One cause for Celestia crashing is that it has loaded too many Addons and surface textures, resulting in it exceeding the 32-bit address limitation. Here are couple of things to try which will reduce its memory usage:
1. Make sure that Celestia's extras directory does not contain any other Addons before you start the EDU Activity.
2. Before running any Activity, start Celestia, select medium resolution surfaces textures, then exit Celestia. This sets the default to be medres and not hires, thus reducing memory usage. If you continue using Celestia and don't exit after changing the default, it will already have loaded some hires textures, pushing it over the memory limit that much sooner.
Another type of crash happens when you've exceeded the amount of graphics memory in your graphics card. Usually, though, Windows will generate a popup explicitly saying that's what's happened. Unfortunately, Celestia does not provide any way to release either its own or the graphics card's memory. When they fill up, it falls over.
Some other things to take into account:
You are supposed to use the appropriate "starting Celestia Here" link because the associated VisualBasic script specifies a particular .cfg configuration file when it starts Celestia, one which is unique to that specific Activity. Among other things, that config file tells Celestia which "built in" databases to load (like the 1M stars database) and which Activity extras folder to use.
Similarly, you are supposed to use the appropriate "spacecraft sound Here" link because it runs a .celx script in the background. This Activity-specific .celx script implements the individual keystroke commands which provide various effects.
If you haven't used the initialization link required by the part of the Activity that you're working on, however, the usual symptom is not a crash, but rather that the Activity's Cel:// URLs take you to empty space. This happens because Celestia doesn't have access to the Activity-specific extras folder and, as a result, hasn't loaded the correct SSC, STC and DSC catalogs. A workaround for this "empty space" symptom is to move the entire Activity-specific extras folder into your default extras folder.
A workaround for not using the Activity's "spacecraft sound Here" link is to manually select (double-click-on) the appropriate sound .celx script.
p.s. Unfortunately, I doubt there will be a Mac version any time soon. In addition to needing a Mac-specific version of Celestia, there are many Windows-specific features used in the doc files and in the various addons, not least of which are case mis-matches.