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By Root 881 0

constituents are very close together and declines very rapidly at distances much greater than a femtometer (which is roughly equal to the size of one proton). But it is not easy to push protons together to that distance because of their electromagnetic repulsion. One way to do it requires the protons to be moving extremely fast, and this in turn means a very high temperature indeed because temperature is essentially nothing more than a measure of the average speed of things; the molecules of water in a hot cup of tea are jiggling around more than the molecules in a cold pint of beer. At the very least a temperature of around 10 million degrees is necessary for fusion to begin, and preferably significantly more. Fortunately for us, there are places in the universe where temperatures meet and exceed those necessary for nuclear fusion—deep in the hearts of stars.

Let us journey back in time to the cosmic dark age, less than half a billion years after the big bang when the universe is filled with only hydrogen, helium, and a sprinkling of the lighter chemical elements. Slowly, as the universe continues to expand and cool, the primordial gases begin to fall in on themselves in clumps under the influence of gravity, picking up speed as they rush toward each other, just as this book will speed up toward the ground if you drop it. Faster-moving hydrogen and helium means hotter hydrogen and helium, so the big balls of gas become increasingly hot and increasingly dense. At a temperature of 10,000 degrees, the electrons are ripped from their orbits around the nuclei, leaving behind a gas of protons and electrons known as a plasma. Together the individual electrons and protons continue to fall inexorably inward, faster and faster in a relentlessly quickening collapse. The plasma is rescued from a seemingly irretrievable fall when the temperature approaches 10 million degrees, when something very important happens, something that transforms the hot ball of protons and electrons into the life and light of the universe; a magnificent source of nuclear energy; a star. Individual protons fuse together to make a deuteron, which itself can fuse with another proton to produce helium, and all the while precious binding energy is released. In this way the new star slowly converts a small fraction of the original mass into energy, which heats up the core of the star and allows it to halt and resist any further gravitational collapse, at least for a few billion years—time enough for cold, rocky planets to be warmed, liquid water to flow, animals to evolve, and civilizations to rise.

Our sun is a star that is currently in just such a comfortable midlife phase: It is burning hydrogen to make helium. In the process, it loses 4 million tons of mass every second of every day of every millennium as it converts 600 million tons per second of hydrogen into helium. This profligacy, the foundation of our existence, cannot continue forever, even for our local ball of plasma, large enough to contain a million earths. So what happens when a star runs out of hydrogen fuel in its core? Without the nuclear source of outward pressure, the star will once again start to collapse, getting hotter and hotter as it does so. Eventually, at a temperature of around 100 million degrees, helium begins to burn and once again the star’s collapse is arrested. We are using the word “burn,” but that isn’t really very precise. What we really mean is that nuclear fusion is taking place and the net mass of the final products is less than the mass of the original fusing material—the loss of mass leading to the production of energy in accord with E = mc2.

The process of burning helium is really worth a closer look. When two helium nuclei fuse, they make a particular form of beryllium, made up of four protons and four neutrons. This form, called beryllium-8, lives for only one ten-millionth-of-a-billionth of a second before it falls apart into two helium nuclei again. The brief life of beryllium-8 is so fleeting that it is very unlikely it will hang around long enough to fuse