Make Orbit Camera (3d game)

I'm making a 3d game.

I've got a star and I want my camera to orbit the star when the user moves the mouse. This is a pretty standard feature for many 3d games and other 3d programs (think 3d studio max or another modeler that allows the user to rotate the view around an object).

What I know is this:
- Start X,Y when the mouse started to move
- End X,Y where the mouse is at 'now'
- The diff between the two

I've got a camera at some XYZ coordinate looking at the object - what I need help with is calculating where the camera should move based on the XY of the mouse movement.

Anyone?