Is there a balance between pressure and gravity in main-sequence Stars, white dwarfs, neutron stars, and black holes?

1 Answer
Oct 22, 2017

All bodies need to reach an equilibrium with gravity to stop further collapse.


Most small bodies, which includes comets and most asteroids, have insufficient gravity overcome the rigidity of their component materials to undergo collapse. This means that they can be any shape.

Larger bodies, such as most moons, all planets and all stars, have sufficient gravity to overcome material rigidity and undergo a level of collapse. This is why these bodies are almost spherical in shape.

Almost spherical bodies are in hydrostatic equilibrium where gravity is balanced out by internal pressure to prevent further collapse.

In the case of main sequence stars, the outward pressure generated by fusion reactions in the core keep the star at a relatively large size. All main sequence stars are in hydrostatic equilibrium.

When a star runs out of fuel for fusion reactions, then gravitational collapse of the core is inevitable.

If the stellar core is less than about 1.4 solar masses then gravitational collapse is stopped by electron degeneracy pressure. This is a quantum effect caused by the Pauli exclusion principle. No two electrons can be in the quantum state. This prevents atoms from having overlaying electron shells. The most degenerate state this allows is the white dwarf star.

If the stellar core exceeds 1.4 solar masses then electron degeneracy pressure fails and protons and electrons are forced to combine into neutrons forming a neutron star. This generates a huge supernova explosion in the outer layers of the star.

Neutron stars are held in hydrostatic equilibrium by neutron degeneracy pressure. This is another quantum effect which prevents two neutrons being in the same quantum state.

If the neutron star is more than about three solar masses then gravity overcomes it. This means that gravity can now collapse the stellar core to its extreme which is a black hole.