At this point all the fuel in the Sun will have been used up and, since it is no longer able to generate an outward radiation pressure, gravitational contraction will begin again.

When the energy of bulk motion dissipates, the air is again static. The Sun is currently in this stable state and will remain so for another 5000 million years.

## INSIDE THE SUN

Nuclear Fusion. The system is a little more complicated for a star, since the interior temperature is not constant, and it depends on the past history of the fluid ensemble. When you pop the balloon, the pressure force between the air inside and the air outside causes the fluid element which had been the inside air to expand.

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The total force on any piece is just the sum of the forces exerted by all the other elements, and this will be nonzero if there is more mass at a given distance on one side of the piece than on the other. The equation for hydrostatic equilibrium is shown below.

## hydrostatic equilibrium

Note, however, that the atmosphere itself is in hydrostatic equilibrium. What this means in plain English is that there are two forces on a hot fluid which is massive enough so that gravity counts, and the pressure force balances the self-gravity so that the total force on each piece of the fluid is zero.

The kinetic energy of each particle is one-half times its mass times the square of its velocity. For Public. ESA uses cookies to track visits to our website only, no personal information is collected. If that is all there is, a fluid element which is at higher pressure than its surroundings will expand until the pressures are equalised, and the speed at which this occurs is the speed at which sound waves travel from one side to the other.

Last Update: The first of these forces is the pressure.

## Hydrostatic Equilibrium

Each piece of fluid exerts gravity towards itself according to its mass, which is just its density times its volume. At any point within a star in hydrostatic equilibrium the underlying pressure supports the weight from the overlying material. Now, if the pressure of the gas is not everywhere the same, there is a net force which at each place acts towards lower pressure. A good every-day example of force balance is a balloon: