Hi Cassandra,
Our star, the Sun, is a fairly ordinary star -- as stars go.
Scientists have longed studies how stars change in time by looking at stars of different ages in our own Milkway Galaxy. So scientist have a pretty good idea of the lifecycle of stars like our Sun. How stars evolve in time depends on how massive they are and on what elements they contain. And now they have pretty good computer models that reproduce the sizes and brightness of the stars we see. So that's how they can predict if the sun will expand or shrink.
Hope this helps
Paulett