Magic Music Visuals

I search for some kind of clipping box, so a animation goes not not outside another element.
Eg. I have a planet and use the starfield but this starfield should be limited to a rounded area. I remember to made something similar with AfterEffects.
So my question is there some kind of "clipping" box/element or is the only option to use the "scale" module?

I think what you are looking for is the Mask module? For an example, go to Help > Open Sample Project, and select "LayersExamples.magic".

Let me know if it helps.

Aah, ok!
The mask uses a input element (eg. an image) and the size etc. can be modified with some of the geometry modules. I've searched for a mask which can be drawn with some drawing primitives like circle, rectangle etc.

How can this be achieved to have some smooth "fade-out" at the border of the mask, so it's not hard cutted?

All you have to do is use a mask image with a soft edge. For example, there is an image that comes with Magic called "radial.png" (in the sample projects folder, but I attached it again below).
It has an alpha channel, so on this web page it looks like solid white, but in Magic it will look like a soft circle. Try masking with it and see if you like it