Turn Right At Orion (28 page)

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 irresistible 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 to 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.

It was difficult to approach Virgo's cluster of galaxies without feeling the sense of awe that a tourist from “the sticks” feels when visiting a big city for the first time. There was no comparison between the Virgo Cluster and the Milky Way's Local Group. Without counting the lesser dwarf galaxies and other mild concentrations of stars that would have received official titles in the Local Group but would hardly be recognized elsewhere, the Virgo Cluster contained more than a thousand galaxies splashed across a region 10 million light-years on a side. If the Local Group, with its two dozen or so galaxies (of which fewer than 10 made a strong visual impression) represented a village or small town, then the Virgo Cluster would be a good-sized metropolis, with multiple business districts and sprawling suburbs. Close up, it was a maze. The glittering giant ellipticals marked the downtown hubs. The spirals took second place, not because they were anemic, but because the ellipticals were so imposing. At the time, I would have compared it to a Los Angeles or London. Only later did I come to appreciate that even Virgo was far from the acme of its class.

Like the Local Group, Virgo had a handful of galaxies that dominated all the others, any of which would have put the Milky Way or Andromeda to shame. But there was another difference
between Virgo and the Local Group that overwhelmed any analogies or distinctions based solely on the scale or arrangement of galaxies: All of the dominant galaxies of Virgo were ellipticals. This much was already apparent from the distant viewpoint of the Milky Way. Whereas a spiral, by virtue of its saucer shape, gives the impression of motion, of slight dynamic imbalance, and perhaps even of tumbling through space (surely an illusion), a dominating elliptical provides a focus, giving a cluster the appearance of stability. Because it is nearly spherical, a dominant elliptical galaxy seems to be stationary and to draw everything else toward it.

The two ellipticals that outshone all the others in Virgo spread out to several times the size of the disk in Andromeda or the Milky Way and seemed to form separate nuclei around which many of the lesser—but still bright—galaxies clustered. Of course, neither of these dominant galaxies or any of the other bright ellipticals could be regarded as the true center of the cluster. Each of them had to be executing an elaborate dance under their mutual gravitational attractions. But their local domains of influence were real. One could carry the urban analogy a step further, identifying neighborhoods within the cluster, each one centered on its local elliptical. These neighborhoods even had distinctive characteristics, with spirals concentrating about one of the elliptical centers, small ellipticals about the other. A third type of galaxy, entirely missing from the Local Group, seemed to be a hybrid that combined a large bulge (or small elliptical) component with a disk that contained stars but little or no gas. These galaxies cast their lots with the small ellipticals.

I felt the cluster's influence long before I reached any of its major concentrations of galaxies. Not the gravity generated by the cluster's enormous mass—I was still moving too fast for that to affect me noticeably—but the cluster's atmosphere. I noticed the onset of its drag because I was especially sensitive to the amount of matter that surrounded my craft, concerned as I was about the fuel supply for my travels. As I am sure you have recognized, it would have been completely impractical for me to
carry all the fuel needed for each stage of my journey. During each acceleration phase, most of the fuel would have to be spent near the end, in reaching the maximal Shangri-La factor. But it would have been prohibitive to accelerate all that fuel from rest, not to mention the equal amount of fuel necessary to slow down—
Rocinante
would have been impossibly heavy.