It is ported to N900's architecture (armel): http://packages.debian.org/lenny/stellarium which means it runs on Linux/ARM (little endian). AFAIK not ported to Maemo 5. You could ask for a port, pay someone to port it to Maemo 5, or try it in Easy Debian. You'll probably have to use stylus though. GPS should work since that uses standard protocols. IIRC it also utilizes OpenGL; might be a problem performance-wise, but hopefully you can 'downgrade' the quality. Your question might be worth a different topic.