Why is hydrostatic equilibrium important

Gas pressure in any layer thus is just equal to the weight gravitational force on all the matter above that given layer, in the same manner that the pressure at any depth in a pool of water equals the weight of the water above that depth, hence the term hydrostatic equilibrium.

An immediate consequence is that gas pressure must increase inward toward the center of a star. Previous Equation of State. Next Thermal Equilibrium. Removing book from your Reading List will also remove any bookmarked pages associated with this title. Are you sure you want to remove bookConfirmation and any corresponding bookmarks?

If the star has a massive nearby companion object then tidal forces come into play as well, further distorting the star into an ellipsoidal shape. For an example of this see Beta Lyrae. The concept of hydrostatic equilibrium has also become important in determining whether an astronomical object is a planet, dwarf planet, or small solar system body. According to the definition of planet adopted by the International Astronomical Union in , planets and dwarf planets are objects that have sufficient gravity to overcome their own rigidity and assume hydrostatic equilibrium.

Since the terrestrial planets and dwarf planets likewise the largest satellites like the Moon and Io have rough surfaces and so are not perfectly in equilibrium this definition evidently has some flexibility, but as of yet a specific means of quantifying an object's shape by this standard has not been announced.

It is also important for the intracluster medium , where it restricts the amount of gas that can be present in the core of a cluster of galaxies. Hydrostatic equilibrium can explain why the Earth's atmosphere does not collapse to a very thin layer on the ground.

In the atmosphere, the pressure of air decreases with increasing altitude. This causes an upward force, called the pressure gradient force , which tries to smooth over pressure differences. The force of gravity, on the other hand, almost exactly balances this out, keeping the atmosphere bound to the earth and maintaining pressure differences with altitude.

Without the pressure gradient force, the atmosphere would collapse to a much thinner shell around the earth, and without the force of gravity, the pressure gradient force would diffuse the atmosphere into space, leaving Earth with hardly any atmosphere.

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