Sun in a Bottle - Charles Seife [23]
In the 1930s, fusion was soon to solve the puzzle that had so vexed Darwin and Kelvin, and it would answer a question that had bothered humans for millennia: Why does the sun shine? Hans Bethe, then a physicist at Cornell University, would uncover the answer.
In retrospect, William Thomson was fundamentally correct. If the sun were, in fact, an incandescent ball of liquid, the energy released by infalling matter would only power it for a few tens of millions of years, far short of the time that Darwin’s theory needed to explain the diversity of life on Earth (and far short of the time that other scientists needed to explain geological processes). But Thomson’s work preceded E = mc2 by decades; nobody had yet puzzled over the nature of radioactivity or understood how fission and fusion turn matter into energy. Fusion held the solution to Thomson’s puzzle and vindicated Darwin. Fusion is the source of the energy that has powered the sun for billions of years.
The clues were already old by the time Bethe set to work on the problem. By carefully analyzing the colors of the light that streams from the sun, scientists already had a pretty good idea of what the sun was made of. Roughly 90 percent of its atoms are hydrogen. About 9 percent are helium atoms; in fact, it was by looking at the sun that scientists discovered helium in the first place. The remaining 1 percent is mostly carbon, nitrogen, oxygen, neon, and a tiny smattering of heavier elements, but almost all of these are lighter than iron. The sun bears all the hallmarks of being powered by fusion. Bethe figured out precisely how that power is generated.
A star begins its life as a cloud of gas: mostly hydrogen and a little bit of helium. Because atoms have mass, they attract each other gravitationally, and because of this mutual attraction, the cloud begins to collapse under its own gravity. As gravity compresses the cloud, the cloud heats up.
If gravity were the only force at play, the cloud would simply get smaller and smaller and eventually collapse into a tiny, massive point. But that is not what happens. As the gas cloud gets denser, atoms of hydrogen bump into each other more and more frequently. The collision rate increases dramatically. And as the cloud heats up, its atoms have more energy and collide more violently. The hydrogen atoms jostle each other harder and harder.
Ordinarily, nuclei try to escape from one another. They are positively charged, so they find other nuclei repulsive. When two atoms “collide,” they don’t usually come into physical contact. Once they get within close range, the repulsive forces send them zooming in opposite directions before they actually touch—something like what happens when you try to make two powerful magnets touch each other despite their mutual repulsion. But if the nuclei are moving fast enough—if both atoms are hot enough—then even the mutual repulsion is not enough to keep the nuclei from hitting each other. The two nuclei slam together with great force. This is where fusion begins, and how a sun sparks to life.
Hans Bethe realized that with all these hydrogen nuclei constantly slamming into one another, two hydrogen nuclei—two protons—might smash together at the same time that one spits out a set of particles, turning itself from a proton into a neutron. They fuse, creating deuterium and releasing energy in the process. The deuterium slams into another proton, making helium-3, and again releasing energy. And when two helium-3 atoms collide with each other, they fuse, making helium-4 and releasing two protons and yet more energy. This process, known as the proton-proton chain, turns four hydrogen atoms into helium-4 and lots of energy. Bethe figured out that this was one way our sun generates power: by turning hydrogen into helium. He also realized that other processes are going on as well; for example, the trace amounts of carbon, nitrogen, and oxygen are involved in a cycle that has the same outcome as the proton-proton chain: this process takes four hydrogens and turns them into helium-4.