Hydrostatic equilibrium occurs when compression, due to gravity, is balanced by a pressure gradient, which creates a force in the opposite direction. In stars, the pressure gradient appears as a result of the huge quantity of thermal energy (which acts outward) created by nuclear fusion reactions. It is gravity and this thermal energy that are in equilibrium.
It's a bit like blowing a balloon up, the inward pressure is counteracted by the external pressure of the atmosphere. In addition, when we consider stars, this means that the larger the mass of the star, the higher the temperature must be to achieve this balance. Larger stars will use up their supply of hydrogen more quickly and live a shorter life.
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Yes, a stable star is in equilibrium, called hydrostatic equilibrium, when the outward pressure from heat caused by core fusion processes balances the inward pull of gravity. There are other factors which alter the form of stars such as their rotation or gravity from external sources such as a nearby mass.
You can consider a dwarf planet to be a special case from an asteroid - one that has achieved hydrostatic equilibrium, which basically means that it has enough gravity to force it into a round shape.
Hydrostatic energy is the energy stored/exibited in liquids under pressure!!
Minor Planet number 90377 Sedna has never been moved from the asteroid classification. The IAU hasn't received enough data to say it is a dwarf planet because observations to check for things like hydrostatic equilibrium is so difficult for an object in the far out scattered disc section it resides in. Asteroids do not have hydrostatic equilibrium whereas Dwarf Planets do. Sedna may very well qualify as a Dwarf Planet; we just don't know yet.
Hydrostatic and Equilibrium
Hydrostatic equilibrium has nothing to do with the mass of the star but the compression, due to gravity, balanced by pressure of the star.See the related question.
Stars are held together by gravity and internal pressure that keeps them from collapsing. Gravitational attraction balances the internal pressures when this happens hydrostatic equilibrium is reached. In stars like the sun, gravity wants to crush the star but pressure from fusion pushes out. Gravity and fusion then equal out and hydrostatic equilibrium is achieved.
electrical forces and gravity
Hydrostatic and thermal equilibrium
Yes, Pluto has been determined to be in hydrostatic equilibrium. Planets must orbit the sun (the first criterion for a planet), and must also be in hydrostatic equilibrium (which Pluto is). Pluto fails the third "planetary entrance test" set by the IAU in that it has not cleared its orbit of debris. A link can be found below to check facts and learn more.
Basically it is because of gravity. The technical explanation is called the " principle of hydrostatic equilibrium ".
Hydrostatic equilibrium. See related question.
Because the temperature of the core keeps the Sun in hydrostatic equilibrium. See related question.
I think it is. Take a look at some pictures of Mimas; it looks pretty round to me.