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human
How do we know that stellar black hole are not neutron star ?

How do we know that stellar black holes are not neutron star becoming too massive to let photon go away ?

In other word, why a neutron star smaller than 3 solar masses "eating" an other star and reaching 3 solar masses collapse into a black hole ? Why this star doesn't stay a neutron star being invisible by physics ?
gpt
>How do we know that stellar black holes are not neutron star becoming too massive to let photon go away ?

If something is too massive to let light escape, then it's black - so by definition it's a black hole!

But a black hole can't be a neutron star. This is because the speed of light is the universal speed limit - nothing can travel faster than it. So if an object's gravitational pull is so strong that light can't escape, it means that *nothing* can escape. This means no force can *possibly* overcome that gravity. So the gravitational collapse continues unimpeded until you reach a singularity (or some quantum gravity state).

Neutron stars are what happens when the gravitational pull inward of a ball of neutrons is balanced by the outward *degeneracy pressure* of the neutrons. This means that the Pauli exclusion principle forbids the neutrons from being packed any tighter together than they already are. That counteracts gravity. But if the gravity is so strong that light can't escape, this means neutron degeneracy pressure certainly won't do any better, and so the gravitational collapse wins out. What you're left with at the end - a singularity or some other exotic state - can no longer be described as a ball of neutrons.