Gravity's Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos
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Within this colder gas are molecules. Most of the molecules, like most of the single atoms, are hydrogen. Two electrically positive protons, bound together by their electromagnetic lust for two negatively charged electrons, form a hydrogen molecule. There are also tiny traces of more complex structures: compounds such as carbon monoxide, a carbon and an oxygen atom also bound together by quantum forces; carbon dioxide, with two oxygen atoms; H2O; and even alcohols. If we sniff very carefully at these whispers of heavy molecular structures, as rare as their constituents are compared to hydrogen and helium, we find that most of them contain the element carbon. In fact, roughly 70 percent of all the heavier molecules that are adrift out in the universe contain carbon. We call these organic compounds. This is surprising, because many of these carbon-based molecules are the same structures that we find here on Earth. What they are doing out in interstellar space is a question that we will visit again later, since it has far-reaching consequences.
All these ions, atoms, and molecules drift out of our sack in an extremely thin haze, tenuous enough to be mistaken for a vacuum. In the emptiest regions of the universe there are just a few hydrogen atoms or molecules per cubic meter. Even when our sack opening lets out some of the denser stuff from places like the rich nebulae, the density only reaches about a trillion atoms per cubic meter. By comparison, the air that we breathe on the surface of Earth contains well over a trillion times more particles, roughly 1025 atoms or molecules per cubic meter. Our whiff of universe from the sack is gossamer thin, like the finest and most exquisitely subtle perfume.
Among the normal matter, the light, and the neutrinos emerging from this piece of universe is something else that we can barely sense. It neither reflects nor absorbs electromagnetic radiation. The only sign of its presence is the gravitational pull of its mass. This is the enigmatic substance we call dark matter. Exactly what it consists of is still a puzzle. The most likely candidate is a variety of subatomic particle that has very weak, very ineffectual interactions with “normal” matter. Much like the neutrino, it can drift right through solid material as if it were nothing more than a thin fog. Unlike the neutrino, though, dark matter moves slowly, and each particle carries a significant mass. Until we take a proper look inside the sack, we won’t see how this mysterious stuff arranges itself within the universe, but by counting what seeps out we can already tell that altogether it outweighs all normal matter by a factor of five. The mass of the universe is dominated by it.
There is one more thing that sneaks out of our sack as we carefully peel it open. Especially in the colder, denser, more sluggish wisps that emerge, for every hundred or so atoms or molecules, we will count one microscopic particle or grain of dust. This is not the same kind of dust that you find under your bed. This is far finer and very different in composition. A typical grain of interstellar dust is only about 0.001 millimeters (one micron) across, and may be composed of compounds such as silicon carbide or graphite. Some grains are even smaller, composed of just a few hundred atoms—barely more than giant molecules.
Where does dust come from in the depths of the universe? In truth, astronomers are still trying to understand the details of its origins, although at least two environments are known to produce these microscopic structures. One is the expanding refuse from big, old stars that are beginning to shut down the processes of nuclear fusion in their cores. As their interior undergoes these changes, their outer parts become bloated and end up being blown outward by the pressure of light itself. As this element-rich gas expands away from the star, it cools off, and just like water condensing out of steam, some of the carbon and silicon and other elements condense into small, solid particles of dust. These are set adrift into the cosmos. The other place where we know dust is made is in the great clouds of material blasted free by the supernova explosions of old stars.
All this seeping and rushing of particles and material occurs before we even look properly into the sack. A lot of stuff in the universe is small and fast-moving or small and drifting. Much of it is effectively invisible to us, either because it doesn’t interact with light, or because, like light itself, we can only sense it when it actually collides with us. Yet if you could see the paths of all these components from afar, the cosmos might appear as an opaque fog.
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Now let us peel away the sack and admire the structures within. We’ve analyzed the major ingredients, but how are they arranged, and what are they doing? The first thing we notice is that space is filled with, well, space. Apart from the flood of photons and neutrinos pervading the universe, it is mostly empty. This is perhaps not surprising, but just how sparsely matter is distributed is not immediately obvious to you or me here on Earth. In particular, the scale of atoms and particles compared to the voids between them is tiny. This is strikingly different from our day-to-day sensory experience of life on our planet. But then again, our senses are limited. We think our bodies—and our houses, villages, and cities—take up significant volumes compared to their immediate surroundings. We can reach across the table for a cookie or put our fingers around a pint of beer looking lonely on the other side of a bar. This apparently high density of matter is all a question of perspective, though.
A hydrogen atom consists of an electrically positive proton and an electrically negative electron. Allowing for the quantum fuzziness of matter on these scales, the proton is roughly a thousand-trillionth, or 10−15, of a meter across. The electron can be thought of as occupying a scale a thousand times smaller than this. Yet the overall size of a hydrogen atom is a ten-billionth, 10−10, of a meter. So the typical space between the proton and the electron is a hundred thousand times the size of the proton. You and I, composed of atoms, are mostly emptiness. This remarkable discrepancy between object size and the space in between extends to much larger scales, too. The distances from the Sun of the planets in our solar system are tens of thousands of times greater than the size of the planets themselves.
As we peer at the distribution of gas in our representative chunk of the universe, there is an average of about one atom of normal matter per cubic centimeter. This means that in any random location an extraordinary gulf typically exists between particles of matter out in the universe. You would have to separate grains of sand by about sixty miles to make an equivalent sparseness. Yet if we step back far enough, clear structures and forms emerge.
We know that our limited human visual organs pick out a very narrow range of wavelengths wherever photons are being generated and reflected. Over the volume of what is contained in the sack, we nonetheless see a glistening arrangement of light. Entire galaxies appear as tiny hazy smudges scattered throughout this three-dimensional space. Their positioning is both random and structured. It is as if the great abstract expressionist artist Jackson Pollock were given carte blanche to paint the universe. Vast gatherings of galaxies emerge as you squint your eyes. In some places there are thousands clustered together within just a few tens of millions of light-years: great cathedrals of light, with galaxies forming vast spherical clouds that increase in density toward the center pulpit. Leading into these glowing monuments are what appear to be strands and sheets. Sometimes they are the barest outlines sketched by the distribution of the small patches of light from galaxies, sometimes bold chains of brightness with galaxy after galaxy tracing out the structure. Then there are huge zones of emptiness, voids like great open soap bubbles bumping up against one another. As in a painting of an optical illusion, the entire contents of the sack can be seen in two ways. Either it is a clump of these dark bubbles, outlined by the light of galaxies, or it is a network of interconnected threads, sheets, and clusters of light surrounded by darkness—the dendrites and neurons of some megalomaniac artist’s impression of a web-like cosmic brain.
Figure 5. A chart of 1.6 million galaxies surrounding our location in the universe. Individual galaxies detected in infrared light are shown as tiny bright, grainy points in a map projection of the entire night sky. The dusty plane of our Mi
lky Way galaxy runs around the edges of this picture and down the middle as a dark streak. The galaxies trace out the web-like and foamy distribution of matter. The bright clump at the center of the map is named the Shapley Supercluster in honor of Harlow Shapley. It is 650 million light-years away and contains more than twenty clusters of galaxies, each containing hundreds to thousands of galaxies.
This is the essence of the large-scale structure of the universe. It is both frothy and stringy, yet mostly empty. Galaxies may be hundreds of thousands of light-years across, but only in their densest clusterings are the spaces between them comparable to their size. The vast bubble-like voids can extend over 100 million light-years, with barely a galaxy within. If we look more carefully, seeking out the atoms and molecules of hydrogen and helium that we know occupy the universe along with the particles of ghostly dark matter, we find that they, too, trace the same structure, the same arrangement that is betrayed by the bright galaxies.
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Now we take out our magnifying glass and peer more closely at these hazy points of light, the galaxies. Most of the visible photons coming from them originate from stars. Sometimes it is reflected, or absorbed and then re-emitted from gas and dust that coexist with the stars, but mostly it comes from the incredibly compact stellar objects that make up the galaxies. And the littlest of the galaxies are dwarfs compared to the biggest, which can be a hundred times greater in diameter. The dwarfs may contain only 100 million discernible stars, compared to the hundreds of billions in a giant galaxy. The stars themselves, across all galaxies, are highly varied. The smallest red and dim objects are barely a tenth the mass of our Sun and twice the diameter of the planet Jupiter, gently trickling out a thousandth of the energy that is produced in our own solar system. The brightest and bluest objects are over ten times more massive than our Sun and have ten to twenty times its girth. They pour out hundreds of thousands of times as much energy, but are far, far rarer than Sun-like stars. Most of the stars throughout all the galaxies in our sack of universe are less than half the mass of our Sun. Then the very rarest of all normal stars are those that are a hundred or so times the mass of our Sun, but their numbers add up to barely a grain or two among the great dunes of stellar objects.
All these very different stars, though, are in the midst of their normal adulthood. Deep inside their cores primordial hydrogen is being fused into heavier elements. For the smallest stars, this is a very, very lengthy process. Our Sun will perform this task for a total of approximately 10 billion years. Stars a tenth of the solar mass are the slow cookers of the universe, taking at least a trillion years before they deplete their primordial nuclear fuel. The most massive stars are gluttons. Super hot in their bellies and gulping through their food, they will consume hydrogen for perhaps only a few million years.
Many more star-like objects also exist in the galaxies inside our sack, but these represent the very young and the very old. At one extreme of the stellar life cycle are protostars, objects not yet ready to fuse elements; at the other extreme are the remains of stars that have lived and died. White dwarfs are cooling lumps of dense, leftover stellar material. Held up by the quantum pressure of electrons that we encountered earlier, they are almost as numerous as stars that are still generating energy. Neutron stars are similarly common, since they are the descendants of the most massive stars. These blazing objects are rare in their prime, but they live fast and die young, littering a galaxy like the Milky Way with their brutally compact and dense corpses.
Then there are the black holes. In a big galaxy, small black holes, a few times the mass of our Sun, may number in the thousands or even tens of thousands. Sometimes matter falls screaming into their gravitational lairs, releasing its energy as brilliant flares of electromagnetic radiation. Much more rare, but much more cosmically important, are the giant black holes—the lords of gravity. Billions of times more massive than our Sun, they sit imperiously within the deep recesses of galaxies, a great mystery for us to explore.
Our own Milky Way is a big galaxy, among the largest of such objects. Galaxies like it contain several hundred billion stars, protostars, and stellar remains. It sounds like an awfully packed environment. While it can become so toward the center, a galaxy, much like the rest of the universe, is still mostly empty space. A star the size of the Sun is about 30 million times smaller than the space between it and the nearest star. Even in the densest regions of our galaxy, the average separation between stars is still about a hundred thousand times their sizes. Mapped by its stellar components, a galaxy is a remarkably open place.
Just like the stars they consist of, the galaxies we find inside our sack of universe have an array of forms and types. In addition to coming in different sizes, they exhibit starkly different outward features. The most noticeable is that galaxies range across two primary structural forms. Some are great flattened, disklike structures with huge curving rivers of stars, as if paint had been casually dribbled onto a spinning, slippery plate. Others are almost spherical, dandelion-like hazes of stars. Those that are great disks amount to perhaps 15 percent of all the galaxies we see, and are fittingly known as spirals, of which our Milky Way is one. The fog-like clouds of stars, some of which are flattened into great ovoids, are known as ellipticals. Because this form includes many of the smallest galaxies, the ellipticals outnumber the spirals several times over. In between these major classes are all manner of hybrids, as well as galaxies that have clearly suffered gravitational trauma. Lumpy, distorted, and shredded, these are collectively known as “irregulars.”
The largest galaxies take the form of giant ellipticals. These cloud-like structures get denser and denser toward their centers, with more and more stars packed ever closer together. Most often these mammoth stellar collections sit at the middle of the great clusters of galaxies, at the apexes of the cosmic webbing that we first noticed. In stark contrast, the spiral galaxies typically avoid these locations, preferring to lurk out in the suburbs and hinterlands of the universe. Unlike the pure ellipticals, the great spiral disks slowly spin, although their swirls of stars move separately from their mass, like a projection of light onto the platter of a turntable. The two types of galaxies also show markedly different stellar colors. Spiral galaxies often contain many young, bright, hot, massive blue stars along with thick gas and dust, while elliptical galaxies are usually composed of older, smaller, red stellar systems, with few of the beautiful nebulae that spirals contain. Whatever has happened to make these great collections of stars so different is deeply rooted in their pasts, their locations, and their gravitational environments.
Within all galaxies, no single star is motionless. In the ellipticals and dwarf galaxies, most stars are flying to and fro, on orbits that are little more than a single track that takes them in close to the galactic center and then out again on the other side. They are like angry hornets flying back and forth across the central hive. In a spiral, the stars out in the great disk follow circular orbits around the center of the galaxy, wobbling in and out and up and down as the lumpy nature of the system pushes and pulls on them. Our own Sun does this in our galaxy, completing a leisurely circuit every 210 million years. Toward the bulging centers of spirals, though, stars begin to buzz around more like those in elliptical galaxies.
All galaxies are themselves in constant motion. Within the great clusters, galaxies behave much as the stars do in elliptical galaxies—flying in and out of the core. Velocities can be huge. A galaxy picks up a lot of speed as it falls toward the center of a cluster’s gravity well, reaching more than six hundred miles a second in many cases. Out in the intergalactic countryside the pace is calmer. Average velocities are only one-third as fast. Yet here too the great sculptural forms of threads, sheets, clusters, and voids prevail. Matter tends to stream along the web-like threads, and around the voids. It is gradually filling the densest, most massive clusters and superclusters with more and more material, the way gullies feed mountain lakes.
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Putting asi
de our magnifying glass, we pick up a powerful microscope to pry ever further into this veritable cosmic zoo of galaxies and stars in our sack. Here again we meet up with Richardson’s intractable mapmaking problem: the endless complexity of borders and coastlines. Within every nebulous cloud of molecular and atomic gas are an infinity of corridors and surfaces. Orbiting more than 50 percent of all stars anywhere are big and small planets. Some have moons and satellites, which in turn have smooth or ragged surfaces, mountains, valleys, and even intricate, endless coastlines. Then there are shadowy components: objects called brown dwarfs that are neither big enough to be stars nor small enough to be comfortably called planets. As we tune in to the further reaches of the electromagnetic spectrum, more and more extraordinary structures light up, coming magically into view. Great radio-emitting loops of magnetically entrained particles extend from stars, and even from entire galaxies. High-energy photons, from the ultraviolet to the X-ray to the most energetic gamma rays, stream from the surfaces of fiercely hot white dwarfs and neutron stars. Ten-million-degree gas trapped in the gravity wells of galaxies and galaxy clusters glows with X-ray light, full of strange forms and structures.
Many phenomena repeat themselves. Here is a star like the Sun; there’s another one, and another, and another. But there are always new wonders, too. We can find something extraordinary almost anywhere we look. Here is a pair of stars orbiting so close together that the gravitational pull of one on the other is significantly greater on their facing sides than their outward sides. Raw stellar material, scorching plasma, is streaming across the gap between them in a tug-of-war that may result in one star gobbling up much of its sister. In a remote corner of another galaxy is a giant old star just minutes away from yielding itself up to gravity’s persistent embrace. When it does, its core, now transmuted to iron and nickel, will implode and collapse inward, only to crash into itself with a backlash that will blow the star apart. A great and brilliant supernova will ignite, perhaps to be glimpsed by some life-form on a planet orbiting another star in another galaxy.