What is the hydrostatic balance?

The volume of fluid, which can be gas or liquid, is said to be in a hydrostatic balance when the strength of the mines is developed by gravity balanced by an ascending force exerted by the pressure of the fluid. For example, the Earth's atmosphere is pulled down by gravity, but towards the surface, the air is compressed by the weight of all air higher, so that the air density increases from the upper part of the atmosphere to the earth's surface. This density difference means that the air pressure with height reduces so that the pressure up from below is greater than the pressure down from the top and this network levels the strength of gravity down, which maintains the atmosphere at more or less constant height. If the volume of fluid is not in hydrostatic balance, it must be downloaded if the gravitational force exceeds the pressure or expands if the internal pressure is greater.

This concept can be expressed as a hydrostatic equilibrium equation. It is usually referred to as DP/dz = −gρ and refers to a layer of fluid in a larger volume in a hydrostatic balance where DP is a change in the layer, dz is thickLayer, G is acceleration due to gravity and ρ is the density of fluid. The equation can be used to calculate, for example, pressure in the planetary atmosphere at a given height above the surface.

The volume of gas in space, such as a large cloud of hydrogen, will initially become due to gravity, with its pressure increasing towards the center. The contraction will continue until the external force equal to the inner gravitational force. This is usually the moment when the pressure in the center is so large that the hydrogen cores are joined together to form a helium in a process called nuclear fusion that releases a huge amount of energy and gives birth to a star. The resulting heat is increased by pressure gas, producing external power to balance the inner gravitational force, so that the star will be in the hydrostatic balance. If gravity increases, perhaps more gas falling into the star also increases the density and temperature of the gas, provides more external pressure and maintains balance.

Stars

remain in the hydrostatic balance for a long time, usually several billion years, but eventually run out of hydrogen and start to combine gradually heavier elements. These changes temporarily pull the star out of balance and cause expansion or contraction until a new balance is determined. Iron cannot be combined into heavier elements, as it would require more energy than the process would produce, so when all the nuclear fuel of the star eventually turned into iron, there is no further fusion and the stellar collapses. This could leave a solid iron core, a neutron star or a black hole, depending on the weight of the stars. In the case of a black hole, no known physical process can generate sufficient internal pressure to stop gravitational collapse, so that the hydrostatic balance cannot be achieved and the stellar contraction is expected to a point of infinite density known as singularity.