With sprites, we can add a billboard in space that will stay with the same orientation relative to the observer. This could be usefull to show the user some pictures with or without text explanations, diagrams, schematics, etc (it could even be some maths and physics equations from LaTeX, writen in white on a transparent background. That would look pretty, floating in space !). This is NOT the same as a "classic" billboard which will rotate while the user moves around it (pictures of nebulae, oriented toward Earth, for example).
Here's a stupid example : a simple picture (a single sprite !) floating on the side of a (fake) planet.
And apparently, the billboards are all well depth sorted :
The addon here can very easily be edited and adapted to the user's needs :
At first, it may feel strange, but I think it may have a great pedagogical value. With several billboards in the same environment (or several billboards defined in the same CMOD file), it may even look like an art exhibition.
Of course, the sprite based billboard could be made invisible by default, and its visibility could be turned ON/OFF with a celx switch, like what I've done before with my "layers".
I wonder whether stars' shells can be done by nebulae method.