Simplify the following text for a Grade 7 native English-speaking person.
Original: Apart from the Sun and stars which act as gravitational lenses , this can be done only in binary and multiple star systems , making the binary stars an important class of stars .
Simplified: One way to tell binary stars and double stars apart is to watch them for a long time . Over a few decades of observation , true binary stars will move around each other , but double stars always stay in the same position .