Collapse of a neutron star - strong nuclear force vs gravity

[brajesh]

TL;DR Summary

If gravity is the "weakest force", how does it collapse a neutron star?

I understand that gravity causes a neutron star larger than about 10 solar masses to collapse into a black hole.

I also understand that gravity is the weakest of the four forces.

So I find this counterintuitive and I'm puzzled that why is it gravity that causes the collapse and NOT the strong nuclear force?

 

[Drakkith]

Gravitation's strength falls off as $1/r^2$. By contrast, the nuclear force (aka the residual strong force) is more like an inverse exponential and beyond a few femtometers, comparable to the diameter of several nucleons, the strength is virtually zero. So any single nucleon only really exerts a meaningful force through the nuclear force to perhaps a few dozen or so other nucleons while it's gravitational interaction only drops to 'insignificant' at a much, much larger distance.

Even though gravitation is much weaker, the inverse exponential behavior of the nuclear force makes it get so small, so rapidly that at large distances gravitation simply dominates despite starting out weaker.

 

[lomidrevo]

why is it gravity that causes the collapse and NOT the strong nuclear force?

Neutrons are fermions, so when the density of the inner core exceeds some critical value, the Pauli exclusion principle takes over the attraction of the strong force, providing pressure needed to stop further collapse. Only when the star is massive enough, this neutron degeneracy pressure cannot support the remnant against gravity, and the result of further collapse is a black hole.

 

[Drakkith]

Neutrons are fermions, so when the density of the inner core exceeds some critical value, the Pauli exclusion principle takes over the attraction of the strong force, providing pressure needed to stop further collapse.

Did you mean gravity? Or am I misunderstanding you?

 

[lomidrevo]

Did you mean gravity? Or am I misunderstanding you?

I mean strong (nuclear) force, but I could have written it much better, indeed. Let me reformulate the sentence: at very short distances the strong nuclear force is not attractive anymore, but repulsive due to Pauli exclusion principle.

In previous post I wanted to describe it differently: pauli exclusion principle (repulsive) acting against strong nuclear force (attractive). But that is probably not correct, according to update of my very limited knowledge of quantum chromodynamics The exclusion principle is already built into the strong force, so I perhaps should not treat them as two different "forces" acting against each other.

Of course, the same principle acts against the gravity (providing the neutron degeneracy pressure), that is the true key point. But for some reason the OP seem to be intrigued by the question: Why NOT strong force is causing the collapse? The brief answer I would provide now is: because strong force becomes repulsive as the nucleons are being compressed together.

 

[Delta2]

My answer to this:(you may laugh, I think its funny anyway)
Gravity is like a HUGE (huge mass that accumulates) but weak Giant, while strong force is like a powerful mosquito. We all understand that a HUGE GIANT no matter how weak can eventually (he might have some trouble in the process) crash a powerful mosquito.