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4
Ground Zero
I was surprised that my arrival close to the Milky Way’s central black hole did not strike me more viscerally. There was never a point at which I could feel, between my head and feet, the stretching force due to the stronger gravity closer to the hole. I knew that these bigger black holes are altogether more gentle on visitors than their smaller counterparts, yet I expected some gut feeling to tell me that I was in the presence of an enormous source of attraction. The visuals were hardly as dramatic as I expected, either. There was no grand disk of superheated gas, crackling with energy, swirling into the black hole. All I could see was a diaphanous, bluish luminescence surrounding what I deduced to be the black hole’s location. I could tell where the hole was by observing the distortions of the stars beyond it as their tight rays curved while crossing the hole’s gravitational field. A lens-like distortion of a distant stellar scene, some subtle gradations of the blue glow—was that all there was to see of the Galactic central black hole?
Yes, I was disappointed, but I should have seen it coming. What made this black hole appear so serene was that it resided in a near vacuum. There was no “donor” star to be plundered for its substance, as I later encountered while visiting the much smaller black hole known as Cygnus X-1. The dense interstellar clouds were few and far between, and they were so stirred up by all the hot stellar winds and supernovae that they responded little to the black hole’s lure. Quite simply, there was very little matter flowing into the black hole at the center of the Milky Way.
I ventured into the outer reaches of the blue-glowing corona. The gas there was so tenuous that I felt safe enough immersing my craft in it. I measured the radiation and found that it was not really “blue.” It merely appeared blue when filtered through my visual range. Electromagnetic radiation was being produced across the entire spectrum, from radio waves through microwaves, infrared, visible, ultraviolet, X-rays, and gamma rays, the bulk of it coming out in the infrared and ultraviolet. I tested the temperature of this gas, which was a measure of the energies (and thus the chaotic motions) of its constituent particles. I was still hoping to accumulate more evidence of the effects of gravity pulling the gas inward, thereby causing the particles to move faster. As expected, the temperature seemed to be going up about inversely with distance from the black hole; it reached a billion degrees when I was roughly as far from the black hole as Pluto is from the Sun. But my instruments did not seem to be operating quite as reliably as usual, and I began to record unsettling temperature swings.
The reason proved to be simple, though bizarre: This gas did not have a temperature, in the sense in which temperature is usually understood. At such low densities and high speeds, it is difficult for particles to share their energies with one another. The roughly equal sharing of energy among all particles in a gas is the hallmark of “temperature.” In Earth’s atmosphere, this kind of sharing occurs so instantaneously that one can be sure an average oxygen molecule will carry the same amount of energy as the average nitrogen molecule and will therefore be moving with only 93 percent of the nitrogen molecule’s speed (the square root of ⅞, which is the inverse ratio of their molecular weights). But here, I found that the electrons had wildly different—usually lower—energies from the protons, despite the fact that all the particle varieties were mixed together. To make matters even more confusing, I found that a small fraction of the electrons had vastly higher energies than the protons.
This weird distribution of energies went hand in hand with the weird spectrum of radiation. I pictured the glowing heat-shield of my craft on the innumerable occasions when I had re-entered the Earth’s atmosphere in early tests. That progression of colors—red, then orange, yellow, and blue-white—is burned into the visual memory of any habitual space traveler, as is its significance: It tells you the heat-shield is getting hotter. Any solid substance glows with a fixed shade of color that depends on its temperature. The same goes for dense gases, like the atmospheres of these blue stars in the Milky Way’s center (20,000–30,000 degrees), the atmosphere of the Sun (5800°C), and even the Earth’s atmosphere (glowing in the infrared at about 300°C above absolute zero). But the gas surrounding the Milky Way’s central black hole is so transparent, so tenuous, and so “non-thermal” that its “color” seems to have nothing to do with its temperature. Or maybe it is more precise to say that it has no well-defined color. Or, if you will, it is so blessed with electromagnetic radiation of all kinds that it can’t decide which color to be.
I followed this multicolored whiff of glowing gas as far I could toward its doom. In the interest of safety, I took care not to venture below the 24-million-kilometer “orbit of instability.” This is not the horizon of the black hole, below which there is no way to escape—that’s at 8 million kilometers. But according to Einstein’s general theory of relativity (the theory that describes black hotels), it is as close as one can orbit without constantly firing thrusters to keep from failing in. I had no confidence that my piloting skills could keep me out of trouble if I went closer. The corona at this distance exhibited less orderly motion than I was later to find in the disk of Cygnus X-1. Gas was rushing around in all directions. In some sectors it was plunging inward; in others it seemed to be blasting outward in a comparably chaotic rush. Superimposed on these turbulent eddies, the gas was swirling around the hole faster and faster the closer I got. The pressure was so high that I expected it to enhance the effect of gravity in sweeping gas into the hole at a prodigious rate, by pushing inward as gravity pulled. Yet just the opposite seemed to be happening. As the gas was pressed toward the hole, it only swirled faster, and that just seemed to make it stiffen, somewhat like hard rubber. The stiffness of the gas seemed to be holding it back, enabling it to resist the black hole’s lure even as it crossed the 24-million-kilometer threshold from which I watched. Where did it begin its final plunge? I peered toward the hole and barely made out, at about 16 million kilometers from the hole, what looked like a sudden drop in the glow from the gas. I convinced myself that this was where the pressurized resistance to gravity failed and the gas thinned out as it was finally sucked in. But let me be honest: The appearance was so subtle and nondescript that to this day I do not know whether I saw the Milky Way’s giant black hole swallow anything of substance.
There is an old saying among astronomers that black holes are the Universe’s most efficient engines. But what I found in the Milky Way’s center seemed to give the lie to this old dictum. What the saying meant was that you could get the most energy out (in whatever form: light, heat, jet propulsion. . . ), for the least amount of fuel, if you let the fuel spiral into a black hole. Of course, there was no way that this energy could come from inside the black hole; the idea was that the very hot gas would release its energy just before sinking beneath the horizon.
It may seem odd to worry about the fuel efficiency of a black hole when the fuel requirements of any astronomical object are . . . well, astronomical. To power the Sun with nuclear reactions, for example, 620 million metric tons of hydrogen must be converted into helium every second in the Sun’s interior. Yet if the Sun were a giant coal furnace, one would have to shovel in more than 20 thousand trillion tons of coal per second to achieve the same power output. Thus nuclear fusion is more than 30 million times more efficient than coal power. Instead of looking forward to a life expectancy of 10 billion years (of which 5 billion have passed already), during which it will use up only 10 percent of its supply of hydrogen, the coal-powered Sun would incinerate itself completely in 3000 years. Common wisdom had been that “only” 43 million tons per second of any kind of matter would need to be fed into a black hole to produce the Sun’s luminosity, and this would make gravitational power 15 times more efficient than its nuclear counterpart. Indeed, I later found a black hole in which energy was being released with such high efficiency.
The big black hole in the center of the Milky Way was not in this league. Granted, the gas drifting into it was releasing 1
000 times more energy per second than the Sun, but compared to the black hole’s mass this was a pittance: The hole weighed more than a million suns. And it was gobbling up not 43 billion tons of fuel per second, as a “fuel-efficient” model would have, but thousands of times more. Surely all that matter was releasing an enormous amount of energy as the black hole’s gravity compressed and accelerated it. But only a tiny fraction of that energy was getting “out.” Where was the other 99.99 percent going? As I pondered this, I realized why I had found it so difficult to spot the demarcation between the gas swirling through the corona and that rushing in toward the black hole. Instead of shining brightly, the former was nearly invisible because it was jealously guarding its store of energy. Much of this energy was being dragged into the black hole, never to emerge. And some—perhaps most—was being shot back out into space as a super-hot wind, helping the massive stars to stir up the Galaxy’s central cavity.
I was discouraged. I had set out to learn all about the organization of the Universe by studying one big black hole, but the one I had picked to study had turned out to be a dud. What was clearly “wrong” with the Galactic Center black hole was that it occupied an anemic environment. For whatever reason (the powerful winds from all those hot stars, the gravitational deflections of the bar—it didn’t really matter what the cause was), the effect was that this black hole was not eating enough to make it interesting. Sure, I could measure the increasing force is I approached the hole; I could see how it sped up and concentrated the motions of stars; I could even study the distortions of rays of starlight as they, too, felt the effects of the hole’s gravity. And I did see something of the enormous capability of a black hole to set gas in motion and got an inkling of its ineffable ability to endow inbound gas with so much energy that the most violent thermonuclear reactions pale in comparison. But I realized—with a touch of irony—that I had learned more about gravity from other structures I had seen than from the black hole. The odd “tumbling peanut” of the Galaxy’s bar and my musings about spiral arms had shown me the crucial role of motion in opposing gravity’s attractions, not to mention the subtle interplay by which gravity works with motion to create structures. And despite its relatively puny energetics, the collision and merger of two ordinary stars had put on a much more spectacular pyrotechnical display than anything I had seen the black hole do.
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.
Part Two
MOTION
5
The Cannibal
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?