Imbalance occur when the hydrogen of the star's core used up.
Explanation:
Both gravity (hydrostatic) and pressure (thermal) create an equilibrium state during star formation. A star is in the state of hydrostatic equilibrium when the pressure that is created due to core burning is in balance with the force of gravity. These two will be in equilibrium when the outer push of core is in equal amount to the inward push of gravity. When the hydrogen that is present in the star's core has been used up, burning stops, which leads to the imbalance of gravity and pressure. This imbalance the star which undergo significant changes in its shape and structure..
