This is probably because they all use the same material.
The Galaxy Behavior is attached to the Galaxy prefab and is set to execute within the editor as well as in run-time. In order for the behavior to update things like scale in edit (non play) mode, it needs to have access to the material. When you instantiate that prefab several times in the editor, however, they will all refer to the same, shared material. This is because the script would otherwise leak new instances of the material into the scene, which is likely not what you want. In run-time, the script is free to refer to its own material instance, however.
If you want to see each galaxy as it is meant to look in run-time while working in the editor, the easiest thing to do is make a material copy per instance of the galaxy and assign them appropriately.