# Neutron Stars

Hi guys, I'm not to great at physics and all but I want to learn more. I hope you guys can help me in that aspect.First of all, how do neutron stars form? I was told their electrons shrink into their nucleuses and therefore the whole star shrinks, but what causes the atoms to behave like that?

mgb_phys
Homework Helper
Gravity.

Stars are balanced between gravity trying to squeeze everything together and some outward pressure trying to push it apart.
When a star runs out of fuel, ie. ends up being made of stuff that dones't give off enough energy in a fussion, there is no outward radiation pressure to push the stuff apart.
In neutron stars the pressure to keep them up comes from degeneracy pressure - basically the neutrons can't be pressed together any tighter.
If the star's gravity is big enough to overcome this it becomes a black hole.

What is the outward pressure? It's not inertia like planets. Is it the process of fusion?

russ_watters
Mentor
You got it - heat from fusion keeps the star from imploding....until the fuel runs out.

Astronuc
Staff Emeritus
malawi_glenn
Homework Helper
What is the outward pressure? It's not inertia like planets. Is it the process of fusion?

The outward "pressure" is the Pauli principle

The outward pressure is neutron degeneracy pressure. Electron degeneracy pressure is what stops white dwarfs collapsing in but when this is less than the gravitational pressure, inverse beta decay creates neutrons. Neutron stars generally have a radius of (electron mass/proton mass)*white dwarf radius which turns out to be tiny, something like 10km!

Collapse is nothing to do with heat from fusion. Even if the heat from a stable neutron star had run out and it reached a few K, it would not collapse because the degeneracy pressure is not a function of temperature. The only way to make a stable neutron star tip over the edge to a black hole is by accretion from another object like a second star in a binary system.

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