To do at least something, and to train myself to XML, I programmed an embryo of Sundog Navigator.
The Java application worked on my PC.
Of course, it is not designed with handhelds in mind, but at least it is something.
The code's maybe not commented enough, but you should focus on the user interface first (I mean run the application).

1. It Shows stars, their names, and user can rotate in 3D with the mouse by right clicking.
   Left-click on a star selects it. Some keys can be pressed to switch of names and such.
2. It takes data about the star in an XML file called DefaultSystem.xml
3. The file astronomy2.java defines Java classes to handle star systems.

If you don't have JRE 1.4.0 installed on your machine, you may have a look at the screenshots instead.

Technical Details :

It is a Java application and not an applet.
To run, the Java Runtime Environment, version at least 1.4.0 I guess, must be installed.
If you have a high-speed connection (or 2 hours to spend) you can download it via java.sun.com
Then, to run my application, unzip Data.zip and CompiledClasses.zip in the some directory, and in that directory, type
> java SundogNavigator
be carefull to type SundogNavigator and not SundogNavigator.java

astronomy2.java defines :
- Sector (set of star system)
- StarSystem (a star and a set of planets)
- Planet (a planet and a set of cities)
- City
The XML reading and writing code is in the class Sector. It uses the SAX implementation provided with Java.