eburacum,
Celestia makes some simplifying assumptions about the distances at which Nebula models can be drawn. There also are bugs in some of Celestia's clipping algorithms. Either could be affecting your swarm.
Celestia draws surface texture images differently depending on where they're specified. In particular, when a surface texture is specified by using a Texture declaration in an SSC file, it's drawn opaque and its Alpha channel is used to define its specularity. If a model specifies the name of a surface texture image internally, the image's Alpha channel is used to define its opacity.
Also, more direct control is available over some of the ways a material is applied to a model if you convert a 3DS model to ASCII CMOD format. You can edit the CMOD model file to change the names of the surface texture images, as well as the opacity, specularity, etc.
The CMOD conversion utilities are available in
http://www.shatters.net/celestia/files/cmodtools-win32-1.0.0.zip
They'll have to be rebuilt from their sourcecode on SourceForge for other platforms. It'd be nice if the Mac port of v1.3.2 (being worked on now) were to include them in its distribution.