Hydrostatic equilibrium?

Question

I was reading that the largest of Saturn's moons are so massive that they have achieved "hydrostatic equilibrium". What exactly does this mean?

Answer 1

It means there's an equlilibrium --balance--between the force of hydrogen gas pushing up and gravity pushing down.

The sum of the two forces --pointing in opposite directions is 0 N.

Answer 2

This refers to the fact that the moons have sufficient mass to have condensed into nearly spherical shapes. Object with low mass typically have odd shapes.

(1) A "planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.    1. (see link below)