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less energy would be gained from the reactor than was put into it.
    This equation is losing math for a star, but a huge gain forus. As a star consumes all of the lighterelements, and marches ever higher in the periodic table in the fuels it consumes, iron accumulates in the center. As more and more iron accumulates, the fuel for fusion is consumed, nuclear fusion reactions cease, and the star begins to emit less heat. Iron nuclei, under the right conditions, can absorb energy, almost like a nuclear explosion in reverse. With so much energy released only to be absorbed, these conditions can set off a massive chain reaction that ends as a vast and catastrophic explosion. In seconds, these explosions release more energy than stars like oursun emit in their entire lifetime.
    We recycle.Hydrogen inside us comes from thebig bang. Other elements come from stars andsupernovae. And there they will return when the elements that compose us get spread around the universe by a future supernova.
    This blast is one kind of supernova (another kind can be triggered by collisions of stars). Supernovae work something likeTeller andUlam’s crude device. Theenergy of one explosion brings new kinds of fusion reactions. Recall those fusion reactions forelements heavier than iron? Supernovae release so much energy that these expensive reactions happen. All the elements heavier than iron, such as thecobalt andcesium in our bodies, derive from supernovae.
    Here comes the important part, at least for us. The blast of the supernova spreads atoms of the dead star across thegalaxies. Supernovae are one engine that powers the movement of atoms from one star system to another.
    The smallest parts of our bodies have a history as big as the universe itself. Beginning as energy that converted to matter, thehydrogen atoms originated soon after thebig bang and later recombined to form ever-larger atoms in stars and supernovae.
    The sky, like a thriving forest, continually recycles matter. With the heavens so full of stars manufacturing elements, then occasionally exploding and releasing them, only to recombine them again as a new star forms, the atoms that reach our planet have been the denizens of innumerable other suns. Each galaxy, star, or person is the temporary owner of particles that have passed through the births and deaths of entities across vast reaches of time and space. The particles that make us have traveled billions of years across the universe; long after we and our planet are gone, they will be a part of other worlds.

CHAPTER THREE

LUCKY STARS

    E ver since the big bang, innumerable stars and galaxies have emerged and disappeared. We are relative newcomers to this party. By “we” I mean our entiresolar system.
    It took big ideas and big science to see how our little patch of the universe came into being. The Swedish thinkerEmanuel Swedenborg was occupied by important questions throughout his life. Born in 1688, he lived most of his eight decades believing he should have one great idea per day. In his early years, he worked as a natural philosopher seeking to intuit the structure of the natural world. He inferred, for example, the presence of nerves and a nervous system. Turning his thoughts to the cosmos, Swedenborg proposed a theory for the origin of the solarsystem. He envisioned that the sun developed from a cloud of gas and dust that collapsed on itself and condensed. As the sun took shape, the primordial dust remained as a disk of debris that swirled around the young star. Over time, portions of this cloud coalesced to form the planets of thesolar system. The idea was to remain dormant until two decades later, in 1755, when the philosopherImmanuel Kant had his go at developing ideas on the origin of the solar system. The theory he ultimately developed was largely similar to Swedenborg’s.
    Pierre-Simon Laplace (1749–1827) was one of the greatest mathematicians of all time, called by some the French Newton. Laplace’s name peppers