When I delete a folder from a catalog, then, later inport it back, all the star ratings are gone. How Can I save the star ratings?
Ratings as well as keywords and captions are kept in the catalog only by default. You can choose to 'write metadata to files' if you want, which stores the same data also on the files themselves and make them available without the organizer.
I understand you have selected all the files in a folder and you have deleted them from the catalog (not also from the disk). You should have written the metadata to the files before deleting, shortcut Ctrl W.