Most of the atoms in your body—including the oxygen in your lungs and the iron in your blood—were born inside a star. In the early universe, hydrogen was just about the only element. Stars are giant fusion reactors that pound together hydrogen atoms, making heavier elements and releasing lots of energy in the process. The ones that went nova spewed their contents far and wide. By the time our solar system started to form, roughly nine billion years after the universe began, enough large atoms existed to make our planet, to make the hands that hold this book and the brain that understands these
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