a Star is effectively a constant tug of war between gravity(trying to collapse it) and nuclear fusion( who's energy causing its outer layers to expand).

for a star like our Sun it spends most of its life cycle fusing Hydrogen into Helium , when it eventually runs out of hydrogen to fuse this tug of war goes in favor of gravity whihc will cause its core to collapse further(becoming denser).

However this collapse caused the now helium core ot be under so much pressure it once against become able ot enter nuclear fusion(as now gravity is strong enough to force helium atoms together.).

the fusion of Helium into carbon outputs more energy than the previous reaction cuasing the star's outlayers ot expand further, but because these layers are now further apart the star " cools off" a bit causing its visible light emissions to shift from yellow towards the red part of the spectrum. the Star became a Red giant.

if you go further than this as in the star exhausts is helium supply, the same process happens again, the core will collapse further but for a star this small gravity isnt capable of collapsing it enough to fuse carbon into oxygen, which causes it to release its outer layers(Fusion won the tug of war mentioned above) in the attempt: the star " died" and it just created a planetary nebula, what's left behind is the now dead core of the star a white dwarf which slowly bleed out whatever energy remains thru radiance.