. The Trapezium cluster consisted of more than just its four bright stars. From my perch 50 light-years out, I could count more than 100 stars that seemed to belong to the cluster. The brightest ones were the most massive and would have the shortest lifetimes. Judging from their temperatures, these stars could not have formed more than 2 or 3 million years ago. But what about the less massive stars, which
could look forward to much longer lifetimes? Did they, too, have to be so young? The stars' slight motions, which I had ignored until now, provided the crucial clue. Even though these motions seemed insignificant, just a few kilometers every second, they were still large enough to overcome the mutual gravitational attractions of the stars for one another. Unlike the cluster at the center of the Milky Way, whose high-speed stars would be tied together indefinitely by gravity, the Trapezium was ephemeral. A few million years hence and the stars will have drifted apart. Their current proximity to one another meant that all of the stars had to have formed en masse. They were all young.

A curious coincidence, I thought. As it was forming 2 or 3 million years ago, the cluster would have been embedded just beneath the surface of the molecular cloud. Had it been embedded more deeply, it would not have emerged yet. BN-KL was still buried, after all. Maybe it was a lucky break that the Trapezium was able to burrow out of its birthplace, to shine for Earth to see. But sheer coincidence was difficult to defend in this case. BN-KL was, in fact, very close to the surface of the cloud, and a census of its stars indicated that it was even younger than the Trapezium. Thus, in 2 or 3 million years, the BN-KL cluster probably will have eaten its way out of the womb, just like the Trapezium. No, the relationship of age to depth inside the cloud seemed a deliberate trend. But what effects could conspire, not only to make stars form in clusters, but also to make them form just inside the surfaces of molecular clouds? Why not in those clouds' deep interiors?

I scanned the skies looking outward from the cloud. If I consciously sought a clue (and I'm not sure I had any well-defined purpose), it was just a wild hunch that I would find it by looking
away
from the nebula. And I found something interesting: a circumstantial clue, to be sure, but one that reinforced my suspicions. Beyond the Trapezium, maybe 200 or 300 light-years away, was another young cluster, and beyond it a similar distance was a third. I remembered that I had seen the third one peeking around the opaque cloud's edge as I first approached
the Orion molecular cloud from the other side. Neither of these clusters appeared to be as young as the Trapezium, if the absence of comparably bright stars was anything to go by. (One could never be certain that massive stars had ever formed in these clusters, but it seemed a good bet.) It also fit that each of these clusters was spread out over a much wider area than the Trapezium cluster, as though their stars had been drifting away from the point of common origin for a much longer time. The two methods of estimating age gave consistent results. I guessed the nearer cluster to be about 7 million years old and the more distant one to be celebrating its 12 or 13 millionth birthday.

It was not hard to imagine that this had something to do with the birth of stars and its relationship to the edges of molecular clouds. The progression was unmistakable. The farther a cluster was from the current wall of molecular gas, the older and more spread out it was. This suggested that the formation of the clusters was indeed related to the retreat of the molecular cloud and did occur near the cloud's edge. The molecular cloud was not just a receding glacier, exposing a moraine of stars as it retreated. The retreat of the molecular cloud was a sign that stars were being vigorously created out of its vast reservoirs of dust and gas. Even more remarkably, once the process got going near one wall of the cloud, it kept renewing itself with successive waves of star formation. All that remained was to figure out how.