A medium stable state star is formed when dust and gas create just enough gravity to cause the process of fusion of hydrogen. Fusion is pushing out, while gravity is pulling in. So two hydrogen start forming together to form helium. Which creates a red giant. During the process of a red giant, a star has exhausted its supply of hydrogen. While mass has decreased so much that the force of gravity pulling mass in, is not enough to balance the explosive force of fusion outward. The star grows in volume.

Medium Stable Star to a Red Giant