The sun right is powered now by fusing two hydrogen atoms and creating helium. The fusion creates a lot of heat and pressure outwards, which is balanced by the pressure from the mass of the star, keeping it from collapsing in on itself.

That hydrogen is finite, and it will eventually run out. “The end of its life” is the or the end of our suns main sequence, when it can no longer keep fusing hydrogen. It will begin collapsing in on itself and the buildup of pressure will begin another reaction — for a very short while it will be fusing helium. This reaction, at the very edge of the suns core will push outwards and overcome the gravity, enlarging the diameter of the sun quite drastically. It is expected that the earth will be eaten up bu the sun. But all the while, as the sun grows, it also cools. Eventually it will shrink and form a white dwarf that will cool and become ever so dimmer over billions of years.