Turn Right At Orion

by Mitchell Begelman

  There is, however, one complication in a galactic disk that does not apply to the Solar System. Unlike the Solar System, where 99 percent of the mass is concentrated in the Sun and the individual planets have minimal effect on one another’s motions, a galaxy’s gravitational field consists of the combined gravity of all its constituent stars and gas clouds. This can make for very complicated orbital motions, because the amount of matter a body is orbiting depends on how far it is from the center, on whether the matter is symmetrically placed about the center, and so forth. Moreover, it is not just the mass of the disk that holds the galaxy together. Theorists have shown what would have happened to the Milky Way if it had consisted of a disk alone. It would have been violently unstable, sloshing from side to side, even buckling, and ultimately losing its disk-like shape altogether. The reason why this does not happen is that the entire disk is embedded in a vast spherical halo consisting of an even larger number of stars; very tenuous, hot gas; and a third, catchall component that is so hard to detect that astronomers still refer to it as “dark matter.” In fact, most of the gravitational pull that keeps the Solar System from shooting off into intergalactic space comes from the halo, not the disk. The stars in the halo are mostly old and faint, which makes even the “non-dark” part father hard to detect. But individual halo stars do occasionally give themselves away because they do not participate in the orderly orbital motion of the disk stars. As I traveled I found interlopers easy to spot, because their motions seemed to be completely random, and they were usually moving much faster than any of the disk stars.

  Even with the halo in place, the galaxy’s disk is still slightly unstable, and it is this slight tendency toward imbalance that generates the graceful spiral patterns. As in most great art, the slight imperfections are what count. I had always admired pictures of well-formed spiral galaxies for their aesthetic qualities, but I knew that the mechanism that leads to spiral structure is prosaic and explicable entirely in terms of the subtleties of gravity. Suppose that by some chance fluctuation, a group of gas clouds and stars bunch up a bit as they orbit the galactic center. The bunched-up matter exerts a little extra gravitational pull on the surrounding stars and gas clouds and slows them down slightly. They then bunch up, while the original bunched-up stars and gas clouds gradually return to their normal, clockwork paths around the galaxy. The newly bunched stars and gas then cause the next group of stars to bunch up, and so forth. A bottleneck is created and perpetuated: a classic rubbernecking delay.

  Thus the spiral “arms” are more aptly called waves, because they are not really objects but rather patterns. They are merely the spiral-shaped loci where the stars and gas of the disk slow down briefly as they march around the galaxy’s center—a kind of interstellar traffic jam. Traffic jams may one day seem quaint anachronisms on Earth, once vehicles are controlled by computer, but it is doubtful that traffic engineers will ever learn to tame gravity.

  What make spiral arms so obvious are the “tracers” that outline them: chains of molecular clouds and clusters of newly formed stars. If spiral arms are traffic jams, then the tracers are the result of accidents caused by the backup. As the gas clouds bunch up, some of them run into one another and merge, creating molecular agglomerations larger and denser than average. With increasing size and density comes an increase in the local gravitational force. These giant molecular clouds are set with a gravitational hair-trigger, primed so that their own self-attraction will overwhelm them given the slightest provocation. Any sudden compression as two clouds collide will cause them to collapse under their own weight. Perhaps that’s all it takes to trigger star formation: cloud collision and—voilà—spiral arms are bejeweled with brilliant strings of young stars.

  As I mused on the nature of the thick cloud layers that surrounded me, it became clear that I had already left the spiral realm behind. These gaseous strata were too thick, too permanent-looking to represent anything as ephemeral and delicate as the passage of a spiral wave. I realized that I was already too close to the Galaxy’s center to encounter spiral structure, and I recalled a map I had once seen, showing the positions of the Milky Way’s spiral as deduced by radio astronomers. According to this map, if you were to look at the Galaxy from above the disk, you would see the arms splayed out openly in the outer regions of the Galaxy. Indeed, I would have crossed better-defined arms had I headed in the direction of Cygnus or Perseus, instead of toward Sagittarius. Closer in to the center of the Galaxy the spiral pattern becomes more tightly wound, less spiral-like and more like a series of concentric rings. I figured that I must have been passing through these rings, just about at the place where they begin to merge into an undifferentiated continuum.

  Suddenly I understood that even this detail was a subtle part of the scheme by which gravity organizes the Galaxy. Gas passing though a spiral arm tends to lose just a little bit of its orbital motion because of the retarding effect of gravity, and consequently, it drifts closer to the center. Thus spiral waves help gravity to achieve its universal goal of attracting all kinds of matter—stars, gas, whatever—toward a common center. In these dense molecular and atomic gas are cloud-layers a few thousand light-years from the Galaxy’s center, much of this drifting gas seems to accumulate.

  I was now expecting a gloomy ride the rest of the way in. Gravity indeed would like to collect ever-denser banks of cloud and dust toward its central focus. Surprisingly, though, the gas doesn’t drift all the way to the center. As I emerged from the innermost molecular band—a couple of thousand light-years from ground zero—the atmosphere was still murky, but I noticed that the clouds were becoming much patchier than in the dense smogbanks I had just traversed. A view of the scene in infrared rays showed why. I was now passing through the “bulge” of the Milky Way Galaxy, where the disk appears to puff up and meld with a sort of inner halo. An ever-increasing fraction of the stars around me were moving randomly, rather than marching in lockstep circular orbits. The Galaxy’s geometric center lay dead ahead, but there was something strange about the pattern of stars on the sky. I hesitated for a moment, disoriented. But then I saw what the problem was. Instead of being symmetrically arrayed under the evenhanded attraction of gravity, as I had intuitively expected, the distribution of stars was lopsided! There were more stars to the left of the Galactic Center than to the right.

  I was face to face with the Galaxy’s stellar “bar,” which resembles nothing so much as a huge tumbling peanut made up entirely of stars. Stars caught up in the gravity of the bar execute motions that are far different from the orderly circular orbits of disk stars, different even from the chaotic dashing of halo stars. These trapped stars trace out semi-repetitive shapes ranging from figure eights, to complicated cat’s cradles, to woven tubelike structures reminiscent of those “Chinese puzzles” that trap your fingers. It is amazing that these patterns can hang together, but bars are remarkably robust and are found in a good half of the spiral galaxies.

  Thanks to the bar, for once the inexorable central pull of gravity is foiled. The bar hinders the inward drift of gaseous debris. The churning gravitational forces produced by the tumbling peanut stir up the motions of any gas clouds that venture inside, driving the clouds slightly farther from the Galactic Center. That explains why I was emerging into a region of the Galaxy where the clouds were becoming sparser. But it does not explain what I saw next.

  3

  A Ballet

  Suddenly I emerged into a clearing only 10 or 20 light-years from the Galactic Center and saw a cluster of stars the likes of which I had never seen before. Nearly a million stars were crammed into a volume that would have been occupied by only a few dozen stars had it been in the vicinity of the Sun. I was immediately struck by—and would lose several night’s sleep to—the many blue-white, blindingly luminous stars that were mixed in with the more common stellar varieties in every direction. These kinds of superstars, though only a few times more massive than the Sun, were so bright that they were doomed to burn themselves o
ut in a blaze of glory, I had seen a few of them en route from the solar neighborhood, but everywhere else in the Galaxy they seemed to be quite rare. This relative rarity was not a surprise, because such stars last less than 10 million years (compared to 10 billion years for a star like the Sun) before throwing off their envelopes in violent convulsions and, if they are heavy enough—more than 8 or 10 times the mass of the Sun—exploding as supernovae. But even before they become violently unstable, they will have spent most of their lives injecting fast, hot streams of gas into their surroundings.

  Given how many massive stars there were, it came as no surprise that the whole Galactic Center was immersed in a kind of hot bubble, similar to the superbubbles I had traversed earlier but lacking their opportunity to vent into the Galaxy’s halo. As a result, the pressure outside my craft had increased enormously, although it was still much lower than any vacuum ever produced on Earth. I had measured it when I arrived in interstellar space near the Sun: There it was 0.000000000000000001 of the mean atmospheric pressure at sea level on Earth. But in the Galactic Center it was thousands of times higher than in the Sun’s vicinity, pumped up by the combined effect of the fast winds and the added jolt of a stellar explosion every thousand years or so.

  How did these stars get here? Because they were burning up inside at such a prodigious rate, they couldn’t have been older than a few million years. Were they formed here? I looked around for likely sites of star formation, giant clouds rich in molecules, shielded from the heating and evaporative influences of hot stars and supernova blasts. But I couldn’t find any. Streamers of compressed gas stretched here and there, squeezed by the high pressure and combed out along magnetic lines of force. A few of the nearby gaseous streamers seemed dense enough to collapse under their own weights, and it’s just possible that I spotted a couple of stars forming. But the impression I had was that the cluster occupied a vast, nearly empty cavity, the space between the stars glowing faintly in X-rays because of the high temperature, which now topped 10 million degrees in places. What gas clouds I saw were being flung about chaotically in all directions—not exactly the optimal conditions for organized gravitational contractions to create, or replenish, a cluster of nearly a million stars. Were conditions very different a million, 10 million years ago? Was the Galactic Center then full of dense, dusty clouds of gas, churning out all these stars in a giant burst of gravitational coagulation?

  I still do not know the answer to this question. The hypothesis offended my Copernican roots, because it would have meant that I was visiting the Galactic Center under special conditions, and it raised more questions than it answered. Why should I have happened upon the Milky Way’s nucleus at just the “moment” (astronomically speaking) when rampant star formation had stopped but its shortest-lived products were still vital? Why was there no molecular gas in reserve anywhere near the present-day star cluster? I searched desperately for alternatives. Then, with little warning, one was thrust upon me: At least some of these young, massive stars must have formed from the collisions and mergers of smaller stars.

  Watching two stars collide and coalesce has to be one of the most spectacular sights in the Galaxy, but you have to be lucky to see it. Stellar collisions occur only once every few thousand years, and only in the centers of galaxies are the stars packed tightly enough for collisions to occur even this frequently. It is a safe bet that no stellar collision has ever occurred in the environs of the Sun.

  The whole concept of a stellar collision sounds violent, but what I witnessed was akin to a ballet. The dance begins tentatively, for only in rare cases do the stars hit head-on. Most often they barely brush one another and, if conditions are just right, glide into a delicate embrace. The collision, then, starts as a kind of pas de deux. The stars in question approached one another more slowly than I expected, at not more than a couple of hundred kilometers per second. I should not have been too surprised, because such speeds are typical of the stars located a few light-years from the Galaxy’s center of mass. But slowness was a crucial element in the encounter. If the mutual approach had been too fast, these stars would have sped past one another with little interaction, or (if aimed just so) they would have hit so hard that the outcome would have been ugly: bits of star splattered everywhere, but no long-term relationship.

  As the stars glided toward one another, their motions were gradually deflected from straight-line indifference to gently converging paths, and they sped up. I noticed the swellings that rose gradually on the sides of the stars facing one another, as well as on the opposite sides. Through these bulges, each star was responding to the other’s gravity, which is stronger on the near side than at the star’s center, and stronger at the center than on the far side. (On Earth we get excited about the barely perceptible tides caused by the Sun and Moon, but they pale beside these distortions.) By now the partners were focused squarely on one another, stretched along their mutual axis. As they swung past each other, the bulges tried to follow, and for the most part they kept up. But stars do not like being distorted, and the deformation took its toll. The friction of each star’s continuous internal readjustment heated its interior, and the bulges began to lag behind the stellar motion. The bulges no longer lined up, and their gravitational attractions for one another stowed the stars and drew them closer together. I held my breath, because I knew that this was the crucial stage in the encounter. More often than not, capture would elude the partners: They would release their gentle grasps and swing apart. But in this case the bulges lagged far enough behind, and gravity had enough leverage: The stars just managed to swing into orbit around each other.

  Like most stars that have newly captured one another, my couple shared a graceful orbit, swinging far apart and then plunging close. When far apart, the stars seemed almost oblivious to one another—in this phase, indeed, passing stars have been known to steal other stars’ partners. But when they plunged close together, the bulges reappeared, and the stars sank deeper into one another’s gravitational influence. By now the incessant heating had inflated the stars’ atmospheres, and their gaseous envelopes had begun to mingle. The stellar nuclei, where nuclear reactions pump out energy, were still distinct, but they were rapidly being subsumed beneath the common envelope. Finally, they merged.

  According to theory, it would take another 1000 years or more for the merged star to settle down. The “new” star would become much brighter than the sum of its two progenitors, and not just because the merging process itself generates a lot of heat. The nuclear reactions that power stars are extremely sensitive to temperature (hence the term thermonuclear), and the temperature inside a star depends on the star’s mass and size in a way that is determined by the necessity of a balance between gravity and pressure. Thus, if the new star is double the mass of the old, it must be roughly twice as large. If it were too small, the temperature in the center would be so high that nuclear reactions would proceed at an explosive pace, the pressure would build up, and the star would expand. If it were too large, the central temperature would be so low that the nuclear reactions would fizzle out, and the star would contract. In this way, the temperature sensitivity of thermonuclear reactions provides an elegant feedback that determines the sizes of stars. But a different effect determines how bright a star is. Energy leaks out faster from a larger star than from a smaller star, because the former has more surface area and is generally more porous. As a result, more massive (and therefore larger) stars put out a lot more light than low-mass stars. A star only twice the mass of the Sun puts out about 16 times more light. The flip side is that a 2-solar-mass star has only twice the fuel supply of the Sun, so it can live only ⅛ as long. When I put the arguments together this way, it no longer seemed like such a crazy idea that the hot young stars in the Galactic Center Star Cluster might well have formed from mergers.

  My theoretical training allowed me to anticipate the future of this newly merged star, but I had no time to watch the final stages unfold. I hadn’t come here to study sta
rs, anyway. I was searching for pure gravity, and pure gravity was to be found in one place: the big black hole at the very center of the Milky Way. But how was I to locate the Galaxy’s exact center of mass? I looked about for some secondary clues and noticed, in one direction, an especially dense concentration of stars surrounding a point of light with a strange, very blue glow. As I headed toward it, I immediately noticed that the stars around me were getting closer together. Their random motions were also speeding up: 500 hundred kilometers per second, 1000, 1500. . . . Any stellar collisions that occurred here would be far from gentle. They certainly would not lead to graceful mergers. The stars would be smashed, their debris dispersed to interstellar space. Could that be where some of the streamers of gas had come from?

  I pulled out my calculator and started taking notes on how the stellar speeds were increasing as I approached the blue glow. At of a light-year the speed was 600 kilometers per second, at of a light-year it was 850, and so forth. Every decrease in distance by a factor of 4 brought a doubling of stellar velocity. I quickly deduced that gravity was increasing just as Isaac Newton had predicted for a body with a mass two-and-a-half million times that of the Sun, all concentrated in one place. I was clearly sensing the black hole’s gravity. Yet my initial sense of satisfaction faded as I recalled how my colleagues had deduced everything I was finding, from the mass of the black hole to the shape of the star cluster, without leaving the comfort of their observatory 26,000 light-years away. (Funny how one never focuses on one’s advantages in these situations. For example, it never occurred to me to gloat that my colleagues hadn’t witnessed a stellar merger.) In any case, I had no time to wallow in these conflicting emotions. If I had not braked hard and gone into orbit about the Milky Way’s central black hole, I soon would have become part of it!