This edition of my "Introduction to Add-ons" has been updated for Celestia Version 1.3.0 (and later). Several new features are available which make it a lot easier to keep track of the Add-ons you've installed and to move them to the next version of Celestia. Older Add-ons still work, but if you're willing to change them a little, they'll be much easier to use and understand.
These example Add-ons describe some imaginary objects. There really is a Rosette Nebula, but we really don't know its three-dimensional shape. Also, we know nothing about whatever planetary systems might be in it.
If you don't tell me that something's missing, unclear or wrong, I can't improve it.