Sun in a Bottle - Charles Seife [24]
FUSION REACTIONS IN THE SUN: Colliding protons release an electron and make deuterium, then deuterium and protons make helium-3, and finally helium-3s make helium-4, producing a lot of energy in the process.
The fusion energy released in the guts of the sun makes it shine. But it also threatens to blow the sun apart. The energy heats the hydrogen gas, making the nuclei slam together harder and harder, and the reaction speeds up, pouring more energy into the cloud. This would appear to lead to a runaway reaction; the furnace should run hotter and hotter and eventually get so energetic that the cloud explodes violently in all directions. However, it turns out that the hotter a cloud of gas is, the more it expands. So when the fusion engine runs hot, the star expands slightly. It becomes slightly less dense and the atoms slam into each other less and less often. The fusion engine slows, and the star cools. Gravity takes over once more, compressing the star, heating it up, and making the fusion energy run hot again. This means that a star is in a delicate equilibrium, caught between the force of gravity and the energy of fusion. The force of gravity tries to collapse the star while the energy of fusion tries to blow it apart.
EQUILIBRIUM IN A STAR: Two forces compete for dominance in every star; gravity tries to crush the star to a point, while the fusion explosion tries to blow the star apart. In a normal star, these two forces are in balance.
When that delicate equilibrium fails, the star dies. A fusion engine, no matter how well-balanced, can only run for as long as it has fuel. As a star gets older, its hydrogen supply begins to run out; the hydrogen fusion cycles sputter to a halt. A large star then turns to other light elements to keep itself from collapsing. It begins to fuse helium, turning it into yet heavier elements, such as carbon and oxygen. As the helium runs out, the star fuses heavier and heavier fuels: carbon, oxygen, silicon, sulfur. The fusion engine is rolling further and further down the fusion hill. Soon it hits bottom. The valley of iron.
Fusion gets its energy by making light elements roll down the hill toward iron. Fission gets its energy by making heavy elements roll down the hill toward iron. Iron, already at the bottom of the hill, can’t yield energy through fusion or fission. It is the dead ashes of a fusion furnace, utterly unable to yield more energy. When a star runs out of other fuels, its iron cannot burn in its fusion furnace. The fusion engine has nothing that it can turn into energy, so it shuts off and the star abruptly collapses. Depending on the star’s nature, it can die a fiery death: the final collapse ignites one last, violent burn of its remaining fuel, blowing up the star with unimaginable violence. A supernova, as such an explosion is called, is so energetic that a single one will typically outshine all the other stars in its galaxy combined. The star spews its guts into space, contaminating nearby hydrogen clouds in the process of collapsing into new stars. This is what happened before our sun was born; it got seeded with the nuclear ash of a supernova explosion. All the iron on Earth, all the oxygen, all the carbon—almost all the elements heavier than hydrogen and helium—are the remnants of a dead fusion furnace. We are all truly made of star stuff.18
When Hans Bethe solved the riddle of the sun’s energy, the idea of a fusion bomb seemed absurd. To ignite a fusion reaction, you need to have a bunch of light atoms that are extremely hot (so they have enough energy to overcome their mutual repulsion and slam into each other) and extremely dense (so they are close enough to one another that they collide frequently). The laws of nature seem to conspire against having those two conditions at the same time: hot things expand, reducing their density. The only reason a star can