Turn Right At Orion (23 page)

It sounded like an awfully cumbersome process. It would be much more helpful, I thought, if a star revealed itself in a more systematic way. Peeling away layer upon layer, it could offer up the results of its nuclear alchemy, the strata of newly cooked elements,
in order. If they had been somehow mixed, the cauldron stirred to ensure that the stew wouldn't be lumpy, then the star's disassembly would reveal that, too. In fact, some stars had to give up not just their outer layers, but also much of their deep interiors, if deep space were to receive enough raw material for the building of planets and, for that matter, new stars.

All this time I had been allowing
Rocinante
to drift away from Betelgeuse. From this distance I could finally take it in as a whole: the way it barely seemed to cohere, the sloshing that threatened to dismantle it at any moment, the unsteady wind that was gradually eroding it. I was far enough away to grasp the crudely spherical but uneven shape of an entire hemisphere. As I watched, it erupted again. A wave seemed to envelop the whole star, it pulsed, and a shell of gas flew off into space. Had I had the time, I would have been tempted to wait for the dénouement : the loss of this star's entire envelope and the final revelation of what was inside.

It's lucky I didn't. I already suspected that this star's fate would be dangerously violent, as I was to appreciate—in an encounter with a different star—later in my journey. Betelgeuse would not suffer a dénouement but rather an apotheosis of self-destruction. But there were plenty of other stars that had dismantled themselves, benignly and recently enough that I could perform just the study I proposed.

All amateur astronomers are familiar with planetary nebulae. just the name conjures up an amalgam of the two most irresistible targets for homespun telescopes. The discoverer of the class, William Herschel, had named them deliberately, only four years after he had discovered a real planet (the first such discovery since ancient times), Uranus, in 1781. These nebulae often presented smooth, bright disks, sometimes so compact that their shapes were difficult to make out in telescopes of his time. They can be bright and glow with an even fluorescence that could well be mistaken for the reflection of stellar light by a planet—not that Herschel ever took them for solid bodies. They also show more complex structures. The first one identified by Herschel possesses two luminous extensions, protruding on opposite sides, which led observers to refer to it as the “Saturn Nebula.” Another, a staple of my days as an amateur stargazer, was known as the “Ring” and famed for its delicate elliptical shape and apparent hole in the middle.

I was headed toward one of the most famous planetaries, known colloquially as “The Dumbbell.” This was one of the largest and least well defined of the planetary nebulae, a full 2 light-years across. The name was apt only insofar as the nebula did not approximate a full disk or ellipse but seemed incomplete,
with large bites taken out of opposite sides. This gave it a linear or box-like appearance; in three dimensions one could imagine it as a narrow-waisted hourglass. At the ends of the hourglass, intact arcs of the remnant circle possessed bright rims, and one could make out—or imagine that one did—a ridge of luminosity connecting the arcs through the middle of the ruined disk. One can only suppose that, to some nineteenth-century astronomer with an imperfect lens, the nebula must have resembled a pair of weights hanging off the ends of a bar.

As viewed from Earth, the Dumbbell lay in the direction of the constellation Vulpecula, the fox, nearly halfway round the sky from Betelgeuse and nearly twice as far from home, 800 or 900 light-years. I took a direct route, which meant that I passed closer to Earth than I had been since leaving—100,000 years earlier, by Earth time!—but in my haste to follow through on my quest, I did not stop. What this route did afford me was a view of the nebula not too different from the one I had known as a child.

Planetary nebulae are justly acclaimed for their great range of fluorescent colors, often arrayed in beautiful and orderly patterns. The extraordinary diversity of colors is no mystery. Just as the hot stars of the Trapezium illuminate the Orion Nebula, each planetary nebula possesses its own illuminating star. But the stars that light up planetary nebulae are hotter than those of ordinary nebulae. The powerful ultraviolet rays that they emit tear into the atoms of the nebula with greater destructive impact. Whereas the light from the Trapezium stars can knock one or two electrons off the oxygen atoms of Orion, fry most of the hydrogen, and wreak mild havoc on atoms of other chemical elements such as neon, sulfur, iron, and helium, it cannot, for example, tear both electrons off helium atoms simultaneously. But in a planetary nebula this is done with ease. The result is that the diverse opportunities for atoms, ions, and electrons to recombine in different permutations yield a much richer stew of atomic activity and hence a richer palette of colors. The gas in a planetary nebula is also hotter, fostering additional fluorescence as the atoms knock into one another with greater force.

There is another, more fundamental difference between the Orion Nebula and a planetary nebula like the Dumbbell. Orion is a patch of raw material from which new stars and planets are condensing—matter derived from a molecular cloud, which had in turn scavenged it from interstellar space. A planetary nebula is at the opposite end of the recycling process. It consists of matter that is being returned to interstellar space. The Dumbbell is precisely the substance of an old star's interior, the insides of a defunct red supergiant, expanded and made visible. The Trapezium cluster consists entirely of newborn stars; the star at the center of the Dumbbell is on its deathbed.

As I approached the nebula, I decided to maneuver my craft so as to enter along the axis of the “dumbbell.” The comparatively smooth distribution of interstellar matter in this locale gave way to a lumpier texture even before I reached the bright rim that marked the threshold of the glowing gas. I surmised that I had already crossed into the region that had been overrun by the wind from the red supergiant. Closer in, I began to perceive the glow of gas being attacked by the vanguard of ultraviolet rays, first in a spotty pattern and then more uniformly. The atoms were not being treated too roughly here, a fact I attributed to the relative mildness of the photons that had managed to penetrate this far. All of the most extreme ultraviolet rays from the star had been absorbed—used up—by the gas closer in and were not reaching this distant outpost of the nebula. Consequently, the dominant colors were those of the atoms and ions that were easiest to knock apart: the reds of hydrogen and of weakly disturbed nitrogen, for example. I was slightly surprised that there was no sensation, other than visual, as I crossed into the outermost luminous arc. I had half-expected a slight bump, conditioned as I had become to associating sharp and bright boundaries with shock waves. But this gas was still more or less the undisturbed effluence from the old supergiant. The flow here was leisurely. As I knew from my experience at Betelgeuse, the wind coming off the extremities of a red supergiant might well have had velocities less than the 20 or 25 kilometers per second I
measured here. The wind's speed also gave away the time that had elapsed—30,000 years—since this gas had taken leave of its parent star and joined the ritual of unraveling that I now witnessed in its latter stages.

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.