Turn Right At Orion (3 page)

I was staring straight in the direction of the Milky Way's center but didn't realize it. What I saw was a warren of bright ridges and dark lanes, broadening and seeming to become more complex and convoluted as I traced the Milky Way's path southward into Sagittarius. There was still no sign of the center, but it was there all the same, hidden by a screen of dust.

In my defense, even the best astronomers of the early twentieth century had had a tough time determining the Milky Way's center, to say nothing of the Galaxy's size and shape. They thought they had it in 1920, when a Dutch astronomer named Kapteyn mistakenly concluded that Earth occupied the place of honor. But “Kapteyn's Universe” turned out to be no more than the local patch of galaxy surrounding Earth. Kapteyn had estimated the distances of stars in different directions around the sky, assuming that the dimmer stars were farther, on average, in inverse proportion to the square root of their brightnesses. (Light bulbs grow dimmer with distance in this way, he reasoned,
and so should stars.) Counting the numbers of stars at different distances, versus direction, he built up a three-dimensional model of the Galaxy. This was a more detailed version of an approach Herschel had tried a century earlier. But for this technique to work, two conditions would have to be met: The mixtures of star types (colors, sizes) would have to be similar at different distances, and the space between the stars would have to be transparent to starlight. The first condition is true enough, but the second is false. What neither Kapteyn nor anyone else knew at the time was that much of interstellar space is filled with a haze of tiny dust particles that progressively obscure stars at increasing distances, blocking their light entirely beyond distances of little more than 5000 light-years. As a result, Kapteyn severely overestimated the distances to the fainter stars, which are dimmed not so much by their remoteness as by a pall of interstellar smog. What Kapteyn thought was the entire Galactic disk was really only the patch within 5000 light-years, just 20 percent of the way to the Milky Way's true center. It is no surprise that the stars looked symmetrically arrayed about the planet Earth.

Who would have guessed that the Galactic disk resembles a smoke-filled room? The dark lanes and “coalsacks” that you can see with binoculars—regions seemingly devoid of stars—are merely places where the dust concentrations are especially high. The particles themselves are not that unlike the particles that make up cigarette smoke. They are about the same size—a fraction of a micron, or a few percent of a hair's breadth—and are somewhat similar in composition. Many of them consist of “soot,” mostly graphite and an admixture of hydrocarbons, with some very tiny particles of “sand” (silicon-based minerals) mixed in. This dust is pollution from supernova explosions, lesser stellar explosions called novae, and the evaporating outer envelopes of giant stars, and it readily accumulates in stagnant pockets of space, where there are no winds to sweep the Galaxy clean.

Fortunately, the Milky Way's dusty disk is less than 5000 light-years thick, so if you can't see along it very far, at least you
can see out of it. That is exactly why the Milky Way forms a distinct band on the sky, and it is why Kapteyn's predecessors, going back to Thomas Wright in 1750 (and later to include the philosopher Immanuel Kant), had guessed correctly that the stars of the Galaxy were confined to a slab. Only by looking “up” and out of the disk did astronomers spot the telltale—and indirect—evidence of where the true center lies. It was Harlow Shapley who guessed that the spray of “globular clusters”—the 100 or so disembodied “droplets” of concentrated Milky Way, each containing as many as a million stars in a tight spherical packet—formed an extensive halo framing the Galaxy's center like a bull's-eye. Earth was far from the center.

By this point, preparations for my voyage were far advanced. The millennium had turned a few years earlier, public (and government) interest in my experiments had been growing, and I realized that I might soon have to cede control of my research facilities to an increasingly nervous group of backers. Time was pressing, yet for an instant I hesitated. I knew from abstract arguments where the Milky Way's center was, but I was frustrated at not being able to see it. Did I really dare to embark on a journey toward an invisible destination? My enthusiasm began subtly to wane. Then, at the last minute, I was granted the preview I needed to cement my resolve—a spectacular image.

My good luck stemmed from the fact that light rays are not infinitely thin beams traveling along precisely straight lines but, rather, are slightly “fuzzy” and always a little bit spread out, no matter how tightly one attempts to focus them. This fuzziness, which increases with the wavelength of the light, means that light can bend very slightly around sharp edges (the phenomenon of diffraction) and cannot be blocked by any object that is much smaller than the wavelength. Interstellar dust grains have sizes comparable to or larger than the wavelengths of visible light, and hence they block these rays rather effectively, but they are smaller than the wavelengths of infrared rays. Thus infrared radiation from the Milky Way's center passes through the dust unimpeded. I recalled that my colleagues who had mapped the
motions of stars in the Galaxy's center had exploited just this selective transparency. With the aid of an infrared telescope orbiting above Earth's atmosphere (the nearest place where one could get an unmuddied view), I peered toward that spot in Sagittarius that had been pinpointed so painstakingly, by triangulation off the globular clusters, many years earlier. I now saw the disk of the Milky Way clearly, converging to a thin band with distance. The Galaxy's bulge became clear, framing the Galactic nucleus. And zooming in, a compact star cluster was unmistakable, as was the glow from the warmed gas clouds that surround it. This distant view rekindled my eagerness. But it hardly prepared me for the real thing.

When I recall my journey between Earth and the Galactic Center, I picture the clouds. I see my vessel darting between and through dense pillars of dust and chemical residue that blot out all view of the stars outside. I recall a vague sense of foreboding and anxiety as I visualize my passage through, the darkest clouds. It was a bit like flying through a thunderstorm, but (for the most part) there was no turbulence, which was eerie in itself. There were stars—they must have been a prominent part of the scene—but somehow they didn't make so great an impression as the dark, imposing clouds.

I was edgy. Faced with mounting pressure, I had left in a hurry and somewhat surreptitiously. The enormity of what I was undertaking hit me fully only after I was en route, and I brooded over everything I might have forgotten. Were all stores and life support systems fully charged and operational? Had my accelerators been set correctly? Had I said my goodbyes to those friends and colleagues I knew I would miss? At these speeds, communication with Earth would be cumbersome at best, and I knew I was unlikely to receive responses to the reports I sent back, half-heartedly, during the first few years of the mission. I also could not forget the rivalries and jealousies that had plagued the support teams. Would someone from Earth give
chase? I worried, even though I knew this was utterly impossible. The risk of sabotage, or a simple mistake, was more real. On guard for a malfunction and too excited to risk missing any of the novel scenery, I hibernated only sporadically. The repeated cycling of dormancy and wakefulness took a toll on my nerves that compounded my more mundane cares.

The terrain seemed familiar as I departed the Sun's vicinity. The region near the Sun is fairly open and not too dusty, consisting largely of warm to hot gas—by which I mean gas at 100,000 to a million degrees Celsius. Such temperatures are high enough to sublimate most of the dust, and even the individual atoms of hydrogen (which account for 75 percent of the gas, by weight) are broken up (“ionized”) by the heat into their constituent protons and electrons. The Sun's immediate surroundings are not typical, however. It seems that a supernova, or some very hot stars, existed in this region a few million years ago, leaving behind a hot bubble of gas. Several hundred light-years from the Sun, I plunged into a more typical mid-Galactic environment: a zone of scattered clouds. If you are puzzling over these meteorological metaphors, I should point out that there is no true vacuum between the stars. Gas fills every cubic centimeter of the Galaxy, coexisting with the dust in the smoggy regions with a gas-to-dust ratio of 100 to 1 by weight. Some of the clouds have temperatures of 10,000°C, “cool” enough for dust to survive, although most of the hydrogen is ionized even in these clouds. Then there are clouds at only a few hundred to a few thousand degrees, and these are cool enough for the protons and electrons to recombine into complete hydrogen atoms. I learned to spot the regions in which the gas consisted of individual hydrogen atoms because they emitted a peculiar radio glow—always at the same wavelength of 21 centimeters, according to my receiver. The origin of this glow reminded me of the complexity to be found in atoms of even the simplest element. It is produced as the lone spinning electron that orbits in each hydrogen nucleus repeatedly flips direction under the influence of unimaginably tiny magnetic fields created by the central proton.

Interstellar clouds are very different from earthly clouds, which are demarcated by changes in the state of suspension of water vapor in the atmosphere. The interstellar variety is characterized by abrupt temperature changes at the cloud boundaries. Nature abhors a temperature difference, and the thermodynamic imperative tries to drive everything toward uniformity. Heat flows from the hotter surroundings into cooler clouds, causing them to evaporate. The breezes that blow everywhere in the Galaxy are perpetually tearing wisps off them. I wondered, then, why all this structure persists. If these clouds are continuously being destroyed, mustn't new ones replace them? Only later did I learn that the clouds are indeed a telltale sign of the great Galactic cycle of birth and death, made from gas squeezed in the shock waves of stellar explosions and from the dense envelopes of gas shed by slowly dying stars.

I gradually began to encounter some very chilly clouds, so cold and dense that the hydrogen atoms paired off into molecules. The 21-centimeter radio glow faded, quenched by the interatomic pairing, but it was replaced by a much more complicated spectrum of radio waves. These molecular clouds were truly imposing, some of them more than 100 light-years across and containing as much matter as 100,000 Suns. Some were bursting with clusters of bright young stars, but most churned quietly, with just a few modest-sized stars forming here and there. As I traversed these clouds, I encountered dust storms—even occasional “snow” (ice-coated dust grains)—in some of the denser pockets, where temperatures dropped as low as 10 degrees above absolute zero. In the rainbow of radio waves I detected the signature of toxic carbon monoxide, as well as other, more complex molecules: alcohols, formaldehyde, even water.

Halfway from Earth to the Milky Way's center—13,000 light-years from home—the concentration of molecular clouds noticeably thickened. Yet I still passed through an occasional clearing, nearly devoid of gas and dust where temperatures soared to a million degrees or more. These holes had been blasted in the interstellar cloud-deck by the combined effects of supernova explosions
and winds from earlier generations of hot, massive stars. These clearings are so much more dramatic than the small bubble in which the Sun is immersed that they had been noticed from Earth and christened “superbubbles.” As I looked up out of the Milky Way's plane—while crossing these regions, I could see the faint X-ray glow where plumes of superheated gas forced their way out through the Galaxy's atmosphere.

But by two-thirds of the way to the center I was socked in, amid a nearly continuous layer of molecular clouds that was itself sandwiched between layers of cool atomic gas. Were these gaseous lanes part of the famed spiral arms? You may have heard of them. Disk galaxies such as the Milky Way seldom took like featureless platters when viewed from above. Often they sport grand spiral patterns that sometimes can be traced across the entire face of the disk, outlined by chains of dense clouds and dazzling clumps of newly formed stars.

What are these spiral arms? Certainly they are not rigid streamers of stars and gas, twirling around the pivot of the galaxy like a pinwheel. Astronomers had once speculated that they were organized structures held together, elastically, by magnetic lines of force, but it quickly became clear that gravity was the only agent capable of organizing matter over the large scales of a galactic disk. The main effect of gravity on a galaxy's disk is straightforward enough. just as in the Solar System, where the nearly circular motions of planets balance the gravitational pull of the Sun, in a galactic disk the pull of gravity is resisted by the nearly circular motions of the stars and gas cloud about the galaxy's center. The planets farther from the Sun take longer to go around once than the planets closer in, and the same is true of stars and gas clouds in a galaxy. (The Sun takes about a quarter of a billion years to orbit the Milky Way once; the orbital time for other stars varies roughly in proportion, to their distance from the center.) The gravitational forces so dominate other effects that you could no sooner maintain a rigid spiral pattern of stars and gas in a galaxy than you could keep the planets rigidly lined up in the Solar System.

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.