Stars Do Not Shine Forever. Simple reasoning explains why. Stars are of finite size. For instance, the Sun's size is 100 times that of the Earth. This is big, but it's still finite.
Finite size means that stars have only finite amounts of nuclear fuel.
Eventually they will exhaust their fuel and fade. You can compare this to driving your car. One tank of fuel lets you drive only a certain number of miles. When the fuel is exhausted, the car will stop. That depends on how much mass they have.