Turn Right At Orion

by Mitchell Begelman

  Given the extreme uncertainty of the planets’ fates, I realized it would be prudent not to invest too much emotional energy in any particular planetary system. I therefore left battered proto-Earth to its future, whatever that might be, and withdrew from the third disk I had investigated. After visiting these three systems, I continued to explore the planetary systems of Orion, but in a more desultory fashion. I seemed to find planets nearly everywhere I looked. Even the fully formed planetary systems, when spied from afar, showed the signatures of dust; soon I learned that this was a quick way to spot the most likely candidates. I remembered that even the Sun’s system had retained a weak, dusty signature after all its billions of years. It was no surprise, therefore, that the younger systems of Orion—tens of millions of years old, or less—should show dust, and the visible signs of larger debris, in abundance.

  I pondered the visible indications of our planetary system’s history. In the Solar System there was, first, the zodiacal light, a shaft of sunlight scattered by interplanetary debris, that shot above the horizon for a couple of hours before twilight began and for a similar period after it was over. People who lived under the darkest skies (such as myself and my fellow stargazing Montanans) used to boast that this faint glow—tracing the ecliptic, the path of the planets on the sky—interfered with their view of the stars. But this material was not exactly a fossil of the era when planets were little more than consolidations of dust. More likely, it was relatively recent debris, thrown into orbit from the occasional collisions of the last few hundreds of millions of years. Second, there were the belts and clouds of debris in the outer Solar System: Kuiper’s belt and Oort’s cloud, extending far outside the orbit of any planet, swarms of icy bodies that had presumably been ejected from the inner planetary system by the gravitational action of Jupiter, as though by a slingshot. They really were fossils of the forming Solar System. These swarms were very important to the denizens of Earth, because they were the sources of comets, whose appearances sometimes had changed the course of history, and whose far less frequent collisions with Earth had surely changed the course of evolution—for example, wiping out the dinosaurs. But not that much of the primordial rubble was left, and what did remain was spread so thin that it would have been difficult to detect from a distance. An incredibly thin tissue connected our Solar System to its origins. But the younger planetary systems wore their aura of dust for hundreds of millions of years.

  Seeking snapshots of the final phase of planet formation, I toured one young system after another in Orion’s vicinity. Each planetary system I chose to visit encircled a star not unlike the Sun, but that merely reflects my sentimental side. Most kinds of stars—with the exception of the hottest, most massive stars and those with a close stellar companion—had planets. Binary stars tend not to have planets, presumably because the gravitational attraction of the companion disrupted planetary orbits or perhaps prevented the dusty disk from settling down. Every system I visited had a unique arrangement of planets, and few of them resembled the Solar System. There were planetary systems in which giant gaseous planets orbited the star close-in, taking only a few days to circle the star. Had these planets spiraled in from much farther away? And if they had, were they about to commit suicide in the star’s furnace, or would something intervene to halt their plunge? Or had the disks in these systems retained their gas much closer to the star, somehow allowing the planets to condense at this small radius?

  Other systems had planets of various sizes on orbits that swung widely from maximal to minimal distance. Such orbits were probably acquired after the planets had condensed, perhaps through some catastrophic encounter, and they were not conducive to life. Still other systems contained a wide array of small planets in haphazard orbits. These systems clearly could have used a Jupiter to police them. As it was, they never outgrew the era of devastating collisions, and their planetary surfaces never had a chance to settle down.

  I gradually grew weary of this cavalcade of planets. I couldn’t see a pattern; rather, I should say that I couldn’t see a pattern that appealed to my anthropomorphic sensibilities. Planets obviously formed easily, provided the raw materials were available. It was astonishing that these tiny particles of dust could cement themselves together, one by one, until they had formed a pebble, a boulder, a mountain, finally an Earth. But it was far from certain that planets would form in the right place, with the right mass and composition to create and foster life. There seemed to be no imperative driving the debris left over from the formation of a star to organize itself according to the blueprints of our well-ordered Solar System. Far from it: If I had to guess on the basis of my travels, I would venture that our planetary system’s structure—the relative stability of its Earth-like planets, in particular—was a fluke. The most one could say was that planetary systems were so universal, and so diverse, that some of them were bound to present the fortuitous conditions favourable to life. How easily life would form once such conditions existed . . . that topic would have to wait for some other exploration.

  I looked back over the trajectory of my journey to this point. It seemed a good time to take stock. I had moved from the most exotic places in the Galaxy—black holes, and eerie neutron stars—to about as close to home as I was likely to get without actually returning home. Yet the diversity of the phenomena I had seen did not strike me as the defining quality of this voyage. What struck me was the unity of what I had seen, and at the same time its complexity. Gravity—a simple force, purely attractive—was capable of creating this enormous diversity of motions and structures. Jets, disks, stars, planets. Black holes with disks and jets, stars with disks and jets. After all that, how could I be astonished at the diversity of planetary systems? What really distinguished planets was not the process by which they formed (this was just another example of the routine operation of gravity) but the stuff they were made of.

  The mighty dust grain. Why were these minerals made of heavy elements—silicon, carbon, oxygen—here at all, and why were they in solid form? This seemingly mundane question, focused on some tiny objects I had regarded as nuisances throughout most of my voyage, suddenly assumed paramount importance. To answer it, I would have to visit some other kinds of objects I had taken for granted.

  Part Five

  EVOLUTION

  21

  The Blob

  Will you excuse my naïveté in believing that stars should be spherical? As a theoretician, what else could I have believed? Because the force of gravity pulls equally in all directions, any star should arrange itself into as perfect a globule as the Sun. There would be no reason for it to stick out more in one direction than in another. Furthermore, the gas that composes a star is extremely fluid. One could not so much as pile up a respectable mountain or dig a deep canyon on a star, and have it last, without heroic efforts at maintenance. Any deviations from a spherical shape would be quickly washed away. If a star were spinning rapidly enough, then I could understand how it might bulge about the equator, the result of centrifugal force flattening it slightly. Jupiter, Which spins so fast that its day is only 10 hours, presents a noticeably oblate profile as viewed from Earth through any small telescope. Yet even that whirling dervish of a planet is pretty close to spherical.

  But not this star. Rocinante was hovering above a vast, heaving plain that filled more than a hemisphere of my vision. I was trying not to get too close for comfort. Although astronomers classified this as a “cool” star, at a temperature of 3300 degrees (little more than half that of the Sun) its surface was hot enough, especially given that this heat was emanating from such a vast area. All told, 200,000 times more power than the Sun emitted was streaming into space through the luminous smog that spread out beneath my craft. What passed for a surface—as far as I could tell—enveloped a body with a diameter about 30 percent larger than Jupiter’s orbit around the Sun. This was a bit hard to take, as attached as I had lately become to the conceit that planets like Earth might exist nearly anywhere. A planet, ek
ing out its existence on an Earth-size orbit, would be submerged beneath more than 80 percent of this star and would have vaporized long ago. I immediately checked my thoughts, realizing that I was begging an important question. If an Earth-like planet had ever had the opportunity to orbit this star, I must now be witnessing the bloated descendant of what had originally resembled a normal star, perhaps like the Sun. I shuddered at the fate that must have befallen any denizens of that planet. Or had the star always been like this? I was here to find out.

  I thought I knew the answer, because this star had been an object of intense study among my colleagues. The famous star Betelgeuse was the garnet that pinned the cloak to Orion’s left shoulder, as viewed from my childhood home. I had voluntarily clipped my wings at this stage in the journey. Although it was not a part of the Orion “star formation” empire, Betelgeuse was near enough to it. Indeed, for a traveler lately departed from the Orion Nebula, it was “on the way back” toward Earth, a mere 500 light-years away from home, only one-third as distant as that marvelous nebula.

  Was it a failure of imagination, fatigue, or the draw of home that led me even closer to Earth on this next leg of my journey? Perhaps some of each, but it was also the openness of this star’s architecture. This red giant—rather, I should say, this body that my colleagues classified as a red supergiant—seemed ready to burst, a loose bag barely able to conceal what was inside. The secrets inside the star were what I sought. All other stars I had seen, even the ones just condensing, had seemed so tightly composed that all hope of probing the interior seemed vain. Here I thought I had a chance, although I was soon to learn how circumscribed that chance was.

  The carbon, the oxygen, the silicon, the calcium, the iron—all the elements that make dust and rocks, giving planets their rigidity, had to be created inside stars. The interiors of stars were nuclear pressure cookers where a kind of permissible alchemy took place: the transmutation of hydrogen and helium into heavier elements. The pristine Universe, without stars, would have consisted solely of those two simplest of elements, salted with a dash of the next element in the hierarchy of atomic weights, lithium. But in which stars, and when in the life of each star, did the more complex raw materials form? And how, once formed, were they dispersed into open space, to become the planetary bodies I had lately encountered?

  My attention was drawn back toward the extraordinary light that bathed my craft. In keeping with the temperature of the star, this light was intensely red, so much so that it gave everything in the cockpit an eerie red glow except the blue materials, which were turned jet black. To say that there was something unsettling about this star would be an understatement. I was hesitant about describing its “surface” for very good reason: There was no crisp limb, such as one associates with the Sun and every other star I had seen. Once my eyes became accustomed to the light, however, I could perceive a structure to the star’s upper layers. There was an opaque “surface” after all, but it was indistinct and submerged beneath a translucent zone, glowing the same deep red but gradually shading to transparent.

  As I watched, both the translucent layer and its opaque undercoat heaved and buckled. None of these bumps and knobs were permanent features. They swished from side to side, and up and down, slightly changing in color and brightness as they moved. Then I noticed the feature that must have made their activity so disconcerting, As the translucent layer heaved upward, toward me, the opaque floor seemed to drop away. The upper layers of the star darkened and cooled, and I had the uncomfortable sense of being suspended above a void.

  The darkened area grew to cover a good quarter of the star’s surface and would not stay still even then. It began migrating around the star, so slowly and deliberately that it would take years to make a circuit. Moving out from under my vantage point, it was replaced by its opposite, as the opaque floor welled up, glowing brighter and protruding into the translucent layer, narrowing it almost to the point of invisibility.

  As I stared at these vast swells I began to grow seasick. What could make the star slosh so vigorously? Wasn’t there supposed to be equilibrium between the inward pull of gravity and the outward push of the star’s interior heat, manifested as pressure? If so, then why all this motion, as though the star somehow could not find its favorite place of repose? I had sorted through the concept of equilibrium—or so I had thought—in its most challenged state, when I had visited the neutron star at the center of the new Crab Nebula. There the atoms had been crushed to the point where the electrons and protons had merged. At the last instant, the degeneracy effect and the nuclear force of repulsion among the remnant neutrons had saved the star from collapsing to a black hole. Surely, this star’s much weaker gravity could manage to preserve a balance, with the ordinary, less violent pressure of heated gas. Yet the equilibrium near the surface of Betelgeuse seemed to exist only in a loose sense. It was an equilibrium that permitted—or required—enormous waves of matter to sweep across the star, allowed frightening pulsations to erupt, and kept one guessing where the star ended and space began.

  I began to doubt whether there was equilibrium at all, or even a boundary to this star. No matter how far I drew back from the star, I never seemed to reach the sparse conditions of interstellar space. It seemed that this star’s atmosphere went on forever. Was it disassembling itself without my realizing it? I had earlier encountered winds from stars. Even the Sun had a puny wind, and the hot, massive stars of Orion belted their surroundings with powerful gales. As I moved about I detected no such motion. Only when I drew to a halt, trying to position Rocinante as steadily as possible with respect to the center of Betelgeuse (insofar as I could determine where that center was), did I detect the outward flow of matter leaving the surface. The wind was slow—a breeze, really, the gas moving only a few kilometers every second, compared to hundreds of kilometers per second for the Sun’s wind and thousands for the winds of Orion. But Betelgeuse’s wind was dense, and I estimated that more than a Sun’s worth of matter would leave this star within 100,000 years.

  Surely the loss of so much matter so quickly would have a devastating impact on a star. But this body was so huge that it was hard to believe it lacked almost infinite stores of matter. I maneuvered Rocinante into orbit around the giant to deduce its mass and received a shock. It was only 20 times the mass of the Sun: At its rate of evaporation, this star would be gone in little more than 1 million years. My shock was short-lived, though. The Sun, I knew, had started out with enough nuclear fuel to last for 10 billion years. It was not so heavy as Betelgeuse, or as the stars of the Trapezium, but it was parsimonious, using up its store of hydrogen slowly. In keeping with its stinginess, the Sun’s wind was so weak that it would have little impact on its development, even over all those billions of years. Betelgeuse and Orion’s hot stars were profligate, burning their nuclear candles at both ends. Yes, they had several times the Sun’s mass, but they burned their fuel supplies thousands of times more quickly. Even without winds, they could not last more than a few million years. I recalled that the winds from the Trapezium stars had also carried away large amounts of matter, perhaps even as much as Betelgeuse was dispensing. It was curious that potent winds seemed to go along with other signs of prodigality.

  Betelgeuse and Orion’s Trapezium stars parted company in at least one important respect. The Trapezium stars were relatively small, with diameters only a few times larger than that of the Sun. They shared the compactness, sharp surface, and concentrated gravitational pull that I had learned to associate with stars. Betelgeuse had a similar amount of matter, but it came in a much larger package. The star was grossly distended, occupying a volume over 2 billion times that of the Sun, and its gravity, spread over such a large distance, was so weak that its interior could not be nearly so hot as that of the Sun—let alone that of a massive hot star like one of the stars in the Trapezium. If it became that hot, it would blow itself apart in a year!

  If only I could dip a thermometer deep into this churning blob, but diffuse
as it was compared to other stars, the heat coming off its surface and its opaque screen of red-glowing gas kept me at bay. Most of the way to the center, the temperature could not have risen much above 100,000 degrees. That would not be enough for nuclear reactions to create the amount of light that was bathing Rocinante and shining into space in all other directions. Thermonuclear reactions need high temperatures, to bang the atomic nuclei together with sufficient force, and high density, to ensure the fierce collisions occur frequently enough. Betelgeuse had neither, at least within its huge mottled envelope. Deep inside this star, however, there had to be someplace where nuclear reactions did occur, and they had to be sustained at a rate that far outstripped stars like the Sun. A prodigious amount of energy was forcing its way through the star so insistently that it kept the star’s outer layers off guard, unable to settle into a passive role as the mere conveyor of stellar luminosity. The envelope of Betelgeuse was actively involved in transporting its energy, a role that rendered it restless.

  The star took another big gulp, and a new abyss seemed to widen under my craft. I half-expected the curtains to part and even this nebulous but opaque surface to give way fully, allowing me to see down deep into the star’s insides. Then the displaced layers of gas rushed back in a tsunami that swept closer to the underside of Rocinante than I could stomach. I pulled farther away from the star.

  22

  Divining the Interior

  The delicate imbalances that characterized the surface of Betelgeuse made it seem fragile, but something about the star hinted at a concealed ferocity that I did not wish to challenge, even if I could. I was not yet ready to depart from the vicinity of Betelgeuse, but it was apparent that I would have to fall back on a much greater dose of theory than I had anticipated. Of course, I had studied red giants and supergiants ages ago. The theory of stellar structure had been de rigeur for students of astronomy. It was the crowning glory of theoretical astrophysics in the mid-twentieth century; the prediction that stars should grow to the enormous dimensions of giants late in life was one of its greatest triumphs. From the remote point of view of an astronomer on Earth, it would have been thrilling to know everything about a certain type of star without having seen one—and the theory was good enough that this was almost possible. But the features of Betelgeuse that struck me most viscerally were those where the theory was weakest. Theoretical sketches of a smoothly distended envelope paled beside the great waves of matter that sloshed around the star as its internal supply of energy struggled to get out. Statistical descriptions of the bubbling and turmoil that went on just below the surface had never really captured the sudden emergence of turbulent cells that I saw growing to cover degrees of longitude. And the theoreticians had never quite been able to predict just how quickly such a star would erode, mostly in spurts and gasps, of its own volition.