Nuclear Fusion & the Elements of Life

I hope you’ll all forgive this brief post today. I’ve been buried under insane levels of work for the last several weeks. However, as I was taking a short break this afternoon, lying on the couch, and enjoying a cool spring breeze wafting across my face from an open window, I picked up a new book I had bought recently and began reading: The Story of Earth by Dr. Robert M. Hazen, an Origins of Life scientist who specializes in the study of minerals. (I really enjoyed his Origins of Life series from The Great Courses, by the way.)

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Anyways, in lieu of an original post, I’d like to share a piece form Hazen’s book, not only to encourage you to pick the book up yourself and read, but also because these few paragraphs really made an impression on me. I’m more a biologist and can’t profess to have much physics nor astrophysics background, so the idea of how nuclear fusion underlies the development of all we know, from galaxies to embryos, truly cuts me to the quick. Enjoy.

First Light

“Gravity is the great engine of cosmic clumping. A hydrogen atom is a little thing, but take one atom and multiply it by ten to the sixtieth power (that’s a trillion-trillion-trillion-trillion-trillion hydrogen atoms) and they exert quite an impressive collective gravitational force on one another. Gravity pulls them inward to a common center, forming a star– a giant gas ball with epic pressures at the core. As an immense hydrogen cloud collapses, the star-forming process transforms the kinetic energy of moving atoms to the gravitational potential energy of their clustered state, which translates into heat once more– the same violent process that occurs when an asteroid impacts Earth, but with vastly more energy release. The core of the hydrogen sphere eventually reaches temperatures of millions of degrees and pressures of millions of atmospheres.

Such temperatures and pressures trigger a new phenomenon called nuclear fusion reactions. Under these extreme conditions, the nuclei of two hydrogen atoms (each with one proton) collide with such force that neutrons are transferred from one nucleus to another, making some hydrogen atoms more massive than others. After several such collisions, a helium nucleus with two protons forms. Surprisingly, the resulting helium atom is about 1 percent less massive than the original hydrogen atoms from which it formed. That lost mass converts directly to heat energy (just as it does in a hydrogen bomb), which promotes even more nuclear fusion reactions. The star “ignites,” bathing its surroundings with radiant energy, while becoming ever richer in helium at the expense of hydrogen.

Large stars, many of them much bigger than our Sun, eventually used up the prodigious supplies of hydrogen in their cores. But extreme interior pressure and heat continued to promote nuclear fusion. Two-proton helium atoms in a stellar core fused to make carbon, the vital element of life with its six protons, even as new pulses of nuclear energy triggered hydrogen fusion in a spherical layer of atoms surrounding the core. Then core carbon fused to make neon, neon to make oxygen, then magnesium, silicon, sulfur, and on and on. Gradually the star developed an onionlike structure, with layer upon concentric layer of fusion reactions. Faster and faster these reactions occurred, until the ultimate iron-producing phase lasted no more than a day. By this point in the first stars’ life cycles, many millions of years after the Big Bang, most of the first twenty-six elements in the periodic table had been brought into existence by nuclear fusion within many individual stars.

Iron is as far as this nuclear fusion process can go. When hydrogen fuses to produce helium, when helium fuses to produce carbon, and during all the other fusion steps, abundant nuclear energy is released. But iron has the lowest energy of any atomic nucleus. As when a blazing fire transforms every bit of fuel to ash, all the energy has been used up. Iron is the ultimate nuclear ash; no nuclear energy can be extracted by fusing iron with anything. So when the first massive star produced its inevitable iron core, the game was over, the results catastrophic. Until that point, the star had sustained a stable equilibrium, balancing its two great inner forces: gravity pulling mass toward the center, nuclear reactions pushing mass outward from the center. When the core filled with iron, however, the outward push just stopped, and gravity took over in an instant of unimaginable violence. The entire star collapsed inward with such switfness that it rebounded off itself and exploded into the first supernova. The star was ripped apart, blasting most of its mass outward. …

[When] the first big stars exploded, cosmic novelty ensued. These fractured bodies seeded space with the elements they had created. Carbon, oxygen, nitrogen, phosphorus, and sulfur– the elements of life– were especially abundant. Magnesium, silicon, iron, aluminum, and calcium, which dominate the compositions of many common rocks and form a large fraction of the mass of Earth-like planets, also abounded. But in the incomprehensibly energetic environment of these exploding stars, these elements fused in new and exotic ways to make all the periodic table– elements way beyond number twenty-six. So appeared the first traces of many rarer elements: precious silver and gold, utilitarian copper and zinc, toxic arsenic and mercury, radioactive uranium and plutonium. What’s more, all these elements were hurled out into space, where they could find one another and clump together in new and interesting ways through chemical reactions” (pp. 8-11).

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