Hi Julia,

The Sun's core experiences a tremendous amount of force from gravity (since there is so much mass wanting to collapse on it), but the core is able to balance this strong gravitational force through nuclear fusion (where it is burning through its hydrogen), which causes a large outward pressure. For the lifetime of the Sun as we know it, the outward pressure force just balances the downward directed gravitational force, and the core does not collapse. However, eventually the hydrogen in the core is all converted to helium and the nuclear reactions stop, causing the core to collapse..... you can think of it as gravity winning in this battle of forces. This will not happen for another 5 billion years when the Sun will start the process of turning into a white dwarf.

Cheers,

Holly