Here's how to get it.
I have installed stellarium on my machine with these same steps to ensure that nothing is broken. It's not as scary as it looks.
Note that this guide will not work if you're using a PowerPC machine such as a PowerBook or PlayStation3. Ubuntu doesn't have OpenGL acceleration on the PS3 anyways, and if you want to build a .deb for yourself on a PowerBook or other PPC based machine, let me know; I can walk you through it. Unfortunately, canonical only builds PPA packages for i386 (x86), amd64 (x86_64), and lpia (low power intel architecture)-- which is just another way of saying ...more x86. Also note that this guide will not work on Ubuntu Hardy; only Intrepid and newer. If you're still waiting for a bug or two to get worked out of Intrepid, just hang onto this for later.
Firstly, add the apt-line to your software sources.
System > Administration > Software Sources
Go to the 'Third-Party Software' tab, click the 'add' button near the bottom left corner of the window.
"Enter the complete APT line of the repository that you want to add as source"
When you close the window, it'll tell you it needs to reload the index of stuff you can install, go ahead and do it.
== at this point, you have added my Personal Package Archive to your Ubuntu installation ==
[If you already had stellarium installed, just run updates and you're done. --I'll assume you don't.]
Next, go to Applications > Add/Remove, and search for stellarium. It should return one result, and that one result is of course the one you want.
Check the box next to it and hit 'Apply Changes'. Enter your password, and it will probably complain about how I haven't been knighted by King Shuttleworth. Ignore this and install anyway. I am responsible to Canonical for my behavior in regards to this PPA, having signed their code of conduct with gpg, and I'm not going to mess with anyone.
After installing this application, you will be provided with the opportunity to run it immediately. Do what you want.
Stellarium will show up under Applications > Education, but you can move it wherever you will with the Main Menu Editor found in System > Prefs.
*Enjoying is not required by the GPL; if you choose not to enjoy it, no one can stop you.