In Celestia, the stars just stop at the limit of the Hipparcos catalogue. The rest of the galaxy (and other galaxies) are volume rendered using luminous blobs, according to their classification and orientation.
(The Milky Way galaxy rendering is customized so that the arms are correctly arranged according to current radio mapping, even though they contain no rendered stars.)
There is of course some unavoidable speculation: we need radii, rotation periods and oblateness just to show extrasolar planets, for instance, so I derived those using equations based on theory. But you won't find any completely imaginary objects in the Celestia distribution package.

Grant Hutchison