A black hole begins, like all stars, as a big ball of hot gas—mostly hydrogen. If left to its own devices, a sufficiently large ball of gas would collapse under the weight of its own gravity; it would crush itself into a tiny lump. Luckily for us, stars don’t collapse because there is another force at work: nuclear fusion. As a cloud of gas collapses, it gets hotter and denser, and hydrogen atoms slam into one another with increasing force. Eventually, the star gets so hot and dense that the hydrogen atoms stick to one another and fuse, creating helium and releasing large quantities of energy.
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