Elements up to the atomic mass of iron are formed in stars NASA PD

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Stars form from a collapsing hydrogen cloud. A thermonuclear fusion reaction that converts the core's hydrogen into helium.

If the core's temperature gets hot enough, it undergoes a second wave of thermonuclear fusion itself, turning its helium into carbon and oxygen.

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The outermost layers are converting hydrogen to helium, a shell beneath it turning helium into carbon and oxygen, a shell beneath that producing heavier elements, a shell beneath that creating even heavier elements, and so on down to a core in which iron is produced.