Ok, all this discussion of the black hole/white fountain has made me realize that, although I thought I understood, I still don't get it.
Urth's sun is dying for some reason. Severian goes to get a White Fountain, but brings it back to a different universe, to Ushas, whose sun is, as far as I know, doing just fine. How does this action help either Urth or Ushas?
I imagine I'm missing something obvious that re-reading Urth or the archives might answer, but would someone mind explaining it one more time?
-Rostrum