Turn Right At Orion

by Mitchell Begelman

  Whatever the fate of the supernova’s gaseous remnant, the star’s imploded core would stand fast as a neutron star, perhaps shining for a time as a pulsar. But was it really there? By now its effect on the debris should have become apparent. I searched in vain for something giving the debris cloud extra oomph, adding some power to it over and above the decaying radioactive cobalt. If the neutron star were sending out a searchlight beam, producing flashes of ordinary light like the Crab Nebula’s pulsar, those flashes should have been visible directly. But there was no sign of any pulsar. It was unthinkable that the entire core had exploded. Something was surely left behind, but it seemed to be invisible. Was there some reason why the neutron star had not begun to pulse? Had something robbed the core of its angular momentum, damping down its spin as it collapsed? Or had its magnetic field failed to develop? Had something more interesting happened? Could enough, matter have fallen back onto the core to drive it over the edge, causing it to collapse to a black hole? I briefly considered investigating for myself but decided against it. I was not inclined to enter the nebula any more deeply. Like the debris, my thoughts were turned outward, and I had little inclination to explore the kind of tortured confines near a neutron star—or black hole—that had proved so anxiety-provoking before.

  One thing was clear: I had come nowhere close to escaping the complexity of the Milky Way by traveling to its satellites. The Magellanic Clouds were more complex, if anything, because they were less self-contained. These galaxies had evolved differently from the Milky Way’s disk and from each other. Was it because of their frequent encounters with one another or because of their incessant plunge through the stars and gas that made up the Milky Way’s halo? Were the Clouds poor in oxygen and iron because their stars had never manufactured the quantities of heavy elements that larded the Milky Way’s disk, or had massive stars once lavished oxygen, silicon, and iron on the Clouds, only to have these elements stripped away along with the gas they permeated? I tried to project the evolution of the Large Cloud into the future, long after the thousands of hot massive stars that filled the Tarantula Nebula had exploded. Would astronomers then say that the Large Cloud had finally caught up to the Milky Way in its chemical richness? Or would much of this oxygen-enriched gas be pulled out of the Large Magellanic Cloud the next time it got too close to the Small Cloud, before it had a chance to form a new generation of stars? Perhaps the gas would be blown out of the Cloud by the force of those concatenated explosions. I wondered whether the outcome of all this stellar activity might be the enrichment, not of the Magellanic Clouds, but of the halo of the hegemonic Milky Way that would soon engulf them. For how long would there be Magellanic Clouds at all?

  Having seen the grandeur of a supernova explosion, and having allowed the debris to sweep over me only a few months after the event—having been inside a supernova debris cloud—I found further exploration of the Tarantula Nebula and its vicinity hopelessly anticlimactic. I was impatient to move on. I knew that by leaving now, I would miss a spectacle beside which the illumination of the red supergiant’s wind would pale. Twenty years after the explosion, the debris shell itself would finally ram the dense ring that formed the waist of the hourglass, and the ring would sparkle brilliantly in every part of the electromagnetic spectrum.

  But a close-up view of this marvel would have to wait for another time and another supernova. There were many pressing questions still to answer. Should I continue to pursue the puzzle of evolution or set it aside in pursuit of other, equally insistent goals? I was bothered that big pieces of the puzzle were missing. For example, this supernova, despite its dramatic nuclear endgame, could not be the major source of cosmic iron. Carbon and nitrogen were ably supplied by the calm outgassing of planetary nebulae; oxygen and many other elements were derived from violent explosions like the one l had just witnessed. But most of the iron had to come from somewhere else. By this time the cobalt had just about finished decaying, and I knew that this supernova would not release much more than the 7 percent of the Sun’s mass in iron that had been produced through radioactivity. The much larger quantity of iron that the star had created in its last week or so before exploding had been crushed to neutrons and locked up forever. There had to be yet another route by which matter could be forged into iron in a nuclear furnace and then released for future use.

  If I wished to seek out the missing iron, I knew where to look. During my long traversal of the Milky Way’s halo, I had noticed, far in the distance, 100 or more flashes at random intervals and seemingly at random locations throughout the halo. This journey, which for me had required only a couple of decades of hibernation, had sampled 160,000 years in the life of the Galaxy. these flashes were appearing every 1000 years or so. I had had the presence of mind to record the signals from some of these brief flares, and now I knew much more about their nature. They, too, were explosions of stars, but explosions of a rather different sort from the one that had produced my supernova. They were brighter and faster, and they showed no signs of the pink hydrogen light that dominated the explosion I had just witnessed close at hand. I remembered my colleagues debating the provenance of these events during the years before I left Earth. It seemed that they were explosions of stars that had completely lost their hydrogen envelopes. Because they were seen to occur in the Milky Way’s halo as well as in the disk, they had to be old stars. People believed that these supernovae were the explosions of white dwarfs.

  How, in the calm reaches of the halo, an ancient, dead star could suddenly blow up—that had been a major mystery. It had been supposed that the explosion occurred when so much matter was dumped onto the surface of a white dwarf that the pressure of its electrons could not support the extra weight. In these remote environs, the fresh supply of matter could come only from a companion, star. The dwarf would start to collapse, but this time, nuclear reactions would intervene before gravity got the upper hand. The entire star would explode like a powder keg, incinerating its store of nuclear fuels in a great burst and leaving no cinder behind. This kind of supernova produced the iron in the Universe.

  I thought how easy it would be for me to determine, once and for all, how these explosions were triggered. Should I strike out into the Milky Way’s halo to pursue this third venue for nuclear alchemy? There was the considerable problem of trying to pinpoint a likely candidate for such an explosion. Positioning myself would be tricky, even given Rocinante’s maneuverability. But there was a more formidable obstacle: I could not work up the enthusiasm for a further foray through the Milky Way, disk or halo. Even though I had never left the Milky Way’s halo, my exploration of the Magellanic Clouds had given me a new perspective on the home Galaxy. I now saw it as though from outside. I dwelt more and more on the relationships of one galaxy to another. Galaxies underwent great cycles of birth and death, all the time evolving into systems more complex, more diverse. But they did not do so in isolation.

  I pondered where to go next. Distant galaxies beckoned, but the home Galaxy represented a kind of security. The disk of the Milky Way began to seem more inviting. It filled half the sky with its mesmerizing spiral pattern, great rows of nebulae, and dark lanes of cloud. It looked like it would contain Everything I might ever want to explore, and I must have lost my nerve momentarily, because the next thing I knew I was heading straight toward the Milky Way’s disk. But I did not stop there. In what seemed like a few moments I plunged through the disk, emerged from the other side, and kept on going.

  Part Six

  HIERARCHY

  30

  Really Leaving Home

  There must be something tremendously liberating about leaving one’s own galaxy. Some of my colleagues, seduced by the intellectual lures of other galaxies and the way they fit together into even larger cosmic structures, had hardly ever visited the Milky Way—metaphorically, that is—during their entire professional careers. I, ensconced physically in the Large Magellanic Cloud, though still in the outskirts of the Milky Way’s halo, had felt an irres
istible urge to escape. But to where? My journey through the halo toward the Magellanic Clouds had been spared from desolation by the constant presence of the Clouds ahead and the reassuring blanket of hydrogen that enveloped them. I was also kept company during my trip by the other two steadfast companions of the Milky Way: the spiral galaxy that lay in the direction of Andromeda on Earth’s night sky and the face-on pinwheel that ordinarily lay beyond the constellation of Triangulum, the carpenter’s triangle. Each of these galaxies was huge, almost a twin of the Milky Way. Andromeda, more than 2 million light-years away (15 times farther than the Magellanic Clouds), was specially honored as the Milky Way’s partner in a minuet. In their orbital dance, now swinging wide apart, later nearly brushing haloes, these two galaxies set the pace for the modest aggregation of galaxies known as the Local Group.

  In Andromeda, I could see at a glance how the Milky Way must look to a voyager far outside its reach. The Milky Way and Andromeda galaxies were similar in many ways—the disk, the dusty dark lanes, the star-forming nebulae, the central black hole. Just as the Milky Way had two substantial attendants in the Magellanic Clouds, so did Andromeda travel with two robust companions in tow. But such similarities made the curious contrasts between them all the more striking. Andromeda’s attendants looked nothing like the Magellanic Clouds. Their shapes—perfectly smooth, slightly flattened balls of stars—pegged them as members of the other great class of galaxies, the “ellipticals.” They boasted no great nebulae, no chains of newly formed massive stars, no spine of stars that resembled incipient spiral arms or a tumbling stellar bar. Where the Magellanic Clouds were lumpy and ragged in appearance, Andromeda’s companions displayed the kind of symmetry and compactness I had hitherto associated only with the jewel-like globular clusters. Yet the companions of Andromeda were thousands of times heavier than any such cluster.

  I was clearly being taught another lesson about the diversity of cosmic structure. How was I to know the extent to which this lesson would prepare me for the next stage of my journey? After all, my destination had not even been chosen yet. In the excitement over my explorations of the Milky Way’s disk, I had taken for granted the special characteristics that set the Milky Way apart. I was well aware that not all galaxies sported pinwheels, but I had not really thought much about what a galaxy would look like if it had no disk.

  Remove the disk from a spiral and you would have a galaxy that was all halo and bulge. This game of “what if” suddenly assumed importance, because at a certain level of description, elliptical galaxies resemble the disembodied haloes or bulges of spirals. Like the halo, they consist mainly of older stars, a fact given away by their reddish colors. Their brightest stars are all red giants or the kinds of supergiants that precede the demise of low-mass stars through planetary nebulae. They have low concentrations of the heavy elements, a fact that fits in neatly with the apparent absence of vigorous recycling of matter through the birth and death of stars. And their stars move chaotically, according to no organized scheme.

  In their ability to shine brightly in the void, however, ellipticals resemble bulges more closely than they do haloes, I was quickly learning that the haloes of spiral galaxies do not count for much, visually. One certainly did not notice them when gazing at distant spirals—Andromeda, for instance. As I made my first crossing of the Milky Way’s halo, I had been misled by the myriad stars that seemed to surround me at all times. Now, on my way out (and much farther away from the disk than I had been when visiting the Magellanic Clouds), I could see the halo for what it was. Compared to the disk or bulge, the sprinkling of bright stars was meager. The halo contained a lot of mass, that much was known from surveys of its gravitational influence, but most of it was very dim, virtually invisible to Earthbound observers and, for that matter, to me. The bulge of the Milky Way, however, shone brightly. It had partaken much more heartily of the carbon, oxygen, and iron produced in the embedded disk, and the reddish cast was less pronounced; in those respects it differed from the ellipticals. But it had the same kind of presence, the concentration of stars, that could make for an imposing galaxy, particularly when these qualities were scaled up to a giant size, as I was soon to discover.

  By sheer coincidence, the Milky Way’s major companions in the Local Group—the Clouds, Andromeda, and Triangulum—all lay to one side of the disk. Now, having crossed the disk, I saw no such landmarks in my sky as I sped away once more, and I began to feel lonely. The few nearby galaxies I did see were scarcely worth the name—loose, anemic agglomerations of stars rightly called “dwarfs.” just as humans in their original state could feel comfortable only on a rock and water planet like Earth, perhaps spacefaring humans would require the comfort of a robust galaxy to feel secure. As I scanned the desolate space in front of me, I knew that I could not rest until I had found another place where the cycles of change operated vigorously, where stars came and went, and (now that I had seen the Magellanic Clouds) where even whole galaxies exchanged stars and gas and sometimes merged.

  I therefore peered out of the Milky Way’s halo into greater distances than I had ever thought to travel, to the spaces where all I could see, dimly, were other galaxies, millions of them. The spirals were the most numerous by far, but the places that arrested my attention were marked by huge elliptical galaxies. The most impressive spot on the sky was dominated by at least five big ellipticals, two of which far outshone the other three. They seemed to have herded a thousand or so smaller galaxies into clustering around them, making up a great congregation of galaxies in the direction assigned on Earth to the star pattern Virgo. I knew immediately that this was to be my next destination.

  It is not too surprising to find hierarchical arrangements of structure in the Universe. Gravity knows no bounds of scale—no maximum distance beyond which its attraction fails—so, given enough time and opportunity, it will build layer upon layer of structure, ad infinitum. What is slightly more surprising is how distinct those levels of structure can be. In the disk of the Milky Way, for example, stars are separated by 10 million times their diameters, and even their planetary systems are separated by distances thousands of times larger than the orbits of the farthest flung planets. (Just look at how long it took humans to bridge those gaps!) But the hierarchy is much less distinct at the level of galaxies, although it seems reasonably secure for the luminous parts—the aggregations of stars, gas, and dust—that were discovered first. Only much later was it found that most of the matter in galaxies is contained in their extensive, invisible haloes. These overlap much more frequently than the luminous parts, linking galaxies to one another physically without much altering the appearance that they are islands in the sky. But my visit to the Magellanic Clouds had shown me the insidious, long-term damage that can be done when galaxies secretly overlap.

  As I passed through the outer quarters of the Milky Way, I considered the hidden relationships that might exist among the galaxies I saw spread before me. Many, but not all, seemed to be bound up into clusters. Some of these appeared to be loose aggregates with few members, like the Local Group. Others were groupings as rich in galaxies as the Pleiades cluster is in stars. The galaxies in rich clusters were plainly separated by just a few times their widths; one could only imagine the gravitational struggles that must be going on among their haloes. In a few cases, one didn’t have to imagine: Pairs of galaxies were locked in fierce battle, spraying out streamers of stars and gas that contained more matter than the entire Magellanic Cloud system. Where these streamers collided, one sometimes saw the signatures of new stars being born. Why, then, did so many of these tightly grouped galaxies maintain their separate identities, given the forces that worked to mix and homogenize them? The only sensible answer seemed to be lack of time. These clusters must represent tracts of matter still coming together, merely the latest scenes of a galaxy formation opera not yet sung to completion.

  Even the clusters of galaxies did not represent the end of the hierarchical sequence. The clusters themselves seemed not t
o be placed randomly but to be lined up, joined into networks, and linked into grainy membranes that were then curved around great voids.

  Thanks to all this structure, I did not have to use dead reckoning to find my way to the Virgo Cluster. I was guided there by one of these metagalactic highways. As I left the Milky Way’s halo behind and emerged truly into intergalactic space for the first time, I could appreciate that the Virgo Cluster was not merely an oasis in an otherwise barren desert. I perceived other groups of galaxies nearer by, most of them not much more populous than the Milky Way’s own meager Local Group, others containing as many as a hundred or more galaxies. They were not spread uniformly through the space to all sides but, rather, formed a rough sort of corridor that drew me onward. It struck me that, far from being an outpost, the Virgo Cluster was the centerpiece of our corner of the Universe. It was the Local Group that was a way station in Virgo’s “supercluster.” I may have been leaving home, but I was definitely not leaving the neighborhood.

  I suddenly remembered Johannes Kepler and his Dream. I had not thought about the Somnium since the early years of my trip, but now one of its powerful metaphors stood in startling contrast to my present situation. In his story, the great astronomer had been conveyed to the Moon along a shaft of darkness, the shadow of a lunar eclipse. Far from being a voyage through barren intergalactic space, my way to Virgo was being paved by light—the light of myriad galaxies, each a microcosm of our own. In a little more than 34 years, according to my clocks, I crossed the 60 million light-years and entered the outskirts of the cluster.