Turn Right At Orion (6 page)

I was desperate to find a way to salvage my mission. I began to experience bizarre daydreams, in which I visualized various ways to make the Milky Way's black hole light up. I imagined all kinds of strange schemes to bring a large quantity of gas into the immediate vicinity of the black hole and to dump it in all at once. Subjected to rational scrutiny, most of these ideas betrayed the kinds of inconsistencies that daydreams usually suffer. But some seemed half plausible, perhaps informed by scholarly tracts I had read before my departure. One in particular seemed highly realistic. What if the black hole swallowed an entire star?

My daydream began with a vision of those stretching forces I had searched for in. vain when I first descended toward the black hole. These had proved too weak to be discernible by a person, but the same cannot be said for ordinary stars that venture too close to the hole. Even though it weighs as much as 2.5 millions Suns, the Milky Way's central black hole is too small, too concentrated to swallow an ordinary star whole. Instead, its gravitational
field will pull apart any star that comes closer than about 10 times the radius of its horizon, or about 80 million kilometers.

As I drifted off, I saw in my mind's eye a doomed star, approaching from out of nowhere. Of course I knew it was really on a highly elongated orbit, jostled into making the plunge by its gravitational interactions with other stars. As it approached, I observed the familiar tidal swelling rise on the star, an effect due this time to the difference in the black hole's pull on the star's near and far sides. But in contrast to the stellar merger, where the tidal forces had bad a gentle touch, here the distortion got greater and greater the closer the star came to the hole. Eventually, the star's gravity could no longer hold it together, and it simply came apart. But not in a gentle way: The black hole's gravity ripped it to shreds.

The dance metaphor, perhaps overworked when I tried to describe stellar mergers, popped back into my head. I saw the debris of the star performing an elaborate dance, only this time it was more like a tango than a ballet. About half of the material was flung off into space at high speed, a shimmering spray, never to return. The rest returned to an elongated orbit, but now as a curtain of matter that swung far from the black hole, then fell back into its grip. I wondered whether it would ever settle down; for a time, it swooped in and out, on ever more surprising trajectories. But settle it did, and what remained in the end was a thick donut of gas, swirling in a regular fashion. It was rotating too fast to fall into the hole, but as I watched, it gradually spread inward as well as outward. Eventually, the inward side reached the black hole and erupted in a blaze of glory. This time, there was no question of all the energy disappearing into the hole. Briefly, the Milky Way's central black hole shone, in my imagination, with a power a billion times greater than its normal blue glow.

My alertness returned; I felt refreshed. Should I wait for a tidal disruption to occur, as it must eventually? No plunging stars were in sight, and I certainly could not wait the 100,000
years—maybe more—until a star committed suicide in this spectacular way. I decided to leave.

I now realized that it wasn't sufficient merely to witness gravity's potential to attract and to set matter in motion. One had to pay careful attention to the nature of that motion if one really wanted to understand how things worked. I should have seen the hints earlier. Every time I had focused on the organizing power of gravity, I had been drawn to its influence on the motions of things, whether it be the eggbeater effect of the Galaxy's bar, the self-sustaining sweep of the spiral arms, or the violent encounter of star with star or star with black hole. No matter how important gravity was, ultimately, as the root of cosmic structure, just floating in space and contemplating a huge black hole was not going to teach me everything I needed to know. I had to find a more dynamic system to study.

Legends had circulated for years about the existence of black holes that constantly gorged themselves, in a kind of cannibalistic ritual, on nearby dying stars. They were said to draw in matter so ravenously that they positively sparkled with X-rays and flickered at rates that could have stroboscopically frozen a hummingbird's wings (if one could have illuminated a hummingbird's wings with X-rays). The strange thing—and here is what made it so hard to believe—was that these black holes, unlike the one I had just visited, were not massive sinkholes lording it over an entire galaxy These well-fed black holes were reputed to be the mere remnants of ordinary stars, barely a few times more massive than the Sun. And yet the pathetic, gaunt aspect of the starved black hole at the Milky Way's center haunted me. I had just seen a million-solar-mass black hole, in the center of it all, not even commanding enough nibbles to keep itself shining brightly. How could these lightweights possibly maintain their gluttonous habits? Deeply skeptical, I set out to investigate.

Being a conventional tourist at heart, I set my course for the first black hole that comes to anyone's mind: the granddaddy of them all, Cygnus X-1. This system is located about the same distance from the Milky Way's center as Earth is, but in a completely different direction; it is roughly 6000 light-years to the
east of the Solar System, as measured along the midplane of the Galaxy's disk. As I drifted into hibernation, I clearly recalled the heady days of the 1970s, when we had pored over each new piece of data as it came in, trying out those convoluted, indirect arguments. Was it a black hole, or not? Yes, it was so luminous that it had to be at least as massive as the Sun; otherwise, its own radiation would have inflated it so grotesquely that it would have burst (or suffered some similarly disgusting fate—we weren't sure what). Yes, it was emitting X-rays and was flickering so fast that it had to be very small. Less than 300 kilometers across. Remarkable. Ah, but neutron stars had been discovered by then, and they were
almost
as compact as black holes. Could it, just possibly, be a neutron star? And then the clincher. That 5.6-day orbital period, the discovery of the companion star: It was a member of a binary system. Then, even more remarkably, the measurement of how fast the companion was moving, or at least how fast it was moving toward or away from us (any sideways motion, along the sky's dome, could not be detected), which enabled one to say something about how heavy the X-ray emitter was. The “mass function” of Cygnus X-1 became the talk of every astronomy department canteen. It was really a lower limit to the mass of the “compact object,” and it did rely on one's having correctly interpreted the companion star's speed, but it was a big lower limit—more than 3 solar masses.

What had been seen, when boiled down to essentials, was encoded in the object's name. Cygnus X-1 was the premier source of X-rays in the entire constellation of Cygnus, the Swan that glides along the Milky Way on a summer's night. It was tiny and too massive to be a neutron star, the only other kind of object that could approach it in compactness. By the process of elimination, it had to be a black hole. That was as well as one could do in those days, I thought, as I approached the system for my first close-up look.

I knew, of course, that the binary nature of Cygnus X-1 did more than provide a convenient method of estimating its mass.
By this point I understood that it was the interaction of a black hole with its surroundings that was the key to anything observable. I had learned that lesson painfully as I retreated, in disappointment, from my encounter with the sterile Galactic Center black hole. Cygnus X-1 shone so brightly
only
because it was devouring its companion. But how could two stars, presumably born in binary harmony—maybe even twins—have descended into such an atavistic and violent relationship?

What was clear was that the two partners of the Cygnus X-1 system must have completed an elaborate dance long before I arrived. I recalled the numerous articles that had speculated on this distressing topic. It was very likely, indeed, that the two members of the Cygnus X-1 system had been born together. They were fraternal, not identical, twins, for it was the remnant of the originally heavier twin that was now dining on its sibling. What was now the black hole had once been an ordinary star—30 or 40 times more massive than the Sun, hotter and a million times more luminous, but otherwise similar. Before its collapse to a black hole, it had spiraled so close to its companion—at the same time swelling in the normal decrepitude of stellar old age—that the two stars had mingled, with the dangerous result that most of the matter in the proto-black-hole had drained into its partner. Now that the matter was draining the other way, I wondered: Was this the payback for earlier sibling treachery?

It is usually assumed that the more massive star of a coeval duo will meet its maker first, because more massive stars burn up more quickly. Therefore, you will be forgiven if you think it paradoxical that the proto-black-hole died and collapsed
after
losing most of its mass to its companion. This common confusion results from a failure of many commentators to distinguish adequately between the initial mass of a star, which determines how fast its interior ages, and the composition and structure of the star's core, which contains only a fraction of its mass. It is the latter that determines the star's immediate fate, and in the case of Cygnus X-1, the proto-black-hole's interior had aged too far to be rejuvenated merely by losing weight. And a good thing,
too, for if the proto-black-hole had collapsed (very possibly with an explosive release of energy in its outer layers) before losing most of its matter to its neighbor, then we would not have had a binary system left to study today. The recoil would have sent the black hole careening off into space, bereft of its brother and left to starve alone.

I synchronized my approach with the rotation of the system as it swung around its center of gravity. It was obvious that the star that had not become the black hole bad aged considerably since its weight gain under dramatic circumstances. This was to be expected, because it was now quite a massive star. It was beginning to bloat, as more and more of its nuclear fuel was expended in producing its now-prodigious luminosity. It also looked far from normal, even for its age and mass. Because the black hole tugged more strongly on the hemisphere facing it, that entire side of the star was drawn into a vast, ungainly bulge, tapering almost to a point. I recalled the tides I had seen risen on the stars approaching collision in the Milky Way's center and the more dramatic distention that preceded the destruction of the star that had ventured too close to the black hole in my daydream. But unlike those doomed bodies, this pointy bulge was not growing with time. It seemed to have reached some kind of stasis, with matter from the star flowing into the distension and then spilling through the point. This black hole, it appeared, was not tearing its companion star apart but instead was subjecting it to a more exquisite, drawn-out form of torture.

The stream of gas being sucked off the star was not going quietly. It was full of turbulent curlicues, swirls, and sprays, with an occasional sudden discharge of gas that reminded me of a colicky water faucet, I tried to focus on individual eddies—swirling, dark spots in the turbulent gaseous fluid—and to follow them as they passed through the constriction and toward the black hole. This was not easy; the contrasts were subtle and the flow complex and unsteady. But I was able to discern some regularity in the flow and saw that the matter did not fall straight toward the black hole once it had passed the constriction. On the side of the
constriction toward the black hole, the flow was strongly deflected from making a beeline for the hole. I supposed that it retained the memory of its original orbital motion, which carried it so far forward that it overshot its mark. Despite the exotic setting, I recognized this as the same phenomenon that kept satellites from falling to Earth, and the name of this orbital memory, “angular momentum,” made it all seem commonplace. In a moment of panic, I feared that none of the matter spilling through the constriction would actually reach the black hole. Was this black hole, too, going to starve, despite being surrounded by a cornucopia of gaseous food? Then I remembered all those X-rays—obviously,
something
was feeding this black hole—and I pressed on.

I followed the deflected stream as it gradually spiraled toward the hole. Now it was quite narrow and hence easier to follow, but I began to notice an increasing amount of swirling matter that was not part of the stream. The motion of this swirling gas was directed nearly at right angles to the straight path into the hole, and it did not diffuse through the entire space, as the tenuous corona surrounding the Galactic Center black hole had done. Rather, it seemed to be confined to a platter, or disk, hugging the plane in which the binary twins gyrated. The stream had to plow through this material, and eventually the resistance it encountered took its toll. The stream began to shred around the margins, to wobble in its path, and to spread from its well-focused trajectory. But it faced a still more formidable obstacle. About two-thirds of the way from the constriction to the black hole, the spiraling stream finished one complete circuit and crashed into itself. I quickly pulled away from the disk as I saw the great splatter of superheated gas looming ahead. Gas was thrown up out of the disk, and ripples spread out sideways as the swirling platter tried to come to terms with this unexpected disturbance.

This was the end of the stream—but the source of the disk. I watched the debris of the collision rock back and forth as it settled down, and I saw it spreading into a ring, which then broadened both toward and away from the hole. It thinned as it spread, until it merged into the very disk-like structure that I had encountered on the way in. The pattern behind its motion was now clear: The gas was distributing itself into nice, regular orbits about the black hole, just like a planet orbiting the Sun or a satellite orbiting the Earth. I thought of the Milky Way's disk, and if I imagined each star as an atom of gas, I could perceive a similar pattern. Just as the stars marched nearly in lockstep about the Galaxy's center of mass, so would the atoms of gas in the disk trace broad, stable circles about the black hole, oblivious to the danger they would face if they ventured too close. Because the disk was so much less massive than the hole, there was not even the risk that something akin to spiral arms or tumbling stellar bars would mar the regimental parade, as I had seen them do in the Galaxy. I knew that the gas could stay in that state of motion virtually forever. But clearly that could not be what was really happening, if gas was draining into the hole at the prodigious rate indicated by the X-rays. In order for it to spiral in, something had to be dragging on the orbiting gas, drawing angular momentum from it as it circled the hole. To find out what this viscous agent was, I had to enter the disk itself.