You can treat that, and scientist often do treat it, as a stationary system with some error bounds.

For example, the concept of homeostasis in biology is like this. Lots of things are happening inside the living body, but it's still effectively at a functional equilibrium.

Similarly, lots of dynamic things are happening inside the Sun (or any star), but from the perspective of Earth, it is more or less stationary, because the behavior of the sun won't escape some bounds for billions of years.