Turn Right At Orion
Page 17
Thus it was with some frustration that I had to recall my rusty theoretical tools and infer the hidden operations of this star, trying wherever possible to enrich my deductions with the sensations afforded by my presence so close to this body. Perhaps it was a fitting irony that what I understood of Betelgeuse, close up, would have to proceed from theory rather than being gleaned via proximity to the beast. Even the space traveler, who could explore a planetary surface or atmosphere with relative ease—perhaps with a robotic probe, but directly nonetheless—was still excluded from direct experience of stellar interiors.
Astronomers had once thought that stars shone by squeezing the heat out of themselves. The squeezing came from gravity, and this nineteenth-century idea foreshadowed the later discoveries that objects with really strong gravitational fields, such as black holes and neutron stars, could become luminous by sucking in matter. But this would not work for ordinary stars, because their gravity was not strong enough. The Sun, if powered by gravitational squeezing alone, would last only 30 million years. Thus the discovery of thermonuclear fusion, the heat-yielding reactions that combine smaller atomic nuclei into bigger ones, must have come as a revelation to my academic forebears. There was a short hiatus in which everything stellar was thought to derive from nuclear power, but then gravity returned in a subtler role. If gravity did not power stars directly, its inexorable pull, combined with the changing chemical makeup of the stars as they burned away their supplies of fuel, made their internal structures, and appearances, change with time. Gravity made stars age.
I recalled the outline of how stars evolve. All stars start out with a core rich in hydrogen and a temperature just high enough to fuse that hydrogen into helium at a moderate rate. More massive stars use up their hydrogen much more quickly than less massive stars; this is why they are so much more luminous and burn themselves out so much more quickly.
At first the temperature is highly regulated and differs little from one star to another. In fact, the center of a young star possesses one of the most elegant thermostats known. If the temperature ever climbs slightly too high, the nuclear reactions run haywire and push the core apart, cooling it and quenching the reactions. If the temperature is too low, then the reactions—extraordinarily sensitive to heat—effectively shut down, allowing the core to contract under its own weight until the thermometer rises and nuclear reactions resume.
All of this proceeds smoothly, as long as hydrogen is evenly distributed throughout the core. But this situation cannot last indefinitely, because stars incinerate themselves from the inside out. Eventually hydrogen becomes scarce in the center of the core. The helium that is left behind may burn later on but for now the temperature is not high enough. The nuclear furnace retreats to a shell at the core’s margins, stuck between the star’s envelope, where it is too cold, and the star’s center, which is starved for fuel. Because the center of the star is no longer burning, the thermostat fails, and there is nothing to stop the core from shrinking. The burning layer—the shell—shriks along with it.
These are the conditions that set the stage for a red giant. Once nuclear reactions begin to run out of fuel, what’s left of the furnace gets pulled more tightly together by gravity. As it is compressed, it gets hotter and burns all the faster, pumping out ever-increasing amounts of heat and light. This was the first conundrum of stellar aging: As stars run out of fuel, they grow brighter. Energy is forced into the outer layers of the star faster than those layers can handle it. The heat gets trapped, so the envelope expands, its surface ballooning so enormously that it actually cools down even as it is pumping out more luminosity. Hence the second conundrum: As stars grow brighter, they grow cooler.
As I recapitulated these classic theoretical arguments, I began to understand the basis for my misgivings about the benignity of Betelgeuse’s deep interior. The soft and fluffy envelope concealed a searing core, shrunk down to near-Earth dimensions—a hundred thousand times smaller than the star! The core was a world unto itself, so self-contained and compact that it cared little what the envelope was doing. Thus the outcome of stellar aging, the legacy of gravity, was a gradual disconnection between the star’s core and its envelope.
No wonder the stability of the envelope seemed so precarious. The shroud surrounding Betelgeuse had been pushed to the limit, inflated to the point where it had cooled down to 3300 degrees, about as cool as a star’s envelope could get. There was a reason why red giants and supergiants could never get cooler than this. It was, perhaps, the third conundrum of red gianthood: If a star got too cool, it would release too much of its energy all at once. This bizarre effect was the result of chemical behavior that began to occur at such relatively low temperatures. It was as though the outer layers of the star were a window made of a strange substance whose transparency depended on how hot it was. If the temperature got too low, the window would become more transparent and lose its insulating powers, allowing the stored heat to stream out faster. The rapid loss of energy would cause the envelope to shrink, but—curiously—a shrinking envelope grew hotter, losing its transparency and beginning to store up heat once again. In this way, the temperature would bounce around she 3300 degree threshold, not growing markedly colder but not remaining very stable, either. This, more than anything, was why I saw the opaque “surface” of the star heaving up and down. What appeared to be vast vertical motions were only partially that. I was also seeing more or less deeply into the star as its transparency fluctuated wildly.
23
Nuclear Alchemy
As though to underscore Betelgeuse’s unpredictability, another swell of luminous garnet fluid converged beneath my craft and surged into the space around me, cooling and darkening as it expanded. This gust of matter, liberated from the star, reminded me of my original motivation in visiting Betelgeuse: to trace the raw materials of planets back toward their roots, I had not succeeded yet. The “red giant story,” as far as I had recounted it, was not useful in producing the elements heavier than helium. The helium at the center of a red giant was an inert mass not hot enough to fuse into anything heavier; the energy source of a red giant was still provided by the conversion of hydrogen—in a shell surrounding the core—into yet more helium.
Who needs stars to manufacture helium? About 1 in 13 atoms in the Universe—1 part in 4 by mass, because each helium atom weighs as much as 4 hydrogens—would have been helium to start with, just after the Big Bang and before any stars had formed. And helium, hardly reactive and nearly always gaseous, is useless for forming solid planets.
But Betelgeuse was creating more than helium. It was no red giant. It was too big, too luminous for that—it was, indeed, a supergiant. I needed to follow the star’s interior saga one episode further. The red giant story fit Betelgeuse in outline but not in detail. The story so far had introduced, elegantly, the character of the bloated envelope, its expansion, why it was red, and the growing disengagement between the assertive core and the passive shroud. All of these features would carry on to the higher level of gianthood, with a vengeance.
Once again gravity is the culprit. As hydrogen burns all around it and helium accumulates, the core continues to shrink and grow hotter. Eventually, it becomes hot enough for helium to fuse into carbon, and then (if the star is heavy enough, as Betelgeuse was) for carbon to fuse into oxygen. At first this would happen only in the center of the core, where the helium is spread evenly. The even burning of helium would create a new thermostat effect, like the one that prevented ordinary stars from turning into giants, The red giant would shrink and would come to resemble an ordinary star once again, only brighter and hotter.
However, the brush with normalcy would be short-lived. Helium at the center of the core is quickly used up—in only a few hundred thousand years, for a star like Betelgeuse—and the nuclear reactions again retreat into shells surrounding the core. But this time the multiple layers of the nuclear inferno are burning so ferociously that the star grows to supergiant size and luminosity.
But something told me that I still didn’t have the full story. As I puzzled through what must be going on in the hidden interior of Betelgeuse, I was forced to accept that Betelgeuse had once been a massive, hot star, perhaps identical to one of the stars in the Trapezium. Such stars are rare, and although each one carries more mass than an average star, they could not, all together, contrive to produce all the carbon of the Universe. Moreover, they do not release into their surroundings all the carbon they do produce. For a star like Betelgeuse, life as a red supergiant, spectacular as that stage may appear, would not be the climax. Its core would go on getting hotter and hotter, its evolution accelerating as it produced successively heavier elements until the center of its core was made of nothing but iron. Before its slow, dense wind could have completed the dispersal of whatever lighter elements are left, its core would have done something much more dramatic. The stage at which it produced and dispersed elements like carbon would be just a milepost along its road to something grander, not the ultimate use of its talents.
Accordingly, it must be the more widespread types of stars, those heavy and old enough to have exhausted their nuclear fuel but not much heavier than the Sun, that spread around the elements such as carbon. Perhaps I should have visited one of them. They would also go beyond the red giant phase to become supergiants and to create carbon, even turning some of the latter into nitrogen in an elaborate nuclear barn dance outside their inert cores. Would they, too, mix their newly formed elements with the unfused gas of their envelopes, dredging the enriched gas up to the surface and releasing it in a wind?
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.
24
The Dumbbell
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 c
enter 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.