Nuclear fusion is the energy source of the stars. The force of gravity compresses this ball of plasma. At its core, matter is so dense that the temperature and pressure reach extreme values, allowing the fusion of the hydrogen atoms. This fusion then produces the energy enabling the star to shine and counterbalance gravity, thus maintaining hydrostatic equilibrium, giving the star a spherical shape.
In simple terms, this video explains the complex processes of fusion and the evolution of the stars which depends on it.
For more information: To learn more about nuclear fusion, you can watch the entire film in the original French (VF) or the original French with English subtitles. CEA also proposes a series of videos going into this subject in greater depth.