I have added text and an animated model (!) to explain the motion of the sprites with two degrees of freedom. These are sprites 1 and 2. Sprite 4 has 1 degree of freedom. Sprite 8 has 3.
The behavior of the 2-degree sprites at zenith is interesting, but the angle of view is very restricted. I tried to use sprites this way to make sparkles, but was disappointed. You'd probably get a better effect just by direct animation. So, my message is still to use sprites in their well-behaved regimes. Not that I'm one to talk about being well-behaved...
Getting all the transparent objects to not interfere with each other was tricky. Everything opaque, including the stellar sphere, has pass index 0. The "alpha atmosphere" sphere, that causes twinkling, has pass index 1. The sprites and static, transparent part of the little model have pass index 2. The moving part of the little model has pass index 3.
I'm delighted with that model and the other sprites, actually. The stars can be seen to twinkle through the sprites. The transparent parts of the model render correctly, even though they intersect.
Enjoy learning, Explorers, or simply dream under the blazing stars.