Nature gave birth to numerous fusion reactors. These reactors have no confining walls, just hydrogen fuel held together by its own gravity; they are the stars that glow in the dark night sky, as well as the Sun during the daytime.
The gain in energy from fusion results from the mass of helium being lower than the mass of the protons and neutrons that fuse to make it, with the deficit being the nuclear binding energy. This is the most startling consequence of Einstein’s equivalence between mass and energy, with the huge conversion factor of the speed of light squared – thankfully generating the sunlight that nurtures life on Earth.
Since protons are positively charged, they only fuse at close approach (or equivalently high momentum by the quantum uncertainty principle) – where nuclear binding overcomes their electric repulsion. This requires temperatures above a few million degrees for ignition.
To get such temperatures at the center of a star compressed by gravity, it must possess more than 7% of the mass of the Sun (except for unusual circumstances that I described in a paper with my former postdoc, John Forbes). The lowest-mass stars burn their fuel slowly and can last up to ten trillion years, a thousand times longer than the current age of the Universe. These are the majority population as most stars are dwarfs. For example, the nearest star to the Sun, Proxima Centauri, has only 12% of a solar mass and will shine hundreds of times longer. This is good news for any lifeforms on its habitable planets, and for this reason, we might consider changing our host fusion reactor from the Sun to a dwarf star in the future.
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