Shadows of Forgotten Ancestors Read Free Page B

explosion in the still more ancient past. Five billion years after the events we are describing, these very atoms may be coursing through your bloodstream.
    Our story begins here in the dark, pullulating, dimly illuminated disk: the story as it actually turned out, and an enormous number of other stories that would have come to be had things gone just a little differently; the story of our world and species, but also the story of many other worlds and lifeforms destined never to be. The disk is rippling with possible futures. 3
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    For most of their lives, stars shine by transmuting hydrogen into helium. It happens at enormous pressures and temperatures deep inside them. Stars have been aborning in the Milky Way galaxy for ten billion years or more—within great clouds of gas and dust. Almost all the placenta of gas and dust that once surrounded and nourished a star is quickly lost, either devoured by its tenant or spewed back into interstellar space. When they are a little older—but we are still talking about the childhood of the stars—a massive disk of gas and dust can be discerned, the inner lanes circling the star swiftly, the outer ones moving more stately and slowly. Similar disks are detectable around stars barely out of their adolescence, but now only as thin remnants of their former selves—mostly dust with almost no gas, every grain of dust a miniature planet orbiting the central star. In some of them, dark lanes, free of dust, can be made out. Perhaps half the young stars in the sky that are about as massive as the Sun have such disks. Still older stars have nothing of the sort, or at least nothing that we are yet able to detect. Our own Solar System to this day retains a very diffuse band of dust orbiting the Sun, called the zodiacal cloud, a wispy remake of the great disk from which the planets were born.
    The story these observations are telling us is this: Stars formed in batches from huge clouds of gas and dust. A dense clump of material attracts adjacent gas and dust, grows larger and more massive, more efficiently draws matter to it, and is off on its way to stardom. When the temperatures and pressures in its interior become high enough, hydrogen atoms—the most abundant material in the Universe by far—rare jammed together and thermonuclear reactions are initiated. When it happens on a large enough scale, the star turns on and the nearby darkness is dispelled. Matter is turned into light.
    The collapsing cloud spins up, squashes down into a disk, and lumps of matter aggregate together—successively the size of smoke particles, sand grains, rocks, boulders, mountains, and worldlets. Then the cloud tidies itself up through the simple expedient of the largest objects gravitationally consuming the debris. The dust-free lanes are the feeding zones of young planets. As the central star begins to shine, it also sends forth great gales of hydrogen that blow grains back into the void. Perhaps some other system of worlds, fated to arise billions of years later in some distant province of the Milky Way, will put these rejected building blocks to good use.
    In the disks of gas and dust that surround many nearby stars, we think we see the nurseries in which worlds, far-off and exotic, are accumulating and coalescing. All over our galaxy, vast, irregular, lumpy, pitch-black, interstellar clouds are collapsing under their own gravity, and spawning stars and planets. It happens about once a month. In the observable Universe—containing as many as a hundred billion galaxies—perhaps a hundred solar systems are forming every second. In that multitude of worlds, many will be barren and desolate. Others may be lush and fertile, on which beings exquisitely adapted to their several circumstances are growing up, coming of age, and attempting to piece together
their
beginnings. The Universe is lavish beyond imagining.
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    As the dust settles and the disk thins, you can now make out what is happening