A black hole begins to form as an aging star abruptly collapses when it ceases to undergo nuclear fusion in its core. This process occurs after the star has consumed its component elements in the following order:
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Hydrogen: The star starts its life by fusing hydrogen into helium in its core through the process of nuclear fusion.
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Helium: Once the hydrogen in the core is exhausted, the star can fuse helium into heavier elements such as carbon and oxygen.
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Carbon and Oxygen: As hydrogen and helium are depleted, the star can begin to fuse carbon and oxygen, creating even heavier elements.
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Neon: In some massive stars, after carbon and oxygen are exhausted, neon fusion can occur.
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Magnesium, Silicon, and Iron: The process continues with the fusion of magnesium and silicon into heavier elements. Ultimately, a massive star will begin fusing elements up to iron (Fe).
Once iron is formed in the core, the process of fusion cannot generate energy because fusing iron does not release energy (it requires energy instead). The core becomes unable to support the weight of the outer layers, leading to a catastrophic collapse. If the remaining mass of the core is sufficient (typically more than about 2-3 solar masses), it will collapse into a black hole.