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Nearly everything in the universe was made by stars. Stars are fueled by nuclear fusion. They fuse atoms of the lightest element, with only one proton—hydrogen—into the next lightest element, with two protons—helium. In every fusion reaction, they’re creating an atom of a new element. When they’ve used up their hydrogen, they begin to fuse the remaining helium into a heavier element, carbon. Then they’ll fuse that into oxygen and progressively heavier elements. Each of these fusion reactions is less energetic, so by the time the star is fusing iron, it’s nearing the end of its life. The star may soon explode in a supernova, dispersing the elements it created into other solar systems where they may be used to form planets. After a supernova, the star may continue to exist as a black hole or an extremely dense neutron star. In this environment, elements like iron can capture free neutrons to form still heavier elements like silver, gold or plutonium. The atoms that came together to form Earth 4.5 billion years ago were once part of stars that lived and died long before. Almost every atom in your body is billions of years old, from the calcium in your bones to the carbon in your DNA, recycled countless times. That song was right: we are stardust. Stars made the elements that make us.
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