> Inside a 'thick' nebula, obscuring all other entities in the sky except for their own star and planets.

Similarly to the alien planet in Eric Flint's "Mother of Demons"; there was a permanent thick cloud cover, and the the slowly moving sky illumination due to the unseen sun was called "the mother of pearl" by the locals.

I also like the idea of living in a dense globular cluster. Interstellar probes and, later, manned missions would be feasible quite early into civilization's technological development - since you're just a few light-weeks (or less) from another star, not a few light-years like us.