"Opaque" billboard Nebula images are transparent to stars.
When using a JPEB image as its surface texture, so there is no alpha transparency channel, the billboard Nebula image is transparent to some things and opaque to others: stars can be seen through it, but galaxies cannot. It should block the stars, too.
Here's an example:
This is a link to a larger image. You'll probably have to look at the large image to see the details.
The 3DS model used to hold the billboard JPEG image is square.
The upper left (black) corner is invisible. It's drowned out in the stars that are shining through it.
In contrast, the lower right corner is clearly outlined against the background of the "blobs" drawn to represent the Milky Way.
Note that the Celestial Grid is hidden behind the billboard, too.
My guess is that this may be due to the order in which the objects are being drawn. In other words, this has nothing to do with any transparency that might or might not be incorporated in the image. Galaxies are drawn first, then the grid, then 3ds models, and finally stars. Presumably this is somehow related to the scale factors involved, so that the Z buffer can't be used optimally.