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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