Science Puzzle
Why Doesn’t the Sun Burn Out Like a Fire?
The Sun has been pouring out light for four and a half billion years and is barely halfway through its life. A bonfire the size of the Sun, burning coal, would exhaust itself in a few thousand years.
What is the Sun doing, if it is not burning?
The Answer
The Sun is not on fire. It is not burning anything, and it could not: burning is a chemical reaction that needs oxygen, and space has essentially none. The Sun is doing something far more powerful.
In its core, the crushing weight of the whole star squeezes hydrogen to unimaginable pressure and around 15 million degrees. Under those conditions the nuclei of hydrogen atoms slam together and fuse, four of them eventually becoming one helium nucleus. The helium weighs a tiny fraction less than the four hydrogens did, and that missing mass is converted directly into energy, following Einstein's famous relation between mass and energy.
Chemistry rearranges the electrons around atoms. Fusion rebuilds the nuclei themselves, and releases millions of times more energy per kilogram of fuel. That is why the Sun can shine for ten billion years, converting four million tonnes of its own mass into sunlight every single second, and still be nowhere near finished.
The principle: Nuclear fusion. Stars release energy by fusing light nuclei into heavier ones, converting a small amount of mass into vast energy, a process millions of times more potent than chemical burning.