I progressed toward the star. More of the intense ultraviolet radiation was able to reach my craft as I put layer after layer of the nebula behind me. The reds of the hydrogen and weakly ionized nitrogen gave way to the green of doubly ionized oxygen (a signature of hot nebulae everywhere) and then to the blue-green of completely demolished helium, that trademark of planetary nebulae. I sought evidence for another trademark of planetary nebulae, the sharp spatial demarcations that often layered the different levels of ionization. In the Ring Nebula, for example, photos taken through blue filters showed that the “finger hole” of the ring, which so beautifully frames its illuminating star, is actually filled with the faint light of doubly ionized helium, whereas the ring itself glows brightly in green oxygen, wound round with red filaments. The idea was that the ultraviolet rays grow gradually weaker, and softer, with distance as they propagate through the nebula. This effect was supposed to be aided by the progression of winds emanating from the central, dying star: first the red supergiant wind, slow and dense, then the gusts getting faster and faster as the star unburdens itself of its shroud and expels matter straight from the nuclear burning layers. The faster winds would plow into the supergiant’s slow breeze, sweeping it into a shell that, in the case of the Ring Nebula and some other famous examples, appears as an annulus on the sky. Inside the shell, the bubble of tenuous gas left over from the fast wind would provide unimpeded access for the harshest ultraviolet rays and thus become the site of battered helium’s bluish glow.
Many planetary nebulae possessed even more complex and dramatic structures. The protrusions—handles, or “ansae”—that graced famous planetaries such as the Saturn Nebula were thought to be narrow jets of matter spurting in opposite directions from the central star. It would be a nice symmetry if brightly glowing jets ushered out a dying star much as I had seen them usher infant stars into the Universe in the Orion Nebula, But I could not perfect the analogy. The young stars had their accretion disks, whose swirling motions seemed to be connected to the creation of jets in such diverse circumstances as protostars and the X-ray binary SS 433. In a planetary nebula there was no disk. Some astronomers speculated that the star had created a chimney within itself, perhaps along its rotation axis, or corralled and focused by the motions and gravitational tugs of an unseen binary companion. The jets would then be streams of matter propelled out through the chimney.
I was never able to determine what caused the elongated symmetry of the Dumbbell Nebula. Any old jet tracks that persisted here were intermittent and indistinct. Had the ejection occurred through a broad cone, or had the star swung a pair of narrow jets on a lazy trajectory across the sky? The view from certain angles, during my approach, had suggested the latter, but it was too late to reconstruct the geometry of the expelled matter with any confidence. As for the layering of the winds, what well-organized structure had once existed was by now washed out by the ravages of time and turbulence. This was an old and homogenized planetary nebula. But it was far from smooth. On close examination, many of the diffuse bright patches proved to consist of aggregations of tiny, dense clumps of gas, presumably condensed out of the supergiant wind. Some of the clumps were so opaque that they harbored molecules and dust grains, which had survived despite the harsh environment. And all the regions of the nebula, clumps as well as the less dense gas that filled the spaces between them, bore the chemical traces of the star’s deep interior. There were places where both helium and nitrogen were highly concentrated, compared to their concentrations relative to hydrogen in, say, the Sun. These patches of gas probably came from layers where hydrogen was still being fused into helium, accompanied by the transmutation of oxygen and carbon into nitrogen—one of the subtle games of nuclear physics played in such regions. In other places, the excess carbon was particularly striking. The stellar debris was mixed in everywhere: What the supergiant wind hadn’t dredged up in the early stages of the nebula’s expansion had been injected forcibly, later on, by the impact of the fast winds.
When I got very close to the central star, within a fraction of a light-year, I finally caught its full glare. The surface was blindingly bright, even in visible light, although nearly all the light came out as very harsh ultraviolet rays. I estimated the temperature of the surface to be well over 100,000 degrees, far hotter than any normal star. But this was nothing compared to conditions a hair’s-breadth beneath the surface, where the temperature had to rise to perhaps 100 million degrees to support the nuclear reactions that were still going on in a thin shell. The sharp blue-white sphere in front of me was tiny, only about as big as Earth, and although my view to the nuclear furnace was still blocked, I sensed that at least part of my quest was accomplished. I had seen inside a red supergiant, all the way to the core.
There could not have been much nuclear fuel left to burn. At this close range I finally encountered a fast wind—some thousands of kilometers per second—rushing away from the surface. It carried little mass and must have been a shadow of the gusts that had once swept through the inner regions of the nebula. Such a wind had to be driven by the nuclear reactions just below the surface, where helium was still being fused into carbon and perhaps a small amount of carbon was being transformed into neon and oxygen. But for a star of this mass, most of the carbon would never become hot enough to burn.
I contemplated the future of the Dumbbell’s central star. It had just about reached its last gasp of nuclear burning. Soon the veneer-thin nuclear furnace would run out of fuel entirely and begin to cool down. What would then prevent the remaining core, an inert ball of carbon, from collapsing? To ask that was to beg the question. This tiny core, which weighed nearly as much as the Sun, was already too cool to support itself against its own gravity, even as it played out its nuclear endgame. Something else was preventing it from shrinking, and I knew what it was.
I remembered puzzling over the equilibrium that allowed neutron stars to resist their enormous gravitational fields. I had pondered this while the harsh metallic glare of the Crab II pulsar beat against Rocinante’s skin. There, it had been the atomic nuclei, crushed down to pure neutrons, that had resisted further compression by virtue of their proximity to one another. It was the pressure arising from the bizarre quantum mechanical effect known as degeneracy. Here the same principles were at work, but it was the degeneracy of the electrons that resisted collapse. Gravity in the core of the Dumbbell was squeezing the electrons so close together that they had no choice but to speed up in a chaotic dance. This motion, though not at all related to temperature, was adequate to prevent the collapse of this star, for now and forever into the future. It would never grow much smaller than it was now, and although it would gradually cool and fade, billions of years would pass before it disappeared completely. For the foreseeable future it would remain white hot—the kind of body astronomers call a white dwarf.
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.
25
Leaving Home
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.
Turn Right At Orion Page 18
