Turn Right At Orion (24 page)

Whatever carbon and other trace elements were now locked up in the core of the Dumbbell Nebula's star would remain there permanently. This material had been taken out of circulation, and it hardly mattered into what chemical elements it had been transmuted. But on balance, this star had returned more than it had kept. Nearly the whole envelope—a quantity of matter larger than the core that remained behind—had been processed into a mixture rich in fresh elements that had been dispersed into interstellar space. The Dumbbell Nebula, already quite spread out, would eventually lose its integrity, its atoms mingling widely with the atoms that had been expelled from other stars—or with gas that had never formed part of a star—elsewhere in the Milky Way Galaxy. This blend of elements could someday find its way into new stars and planets. Some of the stars of the next generation would grow into supergiants, create planetary nebulae, and further enrich the Galaxy's chemistry.

Thus planetary nebulae, and their red supergiant progenitors, proved to be the factories that produce and disseminate most of
the fresh carbon and nitrogen in the Universe. Whenever the substance of such a nebula was incorporated into a new star, that star would have a higher concentration of these elements, compared to the primordial element hydrogen, than the stars that had preceded it. If a star's matter had been recycled through multiple generations of stars in the past, then it would have that much higher a concentration of these elements.

Does the thought that stars are not made of the same raw materials, one generation to the next, give you pause? To me this realization was no more or less astonishing than the understanding, brought home so strikingly in Orion, that there are generations of stars—that the census of stars and planets is not fixed. All kinds of new structures are always being formed, and some last only a short time, by cosmic standards, before dissolving and freeing up their matter to form something else, If they cycle through life and death, then why not grant them evolution, too?

My journey had evidently shifted, from one of discovering how things are to one of perceiving how they change and evolve. As time was passing for me, and much more time was passing for those I had left behind on Earth, so was it passing for the Milky Way. There was no destroying these newly formed elements, no going back.

Investigating Betelgeuse and the Dumbbell Nebula had not completely satisfied my curiosity about the relationships between stars and their environments. It had not shown me where the other heavy elements—oxygen, magnesium, silicon, sulfur, iron—came from, although the theories told me that stars like Betelgeuse (the massive stars) would provide the key if I waited long enough. I would soon confirm this for myself, under harrowing circumstances. Nor did it demonstrate the entire evolutionary cycle, of star birth, death, and rebirth, all in one place. The matter of the Dumbbell Nebula would merge silently with the rest of the Milky Way, that much was clear. But who knew in which quarter of the Galaxy its atoms of carbon or nitrogen would next join a star's envelope or a planet's atmosphere?

My mental picture of the Galaxy, which had started out so simple, was now becoming criss-crossed by a cat's cradle of interconnections. The matter liberated in places like the Dumbbell, over here, affected the formation of stars and planets in places like Orion, 2000 light-years away. The same principles, driven by gravity, motion, and the concept of equilibrium, now punctuated by evolution and the incessant emergence and dissolution of structure, kept reappearing in every place I visited. Yet somehow it was difficult to put it all together. The Milky Way Galaxy was beginning to feel too big to comprehend. I sought a more self-contained setting that felt more like a neighborhood. The Magellanic Clouds seemed just the place.

My motives for visiting the Magellanic Clouds were anything but simple. First, I sought relief from the vastness of the Milky Way's disk, the complexity of which was beginning to overwhelm me. In the Magellanic Clouds I hoped to be able to sort out the complicated interrelationships that had impressed themselves on me of late. In my travels so far I had seen hints—more than hints—of cycles of stellar birth and death, great currents of mass and energy shaping structures that were too large to grasp mentally, let alone in one's real field of vision, and subtle evolutionary trends whose significance for the Galaxy's development were unclear In short, I needed some R&R, the opportunity to reflect on these ideas without being forced to face any new ones. The Magellanic Clouds had some of everything—vast star-forming regions bigger than Orion, planetary nebulae, globular clusters, even some incipient spiral arms—all tied up in a couple of compact packages that could be comprehended as a whole, or so it seemed. In adopting this view, however, I turned out to be hopelessly naïve. The Magellanic Clouds were no more self-contained or independent of the entire Milky Way system than the Orion Nebula was independent of Gould's Belt.

At least I could take in both Clouds in a single visual panorama, as I had done on my first foray deep into Earth's
Southern Hemisphere. I remember thinking what a particularly European bit of chauvinism it was that the Clouds had been named after Magellan. These patches of soft fluorescence are every bit as striking as the band of the Milky Way, perhaps more so because of their isolation in the sky. They figured prominently in the celestial mythologies of the indigenous peoples of the south, long before European traders plied those seas. And because there is no southern counterpart to Polaris, no southern pole star, they had served as guideposts to sailors—from Europe and elsewhere—long before the time of the Portuguese circumnavigator. They seemed as isolated in three dimensions as they did in two, and thus they provided me with just as clear a guiding beacon.

Next, there was a more technical nationalization for the visit. I knew that the stars of both Magellanic Clouds had been surveyed by generations of astronomers before I left Earth and that they exhibited certain interesting peculiarities. The most important was that their concentrations of heavy elements—iron and oxygen, in particular—seemed low, as though the Clouds had indulged less vigorously in multiple generations of star formation and recycling than had the disk of the Milky Way. Gazing out of the disk and toward the Clouds I found this puzzling, because the Large Magellanic Cloud, especially, seemed to be rife with massive young stars. One nebula in particular—labeled on my charts “The Tarantula,” although it didn't look any more like a tarantula than the Crab Nebula looked like a crab (What was it about arthropods that fascinated early astronomers?)—put Orion to shame. If the Large Cloud had ever known anything like this level of star formation in the past, it should have been richly supplied with the elements cooked in the massive stars' furnaces. The Small Magellanic Cloud seemed calmer, its star clusters older and more sedate. Reassuringly, its abundances of the heavier elements were even lower than those measured in the Large Cloud. Could I trace the origins of the peculiarities by examining the Clouds close-up?

Finally there was a thrill I hadn't anticipated, one that sneaked up on me as I turned
Rocinante
in the direction of the
Clouds. For the first time
I was heading out of the Milky Way.
All of my paths so far had been confined to the narrow plane of the Milky Way's disk. Occasionally I had bobbed up and down across the molecular cloud deck, for a better view. But I had never strayed more than a few hundred light-years from the plane that marked the Galaxy's equator. To reach the Magellanic Clouds, I would have to take off at about a 45-degree angle with respect to the disk and keep going. The distance would be a long jump—160,000 light-years each way, more than 6 times the distance from Earth to the Galaxy's center—although. it would add less than 24 years to my travel time, and still less to my age, thanks to the Shangri-La factor and the benefits of hibernation.

Most astronomers regarded the Magellanic Clouds as separate galaxies, distinct from the Milky Way. But this was only partially true. The Clouds were certainly well separated from the Milky Way's
disk
and contained a microcosm of nearly everything the disk contained—stars, gas, the works. Though smaller than the Milky Way (each contained only a few percent of our Galaxy's mass, if that), they could hold their own as respectable galaxies. Yet the Magellanic Clouds were captives of the Milky Way. They lay entirely within the Milky Way's halo, that great domain of faint stars and hot gas in which the disk was also deeply embedded. Had the Clouds once been truly independent star systems that had strayed too close and been captured? Or had they always been destined to merge with the Milky Way? Were they merely bits of the disk that had been unaccountably delayed in joining the Galaxy? In any case, their fates were now sealed. Their presents and futures were being irrevocably shaped by the hostile environment they encountered as they flew through the outer reaches of our Galaxy and by their interactions with one another. In the end there would be no escape.

Traveling toward the Magellanic Clouds entails a different kind of drama than traveling along the plane of the disk. The molecular thunderheads, atomic hydrogen cloud-decks, and dense network of warm wisps are all left behind within the first 2000 or 3000 light-years. The chimneys of hot gas, which form
a warren of tunnels and bubbles through the denser clouds, expand and coalesce, and one soon emerges into open territory. The organized circular motion of the disk is left behind; here the stars move chaotically, every star for itself. The gas that fills the space between the stars is so hot that the Galaxy's gravity seems to have little effect on it and so transparent that it is barely detectable except for a pale X-ray glow.

As I accelerated away, the structure of the disk below unfolded at a tremendous rate. I saw the row of atomic hydrogen clouds forming a broad curve, pushed up against the disturbance of the Orion spiral arm like the banks of clouds thrown up against a coastline by on-shore winds. Behind them the dark and billowing molecular clouds reared up where the hydrogen clouds coagulated. The giant molecular cloud complexes were outlined in silhouette by bluish light scattered around their jet-black edges, the light emanating from hidden regions of star formation, unseen Orions. Its bluish tint was imprinted by the smoky haze of interstellar dust. Shafts of intense pink light—the light of ionized hydrogen—momentarily gleamed through holes in the clouds. As my position changed, these beams came and went so quickly, and through such a complex maze of chinks and gaps, that I found it hopeless to try to identify any of the famous nebulae. For a moment I caught a particularly intense field of pink and green and spotted the flash of three or four white-hot stars in a compact package. Was it the Trapezium cluster? It was gone from view before I could check its position.

The patterns were easier to follow when I looked away from the opaque concentrations of molecular gas. Supergiants were easy to pick out: the rubies, Betelgeuse and Antares, and the diamonds, Deneb and Rigel. I spotted some ordinary red giants—Arcturus, Capella, Aldebaran—and the sharp, colorful disks of a handful of planetary nebulae. Other expanding shells of gas, more ragged than planetaries, marked the sites of violent stellar explosions. Ordinary stars, including our undistinguished Sun, formed a richly textured and multicolored backdrop, filling half the sky from my vantage point.

The immense pinwheel pattern of the Galaxy roared into view. The disk was amazingly flat and thin here and bore a thick and even peppering of stars, but the superimposed spiral pattern of dark cloud banks and bright rims of star formation gave it a deeply incised look. Toward the outer Galaxy, I could see the spiral arm of Perseus and a few regions of star formation beyond. Somewhere out there the disk petered out in a mush of indistinct hydrogen clouds, thickening and warping out of the plane for reasons not entirely clear. Toward the Galaxy's center, I made out the Sagittarius arm and beyond it the tightly wrapped bands of atomic and molecular gas pushed aside by the tumbling stellar motions I had encountered on the earliest leg of my journey. The Milky Way's bulge reared up 20,000 light-years in the distance, encircling the hidden Galactic center and fading gradually into the halo. Studding the bulge were dozens of globular clusters, those marvelously condensed spheres—hundreds of thousands of stars apiece, and only a few light-years across—that were very likely left over from the Galaxy's primordial distillation.

I appreciated the contrast this glorious view made with the relative featurelessness of the halo. There were many stars in all directions, but except for the few globular clusters that orbited the Galaxy this far out, they did not congregate in clusters. Nor were they particularly bright. These were all old stars, formed so long ago that even the ones slightly heavier than the Sun had run out of fuel. They shared (and exhibited to a much greater degree) the Magellanic Clouds' peculiarity that heavy elements were unusually scarce, a fact that made more sense here than in the Clouds because these regions clearly had not benefited from stellar recycling any time within the past few billion years. Bright beacons out here were sparse but welcome sights. There were no supergiants, because these would have burned out long ago, but I noted some yellow-white stars, which must have been fusing helium in their centers, and a few red giants.

Occasionally, a small, isolated cloud no more than a few light-years across would appear out of nowhere and startle me. Once
or twice I passed straight through such a cloud. For the most part, these clouds were of mild temperature and consisted of individual hydrogen atoms with the usual admixture of dust, but a few of them contained dense knots rich in molecules. Were they shards of matter falling into the Galaxy for the first time, or had they been ejected from some explosion in the disk? I could not decide, but I quickly realized that these would not be the last of the clouds I would have to contend with. Like a pilot who climbs through a cloud-deck only to find an inaccessibly high layer of cirrus far above, I perceived that I was heading toward a more ubiquitous layer of atomic hydrogen at about the distance of the Magellanic Clouds.