Amain sequence star does not expand or contract due to the balance between the internal heat pushing outward and the weight of the material pressing inward due to gravity. this state of maintaining a constant size is known as: hydrostatic equilibrium thermal equilibrium dynamic equilibrium

Hydrostatic equilibrium

A main sequence star is a stable one. There is no overall change. This is because the gravity  of the material acting inward is balanced by the pressure gradient of the fluid acting outward. The star is made of gases. Its a fluid system. This kind of equilibrium is known as hydro-static equilibrium.

The correct answer to this question is hydrostatic equilbrium

A main sequence star is always under two equal and self regulating force acting in opposite direction.These are named as force of gravity and pressure due to internal heat.

The star has its own gravity which is destabilizing in nature.It tries to compress the star.It acts in vertically downward direction.

Due to the force of gravity,the star might have collapsed into a black hole which is not happening exactly.The star produces its energy from fusion reaction where hydrogen is converted into helium.In this reaction,much more amount of heat is produced.This increases the internal pressure pushing the star in outward direction.The outward pressure and the inward force of gravity balance each other.

Due to this the balance a hydrostatic equilibrium is maintained inside the star.

your answer is, orbit around the nucleus