4. a main sequences star achieves gravitational equilibrium by balancing gravity and generated by the nuclear fusion at the star's core.

Energy generated by nuclear fusion. When a star is formed, the dominant force is gravity. As nuclear fusion begins the radiation outwards, the inward pull of gravity is resisted by the pressure causing the star to be in equilibrium where energy generated by nuclear fusion at the star's core radiates outward and balancing the force of gravity which is pulling the star inwards. This whole is called the ''gravitational equilibrium''.

b. niger

explanation: