Astronomy — Black holes
If, after a supernova explosion, the mass of the collapsing star is more than about 3 solar masses, no known force in nature can stop the collapse. The object reaches white dwarf density, but the degenerate electrons cannot support the weight, and the collapse continues. When the object reaches neutron star density, the degenerate neutrons cannot support the weight, and the collapse goes on. The object quickly becomes smaller than an electron. No one knows of anything that can stop gravity from squeezing the object to zero radius and infinite density. As an object shrinks, its density and the strength its gravity at its surface increase, and when an object shrinks to zero radius, its density and gravity become infinite. Mathematicians call such a point a singularity. Physically we have difficulty thinking about infinite density and zero-radius objects, but even if such objects exit, they may not be visible to us. Theory predicts they will be hidden inside a region of space called a black hole. Although black holes are difficult to discuss without general relativity and sophisticated mathematics, we can use common sense and some simple physics to see why they form. Finding the velocity we need to escape from gravity around celestial body will help explain how black holes were first predicted theoretically and how the might be detected.
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