The Star In You
"Our planet, our society, and we ourselves are built of star stuff."—Carl Sagan, Cosmos
Here's an amazing fact for your next cocktail party: Every single atom in your body—the calcium in your bones, the carbon in your genes, the iron in your blood, the gold in your filling—was created in a star billions of years ago. All except atoms of hydrogen and one or two of the next lightest elements. They were formed even earlier, shortly after the Big Bang began 13.7 billion years ago.
It's true, according to astrophysicists. You and everything around you, every single natural and man-made thing you can see, every rock, tree, butterfly, and building, comprises atoms that originally arose during the Big Bang or, for all but the lightest two or three elements, from millions of burning and exploding stars far back in the history of the universe. You live because stars died; it's that simple.
How is this so? How can you possibly be a walking galaxy of fossil stardust? Well, the story is not a new one, but it bears retelling, if only because its working out was one of the finest achievements of 20th-century astrophysics—and because it's so astonishing.
The start of it all
The story begins at the beginning, as in the Big Bang. That is when, astrophysicists say, all the hydrogen in the universe came into being. Initially it was just protons, and then, as the young universe expanded and cooled, these became bound to electrons, forming hydrogen atoms. The very hydrogen atoms in the H2O that makes up over half your body were born then. They didn't come from your parents; they came from the early universe. Did you have any idea you have atoms in your body that are over 13 billion years old?
If you could separate one hydrogen atom from one molecule of water in your body, shrink down to its atomically tiny size like the scientists in Fantastic Voyage, then reverse time and follow it back to through its unimaginable lifetime, you would find yourself in the immediate aftermath of the Big Bang. That very hydrogen atom, an atom now inside you as you read this, has remained unchanged since the beginning of time.
The Big Bang also churned out helium, the next lightest element. You don't have any helium in you, unless you just sucked the gas out of a birthday balloon. But helium is the second most common element after hydrogen. Together they make up more than 98 percent of the matter in the universe. (Luminous matter, that is; dark matter is a whole other story.) A smattering of lithium (element 3) and one or two other of the lightest elements also formed in the Bang, but these were negligible.
Everything else, every other chemical element, including carbon, oxygen, nitrogen, and all the other elements essential for your life, is thought to have been fabricated in stars.
How? Well, the story is either simple or horrendously complex depending on whether you're a science writer or a scientist. Here's the simple story:
Table for 118
First, what are we talking about when we talk about an element? A chemical element is a substance that cannot be broken down or changed into another substance using chemical means. It can be changed using nuclear means, which is what happens inside stars.
Every second, the sun converts about 500 million tons of hydrogen into helium.
As we learn in high school chemistry—and can remind ourselves with a quick glance at the Periodic Table—hydrogen, the lightest element, has one proton in its nucleus and thus is given the atomic number 1. Helium has two protons and so is number 2, and so on all the way up to uranium, which, with 92 protons in its nucleus, is the heaviest of the "naturally occurring" elements.
Remarkably, all life on Earth, all everything we see around us, consists of various combinations of those 92 elements. There are still heavier elements, ranging from neptunium (93) all the way up to the unofficially named ununoctium (118), though with the exception of trace amounts of neptunium and plutonium (94), these are not found naturally on Earth.
Stars are born
How did—and do, for the process continues today—all the chemical elements first come into existence?
Several hundred million years after the Big Bang, about 13 billion years ago, the hydrogen and helium in the early universe began coalescing into gas clouds, which, in turn, collapsed into the first stars. Gravity, that not-to-be-denied force, caused these newborn stars to contract, heating their cores to temperatures high enough to ignite their hydrogen and trigger its fusion into helium.
This is the first link in a chain of thermonuclear reactions that, depending on the size of the star and its fate, bring about the genesis of all the other chemical elements up to about californium, element 98. (Heavier elements than that are produced only in particle accelerators, physicists believe.) Imagine starting out in your kitchen with just a single natural ingredient and, after baking it in your oven, winding up with all other possible natural ingredients. This is what the universe has done with hydrogen.
The burning of H to He is what our star, the sun, does for a living. In the searing heat of its core—about 27 million °F—the reaction of four hydrogen nuclei fusing to become one helium nucleus happens over and over and over again, ad infinitum. Every second, the sun converts about 500 million tons of hydrogen into helium. (And for every helium atom formed, roughly a trillion photons are emitted from the sun's surface. This is why we wear sunglasses.)
Cooking elements
Our star enables us to live, but at this stage in its own life, it doesn't give us any elements heavier than helium. It's not massive enough. With stars more massive than ours, and up to about eight times its mass*, gravity is forcible enough to compress the core sufficiently to trigger nuclear reactions that produce heavier elements, starting with carbon (element 6) and oxygen (8). In such cores, the heat is high enough, about 180 million °F, to force three helium nuclei to fuse into a carbon nucleus, or four helium nuclei into an oxygen nucleus, millions of times over. This will happen in the sun when it becomes a red giant in five billion years.
In very massive stars, those of more than eight solar masses, the force of gravity drives the temperature in the core up so outlandishly high that it triggers thermonuclear reactions that create elements all the way up to iron (26). At 1,080 million °F, carbon fuses into neon; at 2,700 million °F, oxygen fuses into silicon; and at 7,200 million °F, silicon fuses into iron.
Iron, alas, marks a major turning point when it comes to fusing ever-heavier elements inside stars. All the way up to iron, every time a new fusion reaction occurs, some heat is released. With iron, no other rearrangement of nuclei can generate any more energy. But stars do form elements heavier than iron, including cherished ones like silver and gold, dangerous ones like radon and uranium, and ones you've never heard of (or could pronounce if you had) like praseodymium and ytterbium.
Two ways to you
Stars have one of two ways to produce these heavier-than-iron elements—and, not incidentally, to get them and all the other elements forged in their nuclear furnaces out into space so they can be incorporated into new stars, planets, and people.
Some of that widely dispersed stardust is holding you up right now.
The first way occurs in red giants. These are stars that have burned up all the hydrogen in their centers. When that happens, the star becomes, as the astrophysicist Craig Wheeler has put it, somewhat schizophrenic: The core loses energy, contracts, and heats up even as the envelope—the rest of the star outside the core—gains energy, expands, and cools (and appears redder). The expansion is quite, well, expansive: When our sun becomes a red giant, it will grow so large that it will engulf and evaporate the inner planets, including the Earth.
Some red giants last long enough to create elements in their cores heavier than iron through something called the s-process, for slow. Over a time scale of thousands of years, the s-process can result in the manufacture of elements all the way up to bismuth (83). These get pulled to the star's surface by convection and sloughed off into space via the star's stellar wind. Some of that widely dispersed stardust is holding you up right now.
A real blast
Elements heavier than bismuth only arise through the r-process, for rapid. How rapid? Seconds flat. The r-process is what happens when a star explodes in a supernova. It's easy for us to think of stars as lasting essentially forever, but the most massive stars survive only a few million years—a cosmic moment, really—and when they go, they go fast.
What happens? When a red giant gets to the stage of having fused all its lighter elements and is left with an iron core, the star can no longer retain its equilibrium—heat energy pushing out as gravity pulls in. Gravity suddenly gains the upper hand, collapsing the core all at once to billions of times the density of the Earth. The star then blows itself apart in an astronomical cataclysm. For a brief period, it shines as brightly as an entire galaxy and releases as much energy as our sun will in its 10-billion-year lifetime.
In the first few seconds, protons in the atoms created during the star's life collide with highly energetic neutrons, fashioning in an instant all the naturally occurring elements heavier than bismuth up to uranium, and even a few short-lived still-heavier elements such as plutonium and californium. All these blast out into space at millions of miles an hour, seeding the interstellar medium with the atoms that eventually end up in new stars, new solar systems, and, in your case, you.
The birth of you
Over time, molecular clouds of gas and dust out in deep space develop from those strewn elements and begin to contract under their own gravity. Such clouds are almost all hydrogen and helium, but they've got a scatter of heavier elements, too. And the most abundant elements begin to assemble into molecules, simple ones like water (H2O) and more complex ones like the sugar glycoaldehyde (C2H4O2). Astronomers can identify these compounds, and individual elements, using spectrometers.
Eventually, a kind of raw-clay star called a proto-star forms, with a disk of material surrounding it that will eventually beget planets. That process happened in our own solar system about five billion years ago, resulting in the sun, the planets, and, five billion years later, you.
Did you have any idea you have atoms in your body that are over 13 billion years old?
Just how those atoms and molecules that ended up on our planet went from non-living to living remains one of the great unanswered questions in science. But where the elements came from to start with has now been worked out, in broad strokes anyway, to astrophysicists' widespread satisfaction. It is an amazing story, isn't it?
*While still being sorted out, current estimates hold that, in time, stars below about eight solar masses collapse into white dwarfs, a very dense ex-star; those between about eight and 30 solar masses explode in supernovae (and release their heavy elements); and, with certain exceptions, those between about 30 and 100 solar masses collapse into black holes (and retain their heavy elements).
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