Planet Earth is finite; beyond it stretches the scant flimsiness of interplanetary space. Our Solar System is also finite; beyond it lies the near-vacuum of interstellar space. And our Galaxy, in turn, is finite; beyond it exists the absolute material void of intergalactic space. Perhaps beyond even that, then, the arrangement of galaxies in space is also finite. Which brings to mind the obvious question: How are the galaxies spread throughout the expansive tracts far from the Milky Way? Is there some boundary, or terminus, beyond which galaxies are no longer seen? Or do they reside everywhere, all the way out to the limits of the observable Universe?

Our Cosmic Neighborhood Within the “neighboring” realm of a few million light-years, astronomers know of a few dozen galaxies. Giant spirals, such as our own Milky Way and Andromeda, are found among small ellipticals and many irregulars, such as the Magellanic Clouds. Surprisingly, some nearby galaxies have been discovered even as recently as the past decade, such as the Sagittarius Spheroid, a newly found dwarf galaxy only 80,000 light-years distant yet mostly obscured by our Galaxy’s central bulge. Evidently, these 40 or so galaxies are bound together by their own mutual gravitational attraction—a mammoth version of the same natural phenomenon that holds stars in galaxies, planets around stars, and people on Earth. In all, these “local” galaxies are clustered within a roughly spherical volume whose diameter is some 5 million light-years. Including our Milky Way, the whole bunch is known as the Local Group. It constitutes our extended neighborhood in space.

Several million light-years comprise a significant chunk of cosmic real estate. Do note two important things about it. First, we have suddenly made a large jump in spatial dimensions, from the 100,000-light-year size of our Milky Way to this 5-million-light-year size of our Local Group. Galaxy clusters represent a distinctly higher level of hierarchical structure in the Universe—structure well beyond that of individual galaxies. Second, the Milky Way doesn’t lie at the center of this cluster of galaxies. Not only is Earth not the center of our Solar System and the Sun not the center of our Galaxy, but our Galaxy is also not the center of the much larger Local Group. Though we might like to think so, humankind resides at no special, unique, or privileged location in the gargantuan, perhaps infinite, Universe.

Beyond Our Local Group Many more than a few dozen galaxies reside in the Universe beyond. Time exposures made with large telescopes, such as that in Figure 2.2, reveal thousands of galaxies within any small field of view. In all, astronomers estimate that ~100 billion other galaxies inhabit the observable Universe. Virtually all of them are much farther away than even the distant members of our local galaxy cluster. For millions of light-years beyond the edge of the Local Group, there appears to be nothing—no galaxies, no stars, no gas or dust—just empty intergalactic space.

FIGURE 2.2 — Nearly every point of light in this photographic time exposure represents a galaxy. Rich galaxy fields like this one, called the Hubble Deep Field, cover an area only ~1% that of the full Moon. Extrapolating across the entire sky, ~100 billion galaxies inhabit the observable Universe. (STScI)

Strive to appreciate the far recesses of deep space outside our Local Group. Searching a seemingly interminable void, we occasionally sight a “field” galaxy scattered lonely here and there. Not until we reach a distance of ~60 million light-years away do we find another galaxy cluster, an unmistakable volume of space brimming with galaxies. This cluster is especially rich, containing not just ~40 galaxies as in our own Local Group; the so-called Virgo Cluster harbors nearly 3,000 galaxies. Try to visualize in mind’s eye thousands of individual galaxies all clustered in a swarm, each one housing about a 100 billion stars. No wonder most people have trouble appreciating the immensity of matter, space, and time in the Universe. Astrophysicists are no different; they, too, share the burden of trying to fathom such humongous sizes and scales, including astronomical numbers of astronomical objects.

Galaxy clusters like these populate the Universe throughout. They are not figments of our imagination, rather the results of numerous observations. Their existence is fact, as hundreds of clusters have now been mapped and cataloged. In much the same way that galaxies are collections of stars, galaxy clusters are collections of galaxies. And beyond them, in turn, galaxy superclusters (or clusters of clusters) apparently also exist on colossal scales of typically hundreds of millions of light-years. Both the Local Group and the Virgo Cluster are mere members of such a very much larger system—perhaps. These truly titanic structures comprise a most lofty rung—the greatest established to date—in the hierarchy of material assemblies within the known Universe: particles, atoms, molecules, dust, planets, stars, galaxies, galaxy clusters, and now galaxy superclusters.

Rich Clusters When contemplating the congested confines of rich galaxy clusters—such as Virgo with its thousands of members, or the appropriately named Hercules Cluster with its estimated 100,000 galaxies—it’s hard to avoid the impression that galactic traffic jams must be common. Just as atoms collide when confined in a closed container or hockey players in a enclosed rink, the random motions of galaxies within a galaxy cluster could conceivably induce phenomenal collisions among these huge material constructs.

Galaxies do indeed collide. A good deal of observational evidence proves that they do so, and quite often. Numerous celestial images, such as Figure 2.3, show two or more galaxies in the act of interacting, some of them tearing each other apart. While, in many photographs, galaxies lie along the same line of sight yet are really very much separated in space, others are physically near one another, especially those within the galaxy clusters. Whether galaxies are colliding head-on or only experiencing close encounters cannot often be easily determined, for detectable motions among the distant galaxies typically take millions of years—which is why no human has ever witnessed the full panoply of a galaxy collision, as much as noting its aftereffects virtually frozen in time.

FIGURE 2.3 — The “Antennae” galaxies (NGC4038,39) are probably in the act of colliding, or at least experiencing a close encounter, thereby rearranging their galactic contents and inducing bursts of star formation, but otherwise not causing much fireworks among the existing stars. They display extended tails as well as double galactic centers separated by only ~1000 light-years; the bluishness in the arms is caused by myriad young, hot stars, many perhaps recently formed in the aftermath of this interaction of cosmic proportions. (STScI)

At first thought, collisions among giant galaxies might be expected to create a mind-boggling crunching of matter, complete with spectacular explosions and superlative fireworks. Surprisingly, that doesn’t happen much at all. Such collisions, in fact, are rather quiescent. The stars in each galaxy more or less just glide past one another as the two galaxies slide through each other. That’s because stars themselves hardly ever collide; they are, after all, small objects by cosmic standards. While astronomers have plenty of direct photographic evidence for galaxy collisions, no one has ever witnessed or imaged a collision between two stars—not even in our own Milky Way that we can see closely and clearly.

This oddity occurs because galaxies within most clusters are bunched fairly tightly. The distance between adjacent galaxies in a given cluster averages a million light-years, which is only about 10 times greater than the size of a typical galaxy. This doesn’t really give them much room to roam around without crashing. By contrast, stars within a galaxy are spread out much more thinly. The average distance between stars in a galaxy is a few light-years, millions of times greater than the size of a typical star. Said another way, if our Sun were the size of an apple, its closest neighbors would be ~2,000 km away. Hence, stellar collisions are extremely rare within any one galaxy, with the possible exception of their center-most regions. When two galaxies collide, the population density of stars merely doubles, still leaving ample space for the stars to meander without sustaining much damage. To be sure, the interstellar contents, and perhaps the stars as well, of each galaxy are likely rearranged by the tidal forces induced by gravitational interactions, but no spectacular explosions result, even if the collision is head-on.

That’s not to say that the pushing, shoving, and shocking of the interstellar gas doesn’t cause any change. In fact, among the loose gas galaxy collisions wreak relative havoc! Bursts of star formation erupt in interacting galaxies while passing each other like hurricanes in a pas de deux. In recent years, astronomers have imaged numerous “starburst galaxies,” where their internal gas has been disturbed and rearranged enough to trigger sudden episodes of new stars in the disks of both galaxies. Additionally, already formed stars appear agitated while oddly orbiting like frenzied moths around a lamp, yet other stars seem to be ejected along streamers stretching as much as a 100,000 light-years from the site of the collision. So, although dramatic fireworks from direct hits among stars are most unlikely, computer simulations do show that the ensuing commotion causes affected galaxies to glow about 50% brighter for ~10⁸ years owing to their many newborn stars. Later in this GALACTIC EPOCH, we shall address the evolutionary implications of mergers and acquisitions as galaxies collide, mutually attract, and agglomerate.

Large-scale Structure Completing our inventory of the large-scale spread of matter in space, we naturally pose the next obvious question: Are there even greater assemblies of matter in the Universe, or do galaxy superclusters top the cosmic hierarchy? At present, astronomers are unsure. Some data imply clusterings of galaxy superclusters—or at least nonrandom arrays of galaxies on the largest scales yet observed—but this evidence is shaky and hence subject to debate. If correct, though, this would mean that our Local Group along with several other galaxy clusters comprise a galaxy supercluster centered near the rich Virgo Cluster, and that, in turn, these tens of thousands of galaxies comprise part of an even larger structure on the order of several hundred million light-years in diameter—which is more than 1000 times larger than the size of the Milky Way itself.

What is most clear from the latest cosmic maps of matter on the largest scales probed thus far (see Figure 2.4) are the irregularities: Galaxies seem to be arranged in networks of filaments, or sheets, surrounded by relatively empty regions of space known as voids. A colossal sponge might be a good visual image, or perhaps an immense bubblebath. The so-called CfA Great Wall, a lengthy arc of several thousand galaxies extending ~500 million light-years across the sky nearly 300 million light-years away, is the nearest and most prominent of these giant features. An even bigger, Sloan Great Wall, sporting nearly a 100,000 galaxies ~1 billion light-years long and nearly a billion light-years away, is currently the largest known structure in the Universe. The bright galaxies’ locations resemble spider webs or the neural structure of the human brain, whereas the dark voids, often measuring hundreds of millions of light-years wide, are almost completely absent of any galaxies. The most likely interpretation of these maps—the largest ones ever made—is that individual galaxies, and even whole galaxy clusters, are spread across the surface of vast “bubbles” in space. Much like soapy water, the gigantic bubbles ostensibly fill the entire Universe, whereas the voids are the interiors of those bubbles. Furthermore, the galaxies seem distributed like beads on strings only because the observed 2-dimensional maps are actually crossectional cuts through the real 3-dimensional bubble-like structures. The densest of the galaxy clusters and perhaps the superclusters apparently lie in regions where several bubbles meet—at intersections and nodes of vast cosmic filaments, namely, at some of the great crossroads of the Universe. The observed, “frothy” patterns of galaxies in deep space might be telling us something about our origins, for those patterns are probably traceable to the earliest parts of the PARTICLE EPOCH.

FIGURE 2.4 – This large-scale galaxy survey of the northern hemisphere consists of ~70,000 galaxies within 3 billion light-years. Many "voids" and "walls" on scales typically as large as 500 million light-years can be seen--including the largest of these, the Sloan Great Wall, marked and ~1 billion light-years across. No larger structures are evident, implying that the Universe is homogeneous on the very largest scales. (SDSS)

All told, individual galaxies contribute little to the large-scale architecture of the Universe as a grand cosmic system—but they are the keys to unlocking the details of that architecture. Each galaxy is essentially a passenger on an expanding, huge and foamy framework, much like humans who have little bearing on the overall tectonics of Earth yet ride along with the drifting continents. On the other hand, galaxies can be used to probe that framework of the Universe, in much the same way that geologists probe the structure of Earth by examining individual rocks. Metaphorically, galaxies resemble billiard balls whose motions can help determine the size and shape of a playing table, or, better yet, golf balls that can survey the curved topology of a putting green. In analogous ways, cosmologists analyze the radiation emitted by distant galaxies to unravel the very fabric of the Universe, a vitally important endeavor for any full appreciation of the cosmos.

We have reached the limits of telescopic exploration—at least as pertains size and scale of organized structures. We have also broached the realm of conjecture at the upper end of those structures. Let’s pause for a moment to recapitulate the mental picture before us: We live on planet Earth, which orbits the Sun. The Sun, in turn, is just one of hundreds of billions of stars in the immense Milky Way. Our Galaxy is moreover only one of many galactic residents of the Local Group, which, further in turn, is merely an undistinguished galaxy cluster near the periphery of what might be an even larger galaxy supercluster. And so on among the filaments, voids, walls, and potentially greater structures in the Universe.

At every level in our inventory, nothing seems special about our Earth, our Sun, our Galaxy, our Local Group. Evidently, mediocrity reigns throughout.

Such is our niche in the Universe.