What stops a star from contracting and collapsing under gravity?
1. The stars are powered by nuclear fusion reaction at their core. The outflowing plasma prevents the star from collapsing.
2. The stars are powered by nuclear fusion reaction at their core. This energy radiating outwards prevents the star from collapsing.
3. The stars are powered by nuclear fusion reaction at their core. As the hydrogen fuses to become helium, the radioactivity of the alpha particles generated in the process prevent the star from collapsing.
4. The stars are powered by nuclear fission reaction at their core. This energy radiating outwards prevents the star from collapsing.