Turn Right At Orion (22 page)

If only I could dip a thermometer deep into this churning blob, but diffuse as it was compared to other stars, the heat coming off its surface and its opaque screen of red-glowing gas kept me at bay. Most of the way to the center, the temperature could not have risen much above 100,000 degrees. That would not be enough for nuclear reactions to create the amount of light that was bathing
Rocinante
and shining into space in all other directions. Thermonuclear reactions need high temperatures, to bang the atomic nuclei together with sufficient force, and high density, to ensure the fierce collisions occur frequently enough. Betelgeuse had neither, at least within its huge mottled envelope. Deep inside this star, however, there had to be someplace where nuclear reactions did occur, and they had to be sustained at a rate that far outstripped stars like the Sun. A prodigious amount of energy was forcing its way through the star so insistently that it kept the star's outer layers off guard, unable to settle into a passive role as the mere conveyor of stellar luminosity. The envelope of Betelgeuse was actively involved in transporting its energy, a role that rendered it restless.

The star took another big gulp, and a new abyss seemed to widen under my craft. I half-expected the curtains to part and even this nebulous but opaque surface to give way fully, allowing me to see down deep into the star's insides. Then the displaced layers of gas rushed back in a tsunami that swept closer to the underside of
Rocinante
than I could stomach. I pulled farther away from the star.

The delicate imbalances that characterized the surface of Betelgeuse made it seem fragile, but something about the star hinted at a concealed ferocity that I did not wish to challenge, even if I could. I was not yet ready to depart from the vicinity of Betelgeuse, but it was apparent that I would have to fall back on a much greater dose of theory than I had anticipated. Of course, I had studied red giants and supergiants ages ago. The theory of stellar structure had been
de rigeur
for students of astronomy. It was the crowning glory of theoretical astrophysics in the mid-twentieth century; the prediction that stars should grow to the enormous dimensions of giants late in life was one of its greatest triumphs. From the remote point of view of an astronomer on Earth, it would have been thrilling to know everything about a certain type of star without having seen one—and the theory was good enough that this was almost possible. But the features of Betelgeuse that struck me most viscerally were those where the theory was weakest. Theoretical sketches of a smoothly distended envelope paled beside the great waves of matter that sloshed around the star as its internal supply of energy struggled to get out. Statistical descriptions of the bubbling and turmoil that went on just below the surface had never really captured the sudden emergence of turbulent cells that I saw growing to cover
degrees of longitude. And the theoreticians had never quite been able to predict just how quickly such a star would erode, mostly in spurts and gasps, of its own volition.

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.

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.

Thus it would seem that Betelgeuse had already produced some of the materials out of which one could mold the Earth and the other planets I had visited. According to the theory, carbon and oxygen would be joined by some other trace elements. But would they be disseminated into space? Could Betelgeuse's wind be the medium of their dispersal? My measurement of the wind's composition, as a fresh gust blew past, seemed to bear out the idea. The wind was cool and dense enough for molecules to form, even for solid grains to condense, and condense they did. There were the common terrestrial gases carbon monoxide and carbon dioxide, some odd rarities such as the oxides of zirconium and titanium, and soot—the carbon dust that could someday fill a protoplanetary disk. The disconnection between
core and envelope evidently had not been so absolute as it had seemed. The churning streams of matter carrying energy to the surface had mixed with some of the freshly cooked elements and swept them aloft.

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?