Turn Right At Orion

by Mitchell Begelman

  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.

  I needed to find a trigger. The gas in a molecular cloud was evidently sitting precariously on the brink, almost ready to crystallize into thousands of stars but not quite able to take the plunge. It needed a shove to nudge it over the edge—a falling domino of star formation at Point A that would cause the molecular gas at Points B, C, and beyond to collapse in sequence. I had assumed that the process of star formation would be solely the province of gravity. But gravity alone proved to operate in too leisurely a fashion to create simultaneously the large numbers of new stars (not to mention the few massive ones) represented by these clusters. It must take an extra squeeze to tip a cloud into wholesale collapse under its own self-attraction. While I pondered the source of this compression, Rocinante drifted across a shock front that was much more vigorous than any of those I had encountered inside the Orion molecular cloud. My craft rocked sideways as it was pummeled by the wind of one of the Trapezium’s hot stars, and I saw the pressure outside jump sharply. I had my answer: The massive stars provided the squeeze themselves.

  My earlier impulse had been to attribute to the stars the warm and fuzzy attributes of a family, as though they were members of a species eager to reproduce. But now I saw they were really more like conquistadors, triggering and organizing star formation not in their own dominions but rather in the neighboring provinces, tens of light-years away. Star formation, marched along inexorably, driven by the violent tendencies of the hottest few stars. The most massive stars in each cluster were the alpha males, shedding highly pressurized winds constantly through their brief but brilliant lives and then topping the winds off with violent explosions as they became supernovae. After 5 million years or so, the combined blasts of these select stars would sweep over a previously quiet piece of the molecular cloud, subjecting it to enormous pressures. Mildly dense patches of gas, which had been in no hurry to attain starhood, would implode, while new concentrations of matter would collect where none had existed before. It was not the only route by which stars formed, as I was soon to learn, but it seemed the most plausible way to create the sequential waves of rapid star formation that had eaten away so much of the molecular cloud.

  The paradoxical aspect of this picture appealed to me. Whereas the hot stellar blasts seeded neighboring territories, the expedition was deadly on the home front. Local concentrations of gas, poised to create stars, must have been shredded, literally blasted out of their nests. The supernovae and hot winds cleared out the leftover gas in the home cluster, quenching star formation locally, while triggering it in some previously quiet locale 20 or 30 light-years away.

  No supernova went off while I stared at the Trapezium, and even if one had, its connection to the next great wave of star formation would have been difficult if not impossible to prove. I decided to consign the whole concept of sequential star formation to the realm of promising hypotheses. There was a gap in my experience that was troubling me more. I hadn’t seen a single star actually in the process of formation, much less a whole cluster of stars. Only gradually did it dawn on me that stars—little ones, for the most part—were forming all around me. And I didn’t have to squint at the Trapezium to find them.

  16

  Points of Darkness Shafts of Light

  They had been there all the time. I had passed dozens of them at fairly close range as I sped through the molecular cloud, but they hadn’t registered, I suppose, because I hadn’t been looking for them. In retrospect, it’s hard to believe that I saw the molecular gas as a monolithic sludge, rather than as the choppy and deeply textured medium it really was. Funny how perception can be conditioned by expectations. I had focused single-mindedly on reaching the massive stars of the Trapezium and must have been blinded by their brilliance, because I scarcely noticed the low-mass stars at first, even when I floated with them in the open cavity cleared by the hot stars’ winds. Yet the little stars outnumbered the four brightest stars more than 100 to 1, and clearly had formed as part of the cluster. They were also forming, alone or in meager groups, everywhere else, from the depths of the molecular cloud to the ho
stile oven of the Trapezium’s extended cavity. Finally I saw the knots of gas, dotted around. They were most obvious as opaque black globules against the luminous background of the nebular veneer. How could I have been so oblivious to them? Maybe they had simply been too difficult to detect in the molecular pea soup, far beneath the glowing wall. But as I re-entered the molecular cloud to check on what I’d missed, they stood out as islands—some light, some dark—in the infrared glow.

  The more I looked, the more structured the gas appeared to be. This was not what I had expected. When I crossed the Milky Way I had seen “weather,” to be sure—billowy interstellar cumulus, striated Galactic cirrus, even nimbus cloud decks. These clouds represented changes in temperature, even sharp boundaries, but they had all existed within prescribed bounds of contrast. The interstellar structures were expansive, their sizes typically measured in light-years. But these dark, dusty molecular knots were so cold, so tightly compressed that they seemed to be closing in on themselves, perhaps harboring some secret.

  If the secret had been no more than the story of how such condensations could have formed, it would have been worth extracting. I was well aware of the obstacles that had to be surmounted in order to form a tightly compressed mass even from the dense, cold substance of a molecular cloud. True, gravity’s imperative was to draw matter together, but I had already seen how rotation—angular momentum—could get in the way. Unlike the matter supplied by an orbiting stellar companion to a black hole or neutron star (a Cygnus X-1 or SS 433, for example), the matter that might ultimately condense into a solitary star need not rotate intensively from the start. But rotate it would, sooner or later, as gravity drew it together. Manifest in even the slightest variations of density, speed of drifting matter, and pressure in the molecular cloud were asymmetries that would inevitably amplify into a whirlpool of motion that would be sufficient to prevent wholesale collapse.

  My concerns seemed to be borne out when I got close enough to several of these clumps to observe their structures. They were flattened (usually a reliable indicator of rotation), and my Doppler measurements detected the signature of rotation, as well: blueshift due to gas coming toward me on one side, redshift due to receding matter on the other side. But I soon began to doubt that the action of rotation was the only effect responsible for the wafer-like structure of these clumps. I didn’t have to make any special efforts to discover that the magnetic field was organized in a way that was quite different from that in any of the rotating disks I had studied before. I could trace the magnetic field because of its strange effects on matter, turning it into springy toffee, extruding it into filaments and streamers. The streamers of gas, in turn, outlined the magnetic lines of force, just as the iron filings that I used to play with as a child could outline the lines of force created by a bar magnet. I had encountered the cosmic visualization of magnetic field lines even before I had left Earth, in the prominences and streamers that frequently erupted from the Sun. I had seen similar arcs of magnetic field erupting from the accretion disks in Cygnus X-1 and SS 433 and even emerging from the anemic swirls of gas near the black hole in the Milky Way’s nucleus. But those were highly chaotic situations, in which the magnetic streamers never stood still. They were tousled and unruly, unpredictable and turbulent, and often explosive.

  Here everything was much more calmly organized. Each wafer was wrapped with combed lines of force, some of which seemed to be matted along its surface. Others protruded like porcupine quills, except that instead of ending in sharp bristles, they spread out and joined smoothly onto magnetic lines of force in the surrounding, uncondensed gas. It was as though these collapsed clumps of gas, having fallen partway in on themselves, could not quite bear to take the final step of detaching themselves fully from the medium that had given them birth.

  The elasticity of the magnetic field endowed these knots with a kind of liveliness. It was not only the rotation that was giving pause to their contraction; they were also tethered to the magnetic lines, which seemed to be snapping them back from the brink of collapsing too quickly. They jiggled and rocked, as though they were bouncing in a cradle of bungee cords, yet there was no doubt about the inevitability of their contraction. As the knots shrank, their rotational motions became more evident and eventually rivaled the magnetic tension in attempting to stymie gravity. Rotation, too, pulled on the magnetic field lines, twisting them. I remembered my old lessons about how magnetic lines of force would often appear to be embedded in gas and to move with it, like stripes in salt-water taffy. Thus it was no surprise to see the magnetic lines pinching inward as the knots sank under their own weight and curving into lazy spirals as the angular momentum began to spin them around. But as the gas in these knots became more condensed, it looked as though their hold on the magnetic field was slipping.

  The effect was subtle, at first. I noticed that the pinching seemed not quite so strong, the curvature not so great, as my intuition had led me to expect. The rotation increased inexorably, but the twist in the curves of magnetic field did not keep pace. It was not that the magnetic field was being left behind without a struggle. There was a price to pay, a drag on the rotation that left the spin just slightly less than it would have been had angular momentum been able to do its work unfettered. It was clear why this was happening. As the gas contracted and became denser, its particles closer enough together, it was also becoming colder. The few atoms that had lost electrons, for whatever reason, greedily retrieved them. This doomed magnetism’s grasp on a clump, because only the particles with electric charge, the ions and electrons—not the ordinary atoms or molecules—“felt” the magnetic field. Those few particles alone had shouldered the entire burden of carrying the magnetic field, making it twist and compress as the gas rotated and shrank. Now, with these particles becoming rarer and rarer, the burden of reining in the magnetic field was just too overwhelming, and the field slipped away.

  At the same time as the magnetic yolk was slipping free, it was taking its most serious toll on the clump’s rotation. So much of the rotation had been shed that the (now much rounder) globe could begin serious contraction. It pulled more tightly together, its surface glowing more brightly as the internal pressure grew. It began to heat up again. I estimated that the temperature of one clump’s core was approaching 100,000 degrees, and it was only a matter of time before it would reach 1 million degrees, the threshold for thermonuclear reactions to start. This would not be the powerful transformation of four hydrogen nuclei into one helium, the reaction that powers the Sun and most stars, but a milder one that initiated formerly benign (cold, dusty, passive. . . ) pieces of molecular cloud into the shining mysteries of active starhood. There was now no doubt that this clump of gas was destined to become a star.

  Without the magnetic coupling to resist its rotation, the clump’s outer layers spun faster as it continued to contract, and as more matter fell into its grasp it once again developed a flattened shape. This time the flattening was much more pronounced, a bulbous core surrounded by a thin disk like the planet Saturn. Then the nuclear initiation began. Deep in the core, nuclei of that rare isotope of hydrogen, deuterium (a duo consisting of one proton and one neutron) grew hot enough and came together with enough force to obviate Nature’s safeguards against nuclear power—the protons’ electrical repulsion—and fused. A wave of turbulent energy spread outward through what I may now be justified in calling the “protostar.” The protostar’s outer shield of molecular gas, matter still drifting in, unaware of the developments below, was presumably overtaken by this reactive wave and shocked into retreat.

  I say “presumably,” because I reconstructed these final events in retrospect. Before the wave of heat had swept over my craft from the newly assertive protostar, I was distracted by something that was of little importance in itself, but showed me the way to the next stage of star formation. I was suddenly aware of a small, tattered bit of glowing nebula coming into my field of view. It was very bright, and from the pattern of spectral colo
rs I could tell that it was not just a disembodied bit of the main Orion nebula, fluorescing under the ultraviolet glare of its hot young stars. This gas was glowing because its atoms were being slammed against one another: It was a fragmentary shock wave, seemingly come out of nowhere. I stopped my slow drift and was now virtually stationary with respect to Orion’s molecular clouds, but this stuff was coming at me at surprisingly high speed (even if I accepted that this was a shock wave), closing in at several hundred kilometers per second. I instinctively looked out my opposite porthole to check its destination and was astonished to see another bright patch, this one a quarter of a light-year off (and therefore appearing much smaller, but otherwise similar) and moving away from me at a similar speed. Beyond that, I saw a string of more distant splotches, a whole train of these disembodied shock waves.

  They were all lined up; it was obvious that they were related. But where were they coming from? It didn’t take me long to find out. At this moment, I crossed their path and felt a sudden impulse pushing Rocinante in the direction in which the knots were traveling. So they were not completely disembodied, after all. There seemed to be a continuous, fast stream of gas that connected them. It wasn’t a steady stream. Its speed and intensity fluctuated, which meant that the faster portions could catch up to the slower segments and ram into them. This game of tag was what led to the shock waves and the violent atomic collisions that engendered the intense colored light.