Einstein didn’t discover the existence of black holes — though his theory of relativity does predict their formation. Instead, Karl Schwarzschild was the first to use Einstein’s revolutionary equations and show that black holes could indeed form.
He accomplished this the same year that Einstein released his theory of general relativity, 1915. From Schwarzschild’s work came a term called the Schwarzschild radius, which is a measurement of how small you would have to compress any object to create a black hole.
In theory, any object can become a black hole if you compress it to a small enough space because when you compress it, you make it more dense, giving that object a stronger gravitational pull.
For example, if you shrunk Earth down to the size of a peanut, it would be dense enough to form a black hole. Different objects must be shrunk down to different sizes — it’s the radius of a the sphere you would have to compress the object down to in order to get a black hole that the Schwarzschild radius refers to.
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