Nuclear fusion

When many people describe the sun as more than a bright object in the sky, they often refer to it as a burning ball of gas 93 million miles away. This is technically incorrect because burning requires oxygen, of which there is little on the sun (0.77% of the photosphere, to be exact). No, the light we see is from incandescent plasma: it's hotter than gas, and it's so hot it glows.

So, if it isn't burning, where does all the energy come from? Good question. The answer is nuclear fusion. This is when two atomic nuclei get together and bind themselves into a heavier nucleus. The fusion of two nuclei lighter than iron or nickel generally releases energy, because iron and nickel have the highest binding energies per nucleon. Here's how it works:

We start out with, say, two isotopes of hydrogen, called deuterium (which has a proton and a neutron) and tritium (a proton and two neutrons). (If you're keeping score, we have ²H + ³H so far.)
These hang out in the sun under such temperature and pressure that they get really close to one another. (I suppose in a chemical equation, this step would be the arrow.)
All of a sudden, before you know it, they have united to form a single helium nucleus, a spare neutron, and some energy that used to be holding the nuclei together. (Basically, by moving in with each other, deuterium and tritium have saved a whole bunch on rent, even though it takes a bigger place to house them both. The extra cash they would have spent individually is our energy output.) The final result, then, is ¹n + ⁴He.

Awesome, right? This is happening in the core of the sun billions of trillions of times per second. Actually, to be precise, this is happening in the core of the sun about a hundred and thirty six tredecillion (135,717,300,293,000,000,000,000,000,000,000,000,000,000,000, or 1.35717300293×10⁴⁴) times per second, and that's a bunch. (Incidentally, only one two-billionth of those reactions, or 6.17058015442×10³⁴ of them, produce energy which reaches Earth.)

Nuclear power, in one form on another, will probably feature prominently in the future. In many ways, nuclear fusion is more stable and safer than nuclear fission (breaking up atomic nuclei), the method currently in use in nuclear power stations. It also comes with fewer harmful byproducts, although the output of neutrons can induce radioactivity within the reactor structure. Additionally, natural tritium is scarce (because it's unstable) and so must be produced from lithium, which is a resource more limited than hydrogen. However, this form of power generation has incredible potential, and I'm excited about its prospects. The only problem is that, currently, no nuclear fusion reactor exists which produces more energy than it consumes. Because temperature and pressure are important, maintaining the reaction is difficult.

For more information about nuclear fusion, you should read the Wikipedia articles on Nuclear fusion and the Sun.