Stuck trying to get certain animation effect

programming flash
1

I saw this today:

http://s0.2mdn.net/1062493/Wellingtoneuromillions728x90.swf

I wonder if anyone might be able to advise please on how to animate the effect where the background changes from blue stars to yellow stars?

I can see it’s done using masks, but I’m stuck trying to animate the 360 degree effect which I suppose would be like an animation of a circle getting thinner and thinner, radiating out from the centre.

I can’t explain it too well, but the effect is there in the swf link above. I’ve also attached an image showing what I mean - the animation would I guess start with a full circle, and then smoothly go to eventually nothing, but starting from the middle!

Ack! I hope the swf link and attachment explain what I mean anyway.

Any advice much appreciated.

Thanks!