Not burning at all
What Is the Sun Burning?
The Sun is not on fire; it squeezes hydrogen so hard that atoms fuse and release energy.
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Step 1 of 5
Burning needs oxygen, and space has none.
A fire is a chemical reaction with oxygen. There is no oxygen around the Sun, so whatever it is doing cannot be burning.
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Step 2 of 5
Its own weight crushes the core.
The Sun is so massive that the weight of everything above squeezes the centre to unimaginable pressure and around fifteen million degrees.
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Step 3 of 5
Under that squeeze, atoms fuse together.
Hydrogen nuclei normally repel each other, but at those pressures they are forced close enough to stick, forming helium.
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Step 4 of 5
The new atom weighs slightly less.
Helium comes out a touch lighter than the hydrogen that made it, and that missing scrap of matter leaves as pure energy.
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Step 5 of 5
It has been doing this for billions of years.
The Sun converts millions of tonnes of matter into energy every second and still has enough hydrogen left for another five billion years.
The Sun turns about four million tonnes of matter into pure energy every second, and has done so for over four billion years.
The short version
The Sun is not burning; its own weight crushes hydrogen into helium, and the tiny loss of mass in that swap is released as energy.
Try it yourself
Hold two magnets north to north and push. That resistance is what the Sun's core overcomes to force hydrogen nuclei together.
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