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The properties of a main sequence star can be understood by considering the various physical processes acting in the interior. First is the hydrostatic balance, also called hydrostatic equilibrium. This determines the [|density] structure of the star as the internal pressure gradient balances against the force of gravity. Another way of thinking about this is to imagine the star as a large number of nested thin spherical shells (sort of like an onion). The inward forces on each shell consist of the gravitational pull from all the shells inside it, and the gas and radiation pressure on the outside of the shell. The only outward force on each shell is the gas and radiation pressure on the inside of the shell; there is no gravitational force from material outside the shell (this is known as Gauss's theorem). In hydrostatic equilibrium, the inward and outward forces must balance. If they don't, the shell will either collapse or expand. The timescale for this to occur is called the 'free-fall timescale', and it is about 2000 [|seconds] for a star like the Sun. Since we know the Sun has been more or less stable over the age of the Earth (several billion years), the hydrostatic balance must be maintained to a very high accuracy. A consequence of hydrostatic balance is that the pressure on each shell from material outside it must be less than the pressure from material inside it. This is because gravity acts only in the inward direction. Thus, the pressure in the star must decrease with increasing radius. This is an intuitively obvious result; the pressure at the center of the star is greater than it is at the surface.