Though pictures of the sun sure look fiery, the sun isn't on fire the way you might think, as when paper burns.
When a piece of paper is set on fire with a match, the atoms (mostly carbon, hydrogen, and oxygen) in the chemical compounds in the paper combine with the molecules of oxygen in the atmosphere to produce the chemical compounds carbon dioxide and water and to release heat and light. This is a chemical reaction that we call combustion.
The sun is carrying out a much different process called nuclear fusion. Each second the sun converts 700,000,000 tons of the element hydrogen into 695,000,000 tons of the element helium. This releases energy in the form of gamma rays. The gamma rays are mostly converted to light eventually. This process does not require oxygen. It does require incredibly high temperatures and pressures.
The temperature at the core of the sun is about 15,600,000 degrees on the Kelvin temperature scale. The sun is 4.5 billion years old and has used up about one half of its hydrogen fuel supply.
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