How do Stars Collapse under their own Gravity?

In stars, some complex nuclear reactions take place. In a star's whole life, during fusion reaction, hydrogen converts into Helium, Helium into Beryllium and eventually Beryllium fuses into Iron. During these reactions, outwards Thermal Radiations keep balancing gravity and the star remains in equilibrium.

As long as a star is burning it's fuel, it is in equilibrium. When it is out of fuel, gravity dominates and the star contracts inwards. There's no more heat energy to fuse heavy elements of Iron further. Eventually, no more outwards Thermal Radiation to balance out the gravity. This enormous inwards force makes a star collapse under its own gravity.