It is extremely difficult to tell what you doing in your composition if you don't provide a screenshot of the whole interface with the modified properties of layers giving you trouble revealed. Just select the layers and press the u key twice so we can see what's going on. I could come up with about 100 different ways to get to the point do your screenshot shows.
I am guessing that you are using a particle effect that does not respect the mask. This is pretty normal. If that is so then you need to either per-compose or duplicate the layer, turn off the particle effect on the top copy and use that as a track matte for your Starfield.
I would second what Rick said - pre-compose before applying the mask.
Did you ever solve this issue? Did our experts' advice assist you? Let us know what happened.